How to use your abacus

Transcription

1 How to use your abacus Reading a number on the abacus. The abacus works on the place value system. Reading it is almost like reading a written numeral. The five beads below the bar each have a value of 1. The two beads above the bar each have a value of 5. The beads which are pushed against the bar represent the number. The number on the abacus is 2,364. Thousands Hundreds Tens Ones Adding on the abacus. Suppose you want to add 2,364+3,473. To do this put 2364 on the abacus. You need to move 3 to the center on the right-hand string. There aren't three singles. Instead, you can move a 5 to the center and move 2 1's back. Move 2 ones and one 5 to the center on the tens string. You have two 5's on the tens string so you can regroup. Move the two 5's on the tens string away from the center and move a 1 to the center on the hundreds string. Now you need to move 4 to the center on the hundreds string. To do this move 5 to the center and one away from the center on the hundreds string. Last step: move 3 ones to the center on the thousands string. You now have five ones at the center on the thousands string, so you move them away and replace them with a 5. Here's the result:

2 MACHINES TO DO ARITHMETIC The term computer dates back to the 1600s. However, until the 1950s, the term referred almost exclusively to a human who performed computations. For human beings, the task of performing large amounts of computation is one that is laborious, time consuming, and error prone. Thus, the human desire to mechanize arithmetic is an ancient one. One of the earliest personal calculators was the abacus, with movable beads strung on rods to count and to do calculations. Although its exact origin is unknown, the abacus was used by the Chinese perhaps 3000 to 4000 years ago and is still used today throughout Asia. Early merchants used the abacus in trading transactions. The ancient British stone monument Stonehenge, located near Salisbury, England, was built between 1900 and 1600 B.C. and, evidently, was used to predict the changes of the seasons. In the twelfth century, a Persian teacher of mathematics in Baghdad, Muhammad ibn-musa al-khowarizm, developed some of the first step-by-step procedures for doing computations. The word algorithm used for such procedures is derived from his name. In Western Europe, the Scottish mathematician John Napier ( ) designed a set of ivory rods (called Napier s bones) to assist with doing multiplications. Napier also developed tables of logarithms and other multiplication machines. The videotape series entitled The Machine That Changed The World is highly recommended by the authors. For information about it, see A Jacquard Loom, Hollerith s tabulator, the ENIAC, UNI- VAC, early chips, and other computer artifacts can also be viewed at the National Museum of American History of the Smithsonian Institution in washington, D.C. Also see this book s Web site for more information about the history of computing. EARLY CALCULATORS 3000 B.C. ABACUS B.C. STONEHENGE 12TH CENTURY: AL-KHOWARIZM 1612 NAPIER S BONES 1

3 The English mathematician William Oughtred invented a circular slide rule in the early 1600s. Slide rules were based on Napier s logarithms, and more modern ones like that shown here were used by engineers and scientists through the 1950s and into the 1960s to do rapid approximate computations SLIDE RULE 1642 PASCALINE The young French mathematician Blaise Pascal ( ) invented one of the first mechanical adding machines to help his father with calculating taxes. It used a series of eight tentoothed wheels (one tooth for each decimal digit), which were connected so that numbers could be added or subtracted by moving the wheels. The Pascaline was a digital calculator, because it represented numerical information as discrete digits, as opposed to a graduated scale like that used in analog instruments of measurement such as nondigital clocks and thermometers. Each digit was represented by a gear that had ten different positions (a ten-state device) so that it could count from 0 through 9 and, upon reaching 10, would reset to 0 and advance the gear in the next column so as to represent the action of carrying to the next digit. Although Pascal built more than 50 of his adding machines, his commercial venture failed because the devices could not be built with sufficient precision for practical use LEIBNIZ CALCULATOR The German mathematician Gottfried Wilhelm von Leibniz invented an improved mechanical calculator that, like the Pascaline, used a system of gears and dials to do calculations. However, it was more reliable and accurate than the Pascaline and could perform all four of the basic arithmetic operations of addition, subtraction, multiplication, and division. A number of other mechanical calculators followed that further refined Pascal s and Leibniz s designs, and by the end of the nineteenth century, these calculators had become important tools in science, business, and commerce. # 2

4 THE STORED PROGRAM The fundamental idea that distinguishes computers from calculators is the concept of a stored program that controls the computation. A program is a sequence of instructions that the computer follows to solve some problem. An income tax form is a good analogy. While a calculator can be a useful tool in the process, computing taxes involves much more than arithmetic. To produce the correct result, one must execute the form s precise sequence of steps of writing numbers down (storage), looking numbers up (retrieval), and computation to produce the correct result. Likewise, a computer program is a precise sequence of steps designed to accomplish some human task. The stored program concept also gives the computer its amazing versatility. Unlike most other machines, which are engineered to mechanize a single task, a computer can be programmed to perform many different tasks that is, the choice of task is deferred to the user. This is the fascinating paradox of the computer: Although its hardware is designed for a very specific task the mechanization of arithmetic computer software programs enable the computer to perform a dizzying array of human tasks, from navigational control of the space shuttle to word processing to musical composition. For this reason, the computer is sometimes called the universal machine. An early example of a stored program automatically controlling a hardware device can be found in the weaving loom invented in 1801 by the Frenchman Joseph Marie Jacquard. Holes punched in metal cards directed the action of this loom: A hole punched in one of the cards would enable its corresponding thread to come through and be incorporated into the weave at a given point in the process; the absence of a hole would exclude an undesired thread. To change to a different weaving pattern, the operator of this loom would simply switch to another set of cards. Jacquard s loom is thus one of the first examples of a programmable machine, and many later computers would make similar use of punched cards. The punched card s present-or-absent hole also marks the early occurrence of another key concept in the history of computing the two-state device,which refers to any mechanism for which there are only two possible conditions. Within a decade, thousands of automated looms were being used in Europe, threatening the traditional weaver s way of life. In protest, English weavers who called themselves Luddites rioted and destroyed several of the new looms and cards. Some of the Luddites were hanged for their actions. (The term Luddite is still used today to refer to someone who is skeptical of new technology.) 1801 JACQUARD LOOM # 3

5 MECHANICAL COMPUTERS 1822 BABBAGE S DIFFERENCE ENGINE 1833 BABBAGE S ANALYTICAL ENGINE # The two fundamental concepts of mechanized calculation and stored program control were combined by the English mathematician Charles Babbage ( ). In Babbage s lifetime, humans involved in almost any form of computation relied heavily upon books of mathematical tables that contained the results of calculations that had already been performed by others. However, such mathematical tables took far too long for humans to produce and were typically rife with errors. Moreover, world travel, the Industrial Revolution, and other new scientific and economic realities had produced an explosion in the need for mathematical computations. It was clear to Babbage that human computers were simply not up to the task of supplying the demand. In 1822, supported by the British government, Babbage began work on a machine that he called the Difference Engine. Comprised of a system of gears, the Difference Engine was designed to compute polynomials for preparing mathematical tables. Babbage continued this work until 1833, when he abandoned this effort having completed only part of the machine. According to Doron Swade, curator of the London Science Museum, the cantankerous Babbage argued with his engineer, ran out of money, and was beset by personal rivalry. In 1833, Babbage began the design of a much more sophisticated machine that he called his Analytical Engine, which was to have over 50,000 components. The operation of this machine was to be far more versatile and fully automatic, controlled by programs stored on punched cards, an idea based on Jacquard s earlier work. In fact, as Babbage himself observed: The analogy of the Analytical Engine with this well-known process is nearly perfect. The basic design of Babbage s Analytical Engine corresponded remarkably to that of modern computers in that it involved the four primary operations of a computer system: processing, storage, input, and output. It included a mill for carrying out the arithmetic computations according to a sequence of instructions (like the central processing unit in modern machines); the store was the machine s memory for storing up to 1, digit numbers and intermediate results; input was to be by means of punched cards; output was to be printed; and other components were designed for the transfer of information between components. When completed, it would have been as large as a locomotive, been powered by steam, and able to calculate to six decimal places of accuracy very rapidly and print out results, all of which was to be controlled by a stored program! Babbage s machine was not built during his lifetime, but it is nevertheless an important part of the history of computing because many of the concepts of its design are used in modern computers. For this reason, Babbage is sometimes called the Father of Computing. 4

6 Ada Augusta, Lord Byron s daughter, was one of the few people other than Babbage who understood the Analytical Engine s design. This enabled her to develop programs for the machine, and for this reason she is sometimes called the first programmer. She described the similarity of Jacquard s and Babbage s inventions: The Analytical Engine weaves algebraic patterns just as the Jacquard loom weaves flowers and leaves. In the 1980s, the programming language Ada was named in her honor ADA AUGUSTA During the next 100 years, little progress was made in realizing Babbage s dream. About the only noteworthy event during this time was the invention by Herman Hollerith of an electric tabulating machine that could tally census statistics that had been stored on punched cards. There was a fear that, because of growing population, it would not be possible to complete processing of the 1890 census before the next one was to be taken. Hollerith s machine enabled the United States Census Bureau to complete the 1890 census in 2 1/2 years. The Hollerith Tabulating Company later merged with other companies to form the International Business Machines (IBM) Corporation. Much of Babbage s dream was finally realized in the Z series of computers developed by the young German engineer Konrad Zuse in the 1930s. Ingeniously, Zuse designed his computers to mechanize arithmetic of binary numbers rather than that of decimal numbers. Because there are only two binary digits, 0 and 1, Zuse could construct his machine from two-state devices instead of ten-state devices, thus greatly simplifying the engineering of his computer. The two-state device Zuse deployed was the electromechanical relay, a twoposition switch that would either complete or break the circuit connecting two phone lines. This mechanism was in wide use in the telephone industry to automate connections previously managed by human operators. However, Zuse ultimately grew dissatisfied with the slow speed at which the relay switched from one state to the other. His assistant, Helmut Schreyer,made the brilliant suggestion of using vacuum tubes, which could switch between states on and off electronically, thousands of times faster than any mechanical device involving moving parts. In the middle of World War II, however, Adolf Hitler was convinced that victory was near and refused to fund Zuse s proposal to build the first fully electronic computer. ELECTROMECHANICAL COMPUTERS 1890 HOLLERITH S TABULATING MACHINE KONRAD ZUSE # 5

7 In addition to building electromechanical computers, Konrad Zuse in 1945 designed a high-level programming language that he named Plankalkül. Although Zuse wrote programs using this language, it was never actually implemented due to a lack of funding. As a result, it lay in obscurity until 1972 when Zuse s manuscripts were discovered. This language was amazingly sophisticated for its time over 15 years passed before its features began to appear in other languages. Zuse designed programs to perform tasks as diverse as integer and floating-point arithmetic, sorting lists of numbers, and playing chess ALAN TURING 1944 MARK I ATANASOFF S ELECTRONIC DIGITAL COMPUTER (ABC) # EARLY ELECTRONIC COMPUTERS World War II also spurred the development of computing devices in the United States, Britain, and Europe. In Britain, Alan Turing developed the universal machine concept, forming the basis of computability theory. (See Chapter 4.) During World War II, he was part of a team whose task was to decrypt intercepted messages of the German forces. Several machines resulted from this British war effort, one of which was the Collosus, finished in The best-known computer built before 1945 was the Harvard Mark I (whose full name was the Harvard IBM Automatic Sequence Controlled Calculator). Like Zuse s Z machines, it was driven by electromechanical relay technology. Repeating much of the work of Babbage, Howard Aiken and others at IBM constructed this large, automatic, general-purpose, electromechanical calculator. It was sponsored by the U.S. Navy and (like Babbage s machines) was intended to compute mathematical and navigational tables. The first fully electronic binary computer, the ABC (Atanasoff Berry Computer), was developed by John Atanasoff and Clifford Berry at Iowa State University during It introduced the ideas of binary arithmetic, regenerative memory, and logic circuits. Unfortunately, because the ABC was never patented and because others failed at the time to see its utility, it took three decades before Atanasoff and Berry received recognition for this remarkable technology. Although the ENIAC ( ) bore the title of the first fully electronic computer for many years, a historic 1973 court decision ruled that Atanasoff was the legal inventor of the first electronic digital computer. 6

8 Grace Murray Hopper ( ) began work as a coder what we today would call a programmer for the Mark I in In the late 1950s, Grandma COBOL, as she has affectionately been called, led the effort to develop the COBOL programming language for business applications. The actual physical components that make up a computer system are its hardware. Several generations of computers can be identified by their type of hardware. First-generation computers are characterized by their extensive use of vacuum tubes. Although they could do calculations much more rapidly than mechanical and electromechanical computers, the heat generated by large numbers of vacuum tubes and their short lifetimes led to frequent failures. The ENIAC (Electronic Numerical Integrator and Computer) is arguably the best known of the early electronic computers (and long thought to be the first). It was designed by J. Presper Eckert and John Mauchly, who began work on it in 1943 at the Moore School of Engineering at the University of Pennsylvania. When it was completed in 1946, this 30-ton machine had 18,000 vacuum tubes, 70,000 resistors, and 5 million soldered joints and consumed 160 kilowatts of electrical power. Stories are told of how the lights in Philadelphia dimmed when the ENIAC was operating. This extremely large machine could multiply numbers approximately 1000 times faster than the Mark I, but it was quite limited in its applications and was used primarily by the Army Ordnance Department to calculate firing tables and trajectories for various types of artillery shells. The instructions that controlled the ENIAC s operation were entered into the machine by rewiring some of the computer s circuits. This complicated process was very time consuming, sometimes taking a number of people several days; during this time, the computer was idle. In other early computers, the instructions were stored outside the machine on punched cards or some other medium and were transferred into the machine one at a time for interpretation and execution. Unfortunately, because of the relative slowness of the moving parts of mechanical input devices in comparison to the electronic parts of the computer dedicated to processing, such computers would always finish executing the instruction long before the next instruction was finished loading. Thus, again, the processing portion of the computer was sitting idle too much of the time GRACE HOPPER FIRST-GENERATION COMPUTERS FIRST-GENERATION COMPUTERS VACUUM TUBES ENIAC # 7

9 1945 JOHN VON NEUMANN S FIRST DRAFT OF A REPORT ON THE EDVAC 1945 COMPUTER BUG In 1945, John von Neumann wrote First Draft of a Report on the EDVAC (Electronic Discrete Variable Automatic Computer) computer in which he described a scheme that required program instructions to be stored internally before execution. This led to his being credited as the inventor of the stored-program concept. The architectural design he described is still known as the von Neumann architecture (although there is evidence that others including Eckert and Mauchly and Zuse had similar ideas several years before this). The advantage of executing instructions from a computer s memory rather than directly from a mechanical input device is that it eliminates time that the computer must spend waiting for instructions. Instructions can be processed more rapidly and more importantly; they can be modified by the computer itself while computations are taking place. The introduction of this scheme to computer architecture was crucial to the development of general-purpose computers. While working on the Mark II computer, Grace Hopper found one of the first computer bugs an actual bug stuck in one of the thousands of relays that has been preserved in the National Museum of American History of the Smithsonian Institution. She glued it into the logbook, and subsequent efforts to find the cause of machine stoppage were reported to Aiken as debugging the computer UNIVAC Eckert and Mauchly left the University of Pennsylvania to form the Eckert Mauchly Computer Corporation, which built the UNIVAC (Universal Automatic Computer). Started in 1946 and completed in 1951, it was the first commercially available computer designed for both scientific and business applications. The UNIVAC achieved instant fame partly due to its correct (albeit unbelieved) prediction on national television of the election of President Eisenhower in the 1952 U.S. presidential election, based upon 5% of the returns. UNIVAC soon became the common name for computers. Soon afterward, because of various setbacks, Eckert and Mauchly sold their company to the Remington Rand Corporation, who sold the first UNIVAC to the Census Bureau in # 8

10 Second-generation computers, built between 1956 and 1963, used transistors in place of the large, cumbersome vacuum tubes, marking the beginning of the great computer shrinkage. These computers were smaller, faster, required less power, generated far less heat, and were more reliable than their predecessors. They were also less expensive. Early computers were difficult to use because of the complex coding schemes used to represent programs and data. A key development during the late 1950s and early 1960s was the development of programming languages that made it much easier to develop programs. SECOND-GENERATION COMPUTERS In 1957, after three years of work, John Backus and his colleagues delivered the first FORTRAN (FORmula TRANslation) compiler for the IBM 704. Their first report commented that a programmer was able to write and debug in four to five hours a program that would have taken several days to complete before. FORTRAN has undergone several revisions and remains a powerful language for scientific computing. In 1958, IBM introduced the first of the second-generation computers (the 7090 and other computers in their 7000 series), vaulting IBM from computer obscurity to first place in the computer industry. Also in 1958, as part of his work in developing artificial intelligence, John McCarthy developed the programming language LISP (LISt Processing) for manipulating strings of symbols, a non-numeric processing language SECOND GENERATION COMPUTERS EARLY TRANSISTORS 1957 FORTRAN 1958 IBM 7090 LISP Since 1952, Grace Hopper had been developing a series of natural-language-like programming languages for use in business data processing. This culminated in 1960 with the development of COBOL (COmmon Business Oriented Language) by an industry-wide team. Since then, more programs have been written in COBOL than in any other programming language. Another language that appeared in 1960 was ALGOL 60 (ALGOrithmic Language), which became the basis of many programming languages that followed, such as Pascal COBOL ALGOL 60 # 9

11 THIRD-GENERATION COMPUTERS Third-generation computers used integrated circuits (IC, chips), which first became commercially available from the Fairchild Corporation. These ICs were based on the pioneering work of Jack Kilby and Robert Noyce. It was also during this period that, in addition to improved hardware, computer manufacturers began to develop collections of programs known as system software, which made computers easier to use. One of the more important advances in this area was the third-generation development of operating systems. Two important early operating systems still used today are Unix (1971) and MS-DOS (1981) THIRD-GENERATION COMPUTERS CHIPS AND INTEGRATED CIRCUITS The IBM System/360, introduced in 1964, is commonly accepted as the first of the third generation of computers. Orders for this family of mutually compatible computers and peripherals climbed to 1000 per month within two years 1964 THE IBM SYSTEM/360 In 1965, Digital Equipment Corporation introduced the PDP- 8, the first commercially successful minicomputer. Because of its speed, small size, and reasonable cost $18,000, less than 20% of the six-digit price tag for an IBM 360 mainframe it became a popular computer in many scientific establishments, small businesses, and manufacturing plants. # 1965 PDP-8 10

12 In 1968, Douglas Engelbart and his research team worked at developing a more user-friendly form of computing, usable by average persons and for purposes other than numerical computation. Engelbart s inventions anticipated many of the attributes of personal computing, including the mouse, word processor, windowed interfaces, integrated Help, and linked text that would later be termed hypertext DOUGLAS ENGELBART: COMPUTER MOUSE, TWO-DIMENSIONAL DISPLAY, EDITING, HYPERMEDIA Disillusioned by how work on the multiuser operating system Multics was proceeding, Ken Thompson of Bell Telephone Laboratories began work in 1969 on a simpler OS aimed at the single user. His first implementation of Unix was written in the assembly language of a spare Digital Equipment Corporation PDP-7 computer. In a pun on the name Multics, the new operating system was named Unix. Unix is still undergoing development today and has become one of the most popular operating systems. It is the only operating system that has been implemented on computers ranging from microcomputers to supercomputers. Another noteworthy event began in 1969 when the Advanced Research Projects Agency (ARPA) of the U.S. Department of Defense introduced the ARPANET, a network linking computers at some of the department s university research centers. Transmissions between the ARPANET computers traveled in the form of packets, each of which was addressed so that it could be routed to its destination. As more and more hosts were added to the ARPANET backbone, it became known as the Internet KEN THOMPSON: UNIX ARPANET THE BEGINNING OF THE INTERNET # 11

13 FOURTH-GENERATION COMPUTERS 1971 INTEL 4004 CHIP Computers from the 1980s on, commonly called fourth-generation computers, use very large-scale integrated (VLSI) circuits on silicon chips and other microelectronic advances to shrink their size and cost still more while enlarging their capabilities. A typical chip is equivalent to millions of transistors, is smaller than a baby s fingernail, weighs a small fraction of an ounce, requires only a trickle of power, and costs but a few dollars. The first chip was the 4004 chip designed by Intel s Ted Hoff, giving birth to the microprocessor, which marked the beginning of the fourth generation of computers. This, along with the first use of an 8-inch floppy disk at IBM, ushered in the era of the personal computer. Robert Noyce, one of the cofounders of the Intel Corporation (which introduced the 4004 microprocessor in 1971), contrasted microcomputers with the ENIAC as follows: An individual integrated circuit on a chip perhaps a quarter of an inch square now can embrace more electronic elements than the most complex piece of electronic equipment that could be built in Today s microcomputer, at a cost of perhaps $300, has more computing capacity than the first electronic computer, ENIAC. It is twenty times faster, has a larger memory, consumes the power of a light bulb rather than that of a locomotive, occupies 1/30,000 the volume and costs 1/10,000 as much. It is available by mail order or at your local hobby shop. To simplify the task of transferring the Unix operating system to other computers, Ken Thompson began to search for a high-level language in which to rewrite Unix. None of the languages in existence at the time were appropriate; therefore, in 1970, Thompson began designing a new language called B. By 1972, it had become apparent that B was not adequate for implementing Unix. At that time, Dennis Ritchie,also at Bell Labs, designed a successor language to B that he called C, and approximately 90 percent of Unix was rewritten in C DENNIS RITCHIE: C ETHERNET HISTORIC COURT DECISION REGARDING FIRST ELECTRONIC COMPUTER # Other noteworthy events in 1973 included the following: Ethernet,the basis for LANs (Local Area Networks), was developed at Xerox PARC by Robert Metcalf A district court in Minneapolis ruled that John Atanasoff was the legal inventor of the first electronic digital computer, thus invalidating Eckert s and Mauchly s patent. 12

14 Noteworthy in 1974: The MITS Altair 8800 hobby-kit computer was invented by Edward Roberts (who coined the term personal computer), William Yates, and Jim Bybee. It was driven by the 8-bit Intel 8080 chip, had 256 bytes of memory, but no keyboard, no display, and no external storage. It sold for $ Bill Gates and Paul Allen wrote a BASIC compiler for the Altair ALTAIR BASIC JOBS & WOZNIAK: APPLE 1 Working in a garage, Steven Jobs and Steve Wozniak developed the Apple I. One of the most popular early personal computers was the Apple II, introduced in 1976 by Steven Jobs and Steve Wozniak. Because of its affordability and the availability of basic software applications, it was an immediate success, especially in schools and colleges APPLE II The first supercomputer and the fastest machine of its day, the Cray I,developed by Seymour Cray, was also introduced in It was built in the shape of a C so components would be close together, reducing the time for electronic signals to travel between them. CRAY 1 Also in 1976, Apple Corporation and Microsoft Corporation were founded. APPLE CORP. MICROSOFT CORP. # 13

15 1981 IBM PC In 1981, IBM entered the personal computer market with the IBM PC, originally called the Acorn. Driven by the Intel 8-bit 8088 chip, it used Microsoft s DOS operating system under an agreement that gave Microsoft all the profits in exchange for their having borne the development costs. MS-DOS thus became the most popular operating system for personal computers, and the PC established a microcomputer standard adopted by many other manufacturers of personal computers. The IBM XT debuted the following year, sporting a 10- megabyte hard disk drive. The IBM AT followed in 1983, driven by the 16-bit Intel microprocessor, the first in the line of Intel s 80x86 chips BJARNE STROUSTRUP: C++ NOVELL ANNOUNCES NETWARE TCP/IP By the late 1970s, a new approach to programming appeared on the scene object-oriented programming (OOP) that emphasized the modeling of objects through classes and inheritance. A research group at Xerox Palo Alto Research Center (PARC) created the first truly object-oriented language, named Smalltalk-80. Another Bell Labs researcher, Bjarne Stroustrup, began the work of extending C with object-oriented features. In 1983, the redesigned and extended programming language C With Classes was introduced with the new name C++. Also in 1983 Novell Data Systems introduced NetWare, a network operating system (NOS), which made possible the construction of a Local Area Network (LAN) of IBM PC-compatible microcomputers. Transmission Control Protocol/Internet Protocol (TCP/IP) became the official protocol governing transmitting and receiving of data via the ARPANET. Later that year, the University of California at Berkeley released a new version of BSD (also known as Berkeley UNIX), which included TCP/IP, thus providing academic computing systems nationwide with the technology to tie into the ARPANET. Explosive growth in the ARPANET resulted. # 14

16 Using a renowned Orwellian advertisement parodying the downtrodden masses subservient to the IBM PC, Apple announced in 1984 the Macintosh, a new personal computer driven by the 32-bit Motorola microprocessor. Inspired by Steve Jobs visit to Xerox PARC in 1979, the Mac brought the graphical user interface (GUI) to personal computing MACINTOSH In 1985, Microsoft introduced Windows 1.0, its graphical user interface for IBM-PC compatibles. It was not until the release of Windows 3.0 in 1990, however, that it gained widespread acceptance WINDOWS In 1986, Intel released the 32-bit chip (better known as the 386 chip), which became the best-selling microprocessor in history. It contained 275,000 transistors. The 80486, released in 1989, had more than a million INTEL 386 CHIP In 1991, CERN (European Organization for Nuclear Research) introduced the World Wide Web, developed by Tim Berners-Lee TIM BERNERS LEE: WWW In 1992, Linus Torvalds developed Linux, a free version of the Unix operating system for PCs LINUX # 15

17 1993 PENTIUM CHIPS POWER PC CHIP MOSAIC APPLE NEWTON 1994 NETSCAPE NAVIGATOR 1.0 YAHOO! Several noteworthy things happened in 1993: Intel introduced the 64-bit Pentium chip containing more than 3 million transistors. The Pentium Pro released in 1995 had more than 5.5 million. The Pentium II followed in 1997 with 7.5 million transistors, and the Pentium III in 1999 with more than 10 million. Motorola shipped the first PowerPC chip. The National Center for Supercomputing Applications (NCSA) at the University of Illinois released the first version of Mosaic, the first graphical Web browser. Apple introduced the Newton, the first palmtop computer. In 1994 Netscape Navigator 1.0 was released. Yahoo!, the first major Web index, went online. It was started in April 1994 by Electrical Engineering Ph.D. candidates at Stanford University, David Filo and Jerry Yang, as a way to keep track of their personal interests on the Internet. Jeff Hawkins and Donna Dubinsky founded Palm Computing. The first Pilot was shipped in JAMES GOSLING: JAVA WINDOWS 95 INTERNET EXPLORER INTERNET GOES COMMERCIAL In 1995, the new C++-based object-oriented programming language Oak, developed at Sun Microsystems by James Gosling, was renamed Java and burst onto the computer scene. Applications created in Java can be deployed without modification to any computing platform, thus making versions for different platforms unnecessary. Other important events in 1995 Microsoft introduced Windows 95. Microsoft released Microsoft Internet Explorer 1.0 to compete with the unforeseen popularity of Netscape. The U.S. Government turned the maintenance of the Internet backbone over to commercial networking companies. Commercial traffic was now allowed on the Internet. America Online, Compuserve, and Prodigy brought the Internet to the public. # 16

18 In 1999 more than $300 billion was spent worldwide in the years leading up to Jan. 1, 2000 to solve the Y2K problem (also known as the millennium bug) the inability of old hardware and software to recognize the century change because years were stored with only two digits. Apple released the PowerMac G4 In 2000 Microsoft launched Windows 2000 AMD's Athlon and Intel's Pentium III broke the 1GHz barrier. In 2001 Apple released MacOS X Microsoft released Windows XP IBM's Almaden Research Center unveiled a quantum computer Y2K PROBLEM POWERMAC G WINDOWS GHZ PROCESSORS 2001 MAC OS X WINDOWS XP 2002 QUANTUM COMPUTER This summary of the history of computing has dealt mainly with the first two important concepts that have shaped the history of computers: the mechanization of arithmetic and the stored program concept. Looking back, we marvel at the advances in technology that have, in barely 50 years, led from ENIAC to today s large array of computer systems, ranging from portable palmtop, laptop, and notebook computers to powerful desktop machines known as workstations, to supercomputers capable of performing billions of operations each second, and to massively parallel computers, which use thousands of microprocessors working together in parallel to solve large problems. Someone once noted that if progress in the automotive industry had been as rapid as in computer technology since 1960, today s automobile would have an engine that is less than 0.1 inch in length, would get 120,000 miles to a gallon of gas, have a top speed of 240,000 miles per hour, and would cost $4. We have also seen how the stored program concept has led to the development of large collections of programs that make computers easier to use. Chief among these is the development of operating systems, such as UNIX, Linux, MS-DOS, MacOS and Windows, that allocate memory for programs and data and carry out many other supervisory functions. They also act as an interface between the user and the machine, interpreting commands given by the user from the keyboard, by a mouse click, or by a spoken command and then directing the appropriate system software and hardware to carry them out. The Graphical User Interface The third key concept that has produced revolutionary change in the evolution of the computer is the graphical user interface (GUI). A user interface is the portion of a software program that responds to commands from the user. User interfaces have evolved greatly in the past two decades, in direct correlation to equally dramatic changes in the typical computer user. # 17

19 In the early 1980s, the personal computer burst onto the scene. However, at the outset, the personal computer did not suit the average person very well. The explosion in the amount of commercially available application software spared computer users the task of learning to program in order to compose their own software; for example, the mere availability of the Lotus spreadsheet software was enough to convince many to buy a PC. Even so, using a computer still required learning many precise and cryptic commands, if not outright programming skills. In the early 1980s, the Apple Corporation decided to take steps to remedy this situation. The Apple II, like its new competitor, the IBM PC, employed a command-line interface, requiring users to learn difficult commands. In the late 1970s, Steve Jobs had visited Xerox PARC and had viewed several technologies that amazed him: the laser printer, Ethernet, and the graphical user interface. It was the last of these that excited Jobs the most, for it offered the prospect of software that computer users could understand almost intuitively. In 1995 interview he said, I remember within ten minutes of seeing the graphical user interface stuff, just knowing that every computer would work this way some day. Drawing upon child development theories, Xerox PARC had developed the graphical user interface for a prototype computer called the Alto that had been realized in The Alto featured a new device that had been dubbed a mouse by its inventor, PARC research scientist Douglas Engelbart. The mouse allowed the user to operate the computer by pointing to icons and selecting options from menus. At the time, however, the cost of the hardware the Alto required made it unfeasible to market, and the brilliant concept went unused. Steve Jobs saw, however, that the same remarkable change in the computer hardware market that had made the personal computer feasible also made the graphical user interface a reasonable possibility. In 1984, in a famous commercial first run during half-time of the Super Bowl, Apple introduced the first GUI personal computer to the world: the Macintosh. In 1985, Microsoft responded with a competing product, the Windows operating system, but until Windows version 3.0 was released in 1990, Macintosh reigned unchallenged in the world of GUI microcomputing. Researchers at the Massachusetts Institute of Technology also brought GUI to the UNIX platform with the release of the X Window system in The graphical user interface has made computers easy to use and has produced many new computer users. At the same time, it has greatly changed the character of computing: computers are now expected to be user friendly. The personal computer, especially, must indeed be personal for the average person and not just for computer programmers. Networks The computer network is a fourth key concept that has greatly influenced the nature of modern computing. Defined simply, a computer network consists of two or more computers that have been connected in order to exchange resources. This could be hardware resources such as processing power, storage, or access to a printer; software resources such as a data file or access to a computer program; or messages between humans such as electronic mail or multimedia World Wide Web pages. As computers became smaller, cheaper, more common, more versatile, and easier to use, computer use rose and with it, the number of computer users. Thus, computers had to be shared. In the early 1960s, timesharing was introduced, in which several persons make simultaneous use of a single computer called a host by way of a collection of terminals, each of which consists of a keyboard for input and either a printer or a monitor to display out- 18

20 put. With a modem (short for modulator/demodulator, because it both modulates binary digits into sounds that can travel over a phone line and, at the other end, demodulates such sounds back into bits), such a terminal connection could be over long distances. Users, however, began to wish for the ability for one host computer to communicate with another. For example, transferring files from one host to another typically meant transporting tapes from one location to the other. In the late 1960s, the Department of Defense began exploring the development of a computer network by which its research centers at various universities could share their computer resources with each other. In 1969, the ARPANET began by connecting research center computers, enabling them to share software and data and to perform another kind of exchange that surprised everyone in terms of its popularity: electronic mail. Hosts were added to the ARPANET backbone in the 1970s, 1980s, and 1990s at an exponential rate, producing a global digital infrastructure that came to be known as the Internet. Likewise, with the introduction of microcomputers in the late 1970s and early 1980s, users began to desire the ability for PCs to share resources. The invention of Ethernet network hardware and such network operating systems as Novell NetWare produced the Local Area Network, or LAN, enabling PC users to share printers and other peripherals, disk storage, software programs, and more. Microsoft also included networking capability as a major feature of its Windows NT. The growth of computer connectivity continues today at a surprising rate. Computers are becoming more and more common, and they are used in isolation less and less. With the advent of affordable and widely available Internet Service Providers (ISPs), many home computers are now wired into a growing global digital infrastructure. 19

21 Vacuum Tube Theory, a Basics Tutorial Page 1 Vacuum Tubes or Thermionic Valves come in many forms including the Diode, Triode, Tetrode, Pentode, Heptode and many more. These tubes have been manufactured by the millions in years gone by and even today the basic technology finds applications in today's electronics scene. It was the vacuum tube that first opened the way to what we know as electronics today, enabling first rectifiers and then active devices to be made and used. Although Vacuum Tube technology may appear to be dated in the highly semiconductor orientated electronics industry, many Vacuum Tubes are still used today in applications ranging from vintage wireless sets to high power radio transmitters. Until recently the most widely used thermionic device was the Cathode Ray Tube that was still manufactured by the million for use in television sets, computer monitors, oscilloscopes and a variety of other electronic equipment. Concept of thermionic emission Thermionic basics The simplest form of vacuum tube is the Diode. It is ideal to use this as the first building block for explanations of the technology. It consists of two electrodes - a Cathode and an Anode held within an evacuated glass bulb, connections being made to them through the glass envelope. If a Cathode is heated, it is found that electrons from the Cathode become increasingly active and as the temperature increases they can actually leave the Cathode and enter the surrounding space. When an electron leaves the Cathode it leaves behind a positive charge, equal but opposite to that of the electron. In fact there are many millions of electrons leaving the Cathode. As unlike charges attract, this means that there is a force pulling the electrons back to the Cathode. Unless there are any further influences the electrons would stay in the vicinity of the Cathode, leaving the Cathode as a result of the energy given to them as a result of the temperature, but being pulled back by the positive charge on the Cathode.

22 Vacuum Tube Theory, a Basics Tutorial Page 2 The Diode the simplest tube In a Diode Vacuum Tube there is also another electrode called the Anode. If a positive potential is applied to this electrode, the electrons will be attracted by this potential and will move towards it if it is at a higher potential than the Cathode. For the optimum performance the space between the Cathode and the Anode should be a vacuum. If there are any gas molecules in the space in which the electrons travel, collisions will occur and this will impede the flow of electrons. If an appreciable amount of gas is present, the electrons will ionise the gas, giving rise to a blue glow between the electrodes. In the early days of valves, it was thought that a certain amount of gas was necessary in the envelope. Later this was discovered that this was not the case and new "hard valves were made that had a superior performance to the older "soft valves. Very early radio receivers often used a soft valve for the detector stage and hard valves for the other stages. Space charge The electrons flowing between the Cathode and the Anode form a cloud which is known as the "space charge". It can tend to repel electrons leaving the Cathode, but if the potential applied to the Anode is sufficiently high then it will be overcome, and electrons will flow toward the Anode. In this way the circuit is completed and current flows. As the potential is increased on the Anode, so the current increases until a point is reached where the space change is completely neutralised and the maximum emission from the Cathode is reached. At this point the emission can only be increased by increasing the Cathode temperature to increase the energy of the electrons and allow further electrons to leave the Cathode. Concept of vacuum tube diode with cathode and anode If the Anode potential is reversed, and made negative with respect to the Cathode it will repel the electrons. No electrons can be emitted from the Anode as it is not hot and no

23 Vacuum Tube Theory, a Basics Tutorial Page 3 current flows. This means that current can only flow in one direction. In other words the device only allows current in one direction, blocking it in the other. In view of this effect, the inventor of the Diode vacuum tube, Professor Sir Ambrose Fleming called it an "oscillation valve" in view of its one way action?. Control of current flow Although the basic concept of the vacuum tube enabled a rectifier to be made, it does not allow for another form of control of the flow of electrons in the Anode circuit. However it was discovered that is a further potential was placed between the Cathode and the Anode this could be used to control the flow of electrons between the Cathode and Anode. Once the theoretical idea was devised, it was necessary to implement a way of placing this potential in the right place. An electrode known as a Grid, in the form of a thin mesh or wire through which the electrons could pass, was inserted between the Cathode and Anode. It was found that by varying the potential on the Grid, this could alter the flow of electrons. The Grid is normally placed at a voltage below that of the Cathode so that it repels the electrons and counteracts the effect of the pull on the electrons from the potential on the Anode. If the voltage on the Grid is varied then it will vary or control the level of current flowing between the Cathode and the Anode. As such, this form of grid is known as a Control Grid. It makes the vacuum tube into an active device that is capable of amplifying signals. Further grids The basic thermionic tube with three electrodes is called a Triode in view of the number of electrodes. To improve the performance of the tube, further grids may be added. These tubes are given generic names that describe the number of electrodes, and thereby giving an indication of the type of tube and performance. Number of grids Total number of electrodes Generic name 1 3 Triode 2 4 Tetrode 3 5 Pentode 4 6 Hexode 5 7 Heptode 6 8 Octode The basic concept of the vacuum tube outlined here enables signals to be rectified and amplified. Many refinements have been added in the form of further grids to enable much better performance to be obtained, but the principles involved are all the same.

24 Vacuum Tube Theory, a Basics Tutorial Page 4 Vacuum tube electrodes The Cathode There is a variety of different types of Cathode that are used in vacuum tubes. They differ in the construction of the Cathode and the materials used. One of the major ways in which Cathodes can be categorised is by the way they are heated. The first type to be used was what is termed directly heated. Here a current is passed through a wire to heat it. In addition to providing the heat it also acts as the Cathode itself, emitting the electrons into the vacuum. This type of Cathode has the disadvantage that it must be connected to both the heater supply and the supply used for use in the Cathode - Anode circuit itself. This has disadvantages because it limits the way the circuit can be biased unless each heater is supplied separately and isolated from each other. A further disadvantage is that if an alternating current is used to provide the heating, this signal can be superimposed upon the main Cathode - Anode circuit, and there is a resultant hum at the frequency of the heater supply. The second type of Cathode is known as an indirectly heated Cathode. Here the heater is electrically disconnected from the Cathode, and heat is radiated from the heater to heat the Cathode. Although as a rule it takes longer for these types of tubes to warm up, they are almost universally used because of the flexibility this provides in biasing the circuits, and in isolating the Cathode - Anode circuit from the effects of hum from the heater supply. The earliest type of Cathode is known as a bright emitter Cathode. This type of Cathode uses a tungsten wire heated to a temperature of between 2500 and 2600 K. Although not widely used these days, this type of Cathode was used in high power transmitting tubes such as those used for broadcasting. It suffers a number of drawbacks, one being that it is not particularly efficient in terms of the emission gained for the heat input. The life of the Cathode is also limited by the evaporation of the tungsten with failure occurring when about 10% of the tungsten has gone. A further type of Cathode is known as a dull emitter. These Cathodes are directly heated and consist of thoriated tungsten. They provide more emission than a tungsten Cathode and require less heat, making the overall efficiency of the tube greater. Typically they run at a temperature of between 1900 and 2100 K. Although these Cathode normally have a relatively long life, they are fragile and any valves or tubes using them should be treated with care and they should not be subjected to technical shocks or vibration. The type of Cathode that is in by far the greatest use is the oxide coated Cathode. These may be used with indirectly heated cathodes, unlike the tungsten and dull emitter Cathodes that must be directly heated as a result of the temperatures involved. This type of Cathode is normally in the form of nickel in the form of a ribbon, tube or even a small cup shape. This is coated with a mixture of barium and strontium carbonate, often with a trace of calcium added. During the manufacturing process the coating is heated to reduce it to its metallic form and the products of the chemical reaction are removed when the tube is finally evacuated. In this Cathode it is the barium that acts as the primary emitter and it operates at a much lower than the other types being in the region of K.

25 Vacuum Tube Theory, a Basics Tutorial Page 5 A wide variety of electron tubes have used radioactive material as a cold cathode - voltage regulators, spark-gap tubes, voltage sensitive switching tubes, glow lamps, etc. In general, such tubes consist of a gas filled glass envelope, a radioactive source, an Anode and an unheated (cold) Cathode. An interesting cold cathode tube was the 0Z4, a rectifier tube that was often used in tube car radios in the 1950 s. This tube did not use a radio active Cathode, it utilised a starter electrode and an Ionically heated cathode More information on radio active cold Cathode devices on this web site: The Anode (called the Plate in the early days of tube technology) The Anode is generally formed into a cylinder so that it can surround the Cathode and any other electrodes that may be present. In this way the vacuum tube can be constructed in a tubular fashion and the Anode can collect the maximum number of electrons. For the smaller tubes used in many radio receivers, the Anodes are generally made of nickel plated steel or simply from nickel. In some instances where larger amounts of heat need to be dissipated it may be carbonised to give it a matt back finish that enables it to radiate more heat out of the tube. For applications where even higher powers are required, the Anode must be capable of dissipating even more heat, and operating at higher temperatures. For these tubes, materials including carbon, molybdenum, or zirconium may be used. Another approach is to build heatsink fins into the Anode structure to help radiate the additional heat. This approach is naturally limited by the construction of the device and the fact that the tube needs to be contained within its glass envelope. However a large heatsink structure will require the glass envelope to be much bigger, thereby increasing the costs. To overcome this problem the Anode may be manufactured so that heat can be transferred outside the valve and removed using a forced air or a water jacket. Using this approach the envelope of the tube can be made relatively small, while still be able to handle significant levels of power. The Grid We have already discovered the Grid is the electrode by which the current flowing in the Anode circuit can be controlled by another potential. In the most basic form a vacuum tube may have one Grid. It is possible to use more than one to improve the performance or to enable additional functions to be performed. Accordingly tubes are named by the number of electrodes they contain that are associated with the electron flow. In other words the filaments or heaters and other similar elements are omitted. A Grid is normally constructed in the form of a gauze mesh or a wire helix. If made of wire, it normally consists of nickel, molybdenum or an alloy and is wound using supporting rods that keep it clear of the Cathode. As such they may be wide, possibly oval in shape and they are generally made from copper or nickel.

26 Vacuum Tube Theory, a Basics Tutorial Page 6 To achieve a high level of performance that is repeatable, the tolerances within the vacuum tube must be maintained from one device to the next. In addition to this it is often necessary to mount the Grid only fractions of a millimetre away from the Cathode or other Grids. To be able to maintain these dimensions one approach that is adopted is to use a stiff rectangular frame and then wind the grid wire onto this under tension. This structure then needs to be fixed by the use of glazing or even gold brazing so that it remains firmly in place. Under some circumstances it may even be necessary to grind the cathode surface coating to ensure its flatness. This form of Grid is known as a Frame Grid. Look inside most tubes and you will see mica structures the support the elements. One important aspect of the design of vacuum tubes is to ensure that the Grid does not overheat. This could lead to mechanical distortion and failure of the whole tube. To assist in the removal of heat the Grid wire may be carbonised, and often cooling fins may be attached to the Grid supporting wires. These supporting wires may also be welded directly to the connection pins in the base of the tube so that heat may be conducted away through the external connections. A wide variety of vacuum tubes are available even today. Using the techniques that have been developed over many years they are able to offer excellent repeatability, performance and reliability. The above information was adapted from the site Naming the Grids The first Grid is called the Control Grid (Grid 1), the second grid is called the Screen Grid (Grid 2) and in a Pentode the third grid is called a Suppressor Grid (Grid 3). With tubes with more than three Grids the other grids are usually named in the same way, Grid 4 Grid 5 etc. In many special purpose tubes with more Grids, some of the Grids may be internally connected to other elements. The Triode The Triode has a Cathode, a Control Grid and an Anode. Any where you have two conductors separated by an insulator you have capacitance, As a result, the Anode and Grid in a Triode Tube have capacitance, ( referred to as parasitic capacitance) between them. Because the tube inverts the signal the capacitance appears to be much bigger than it actually is. This is known as the Miller effect and accounts for the increase in the equivalent input capacitance of an inverting voltage amplifier due to amplification of the effect of capacitance between the input and output terminals. The Miller capacitance between the input and the output of active devices like Vacuum Tubes is a major factor limiting their gain at high frequencies. Miller capacitance was identified in 1920 in T vacuum tubes by John Milton Miller. The Miller Capacitance also can cause instability in high frequency/high gain circuits. This same effect also applies to Transistor circuits.

27 Vacuum Tube Theory, a Basics Tutorial Page 7 The Tetrode, Pentode and Beam Tetrode To combat the stability problems and limited voltage gain due to the Miller effect, the physicist Walter H. Schottky invented the Tetrode tube in He showed that the addition of a second Grid, located between the Control Grid and the Anode, known as the Screen Grid, could solve these problems. "Screen" in this case refers to electrical "screening" or shielding, not physical construction - all "Grid" electrodes in between the Cathode and Anode are "screens" of some sort rather than solid electrodes since they must allow for the passage of electrons directly from the Cathode to the Anode). A positive voltage slightly lower than the Anode voltage was applied the Screen Grid, and was bypassed (for high frequencies) to ground with a capacitor. This arrangement decoupled the Anode and the Control Grid, essentially eliminating the Miller capacitance and its associated problems. Consequently, higher voltage gains from a single tube became possible, reducing the number of tubes required in many circuits. This two-grid tube is called a Tetrode, meaning four active electrodes, and was common by However, the Tetrode had one new problem. In any tube, electrons strike the Anode with sufficient energy to cause the emission of electrons from its surface. In a Triode this socalled secondary emission of electrons is not important since they are simply re-captured by the more positive Anode. But in a Tetrode they can be captured by the Screen Grid also acting as an Anode, since it is also at a high voltage, thus robbing them from the Anode current and reducing the amplification of the device. Since secondary electrons can outnumber the primary electrons, in the worst case, particularly as the Anode voltage dips below the Screen voltage, the Anode current can decrease with increasing Anode voltage. This is the so-called "Tetrode kink" and is an example of negative resistance which can itself cause instability. The otherwise undesirable negative resistance was exploited to produce an extremely simple oscillator circuit only requiring connection of the plate to a resonant LC circuit to oscillate; this was effective over a wide frequency range. The solution was to add another Grid between the Screen Grid and the Anode, called the Suppressor Grid, since it suppressed secondary emission current toward the screen grid. This grid was held at the Cathode (or "ground ) voltage and its negative voltage (relative to the Anode) electrostatically repelled secondary electrons so that they would be collected by the Anode after all. This three-grid tube is called a Pentode, meaning five electrodes. The Pentode was invented in 1926 by Bernard D. H. Tellegen and became generally favoured over the simple Tetrode. Pentodes are made in two classes: those with the suppressor grid wired internally to the Cathode and those with the Suppressor Grid wired to a separate pin for user access. An alternative solution for power applications is the Beam Tetrode or "Beam Power Tube. This is a type of Tetrode vacuum tube with auxiliary beam-focusing Plates designed to

28 Vacuum Tube Theory, a Basics Tutorial Page 8 augment power-handling capability and help reduce unwanted emission effects. These tubes are usually used for power amplification, especially at audio-frequency. Multifunction and multisection tubes Superheterodyne receivers require a local oscillator and mixer, can use a tube that combines these two functions into a single Pentagrid Converter tube. Various alternatives such as using a combination of a Triode with a Hexode and even an Octode have been used for this purpose. The additional Grids include both Control Grids (at a low potential) and Screen Grids (at a high voltage). Many designs used such a Screen Grid as an additional Anode to provide feedback for the oscillator function, whose current was added to that of the incoming radio frequency signal. To further reduce the cost and complexity of radio equipment, two separate structures (Triode and Pentode for instance) could be combined in the bulb of a single multisection tube. An early example was the Loewe 3NF. This 1920s device had three Triodes in a single glass envelope together with all the fixed capacitors and resistors required to make a complete radio receiver. As the Loewe set had only one tube socket, it was able to substantially undercut the competition since, in Germany, state tax was levied by the number of sockets. However, reliability was compromised, and production costs for the tube were much greater. In a sense, these were akin to integrated circuits. In the US, Cleartron briefly produced the "Multivalve" triple triode for use in the Emerson Baby Grand receiver. This Emerson set also had a single tube socket, but because it used a four-pin base, the additional element connections were made on a "mezzanine" platform at the top of the tube base. By 1940 multisection tubes had become commonplace. There were constraints, however, due to patents and other licensing considerations (see British Valve Association). Constraints due to the number of external pins (leads) often forced the functions to share some of those external connections such as their cathode connections (in addition to the heater connection). The RCA Type 55 was a Double Diode Triode used as a detector, automatic gain control Detector and audio preamplifier in early AC powered radios. These sets often included the 53 Dual Triode Audio Output. Other early type of multi-section tubes, the 6SN7 and 6SL7 Octal based "Dual Triodes" performed the functions of two Triode Tubes, while taking up half as much space and costing less. The Miniature Tube Bases Early tubes used a metal or glass envelope fixed to an insulating Bakelite or a ceramic base. In 1938 a technique was developed to use an all-glass construction with the pins fused in the glass base of the envelope. This was used in the design of a much smaller tube outline, known as the miniature tube base, having 7 or 9 pins. The introduction of these miniature tube bases, more than previously available, allowed other multi-section tubes to be introduced. The 12AU7, 12AT7 and 12AX7 dual triodes in a nine pin Noval miniature envelope, became widely used audio signal amplifiers. The 12AX7

29 Vacuum Tube Theory, a Basics Tutorial Page 9 was the "high mu" - highest voltage gain device of the three. Another popular combination was a Triode-Pentode such as the 6BL8, 6U8 and 6GH8. These tubes became popular in domestic radio and television receivers. The desire to include even more functions in one envelope resulted in the General Electric Compactron which had 12 pins. A typical example, the 6AG11, contained two triodes and two diodes. Compactrons were used in the last tube television receivers, built mostly for the American market and was the last gasp of the tube technology. Subminiature tubes Many very small tubes were constructed for specialised functions, for example, tubes roughly the size of half a cigarette were used in hearing-aid amplifiers. These tubes did not usually have pins plugging into a socket but were soldered in place. The "Acorn" tube (named due to its shape) was also very small, and was developed during the 1940 s for very high frequency radio equipment being used during World War II. There is also the metal-cased RCA Nuvistor from 1959, about the size of a thimble. The Nuvistor was developed to compete with the early transistors and operated at higher frequencies than those early transistors could. The small size supported especially highfrequency operation - nuvistors were used in UHF television tuners and some Amateur Radio receivers. A look at this Wikipedia web site will show how tube sockets have evolved over the years Tube numbering List of vacuum tubes From Wikipedia, the free encyclopedia

30 Bipolar Transistor Basics In the Diode tutorials we saw that simple diodes are made up from two pieces of semiconductor material, either silicon or germanium to form a simple PN-junction and we also learnt about their properties and characteristics. If we now join together two individual signal diodes back-to-back, this will give us two PN-junctions connected together in series that share a common P or N terminal. The fusion of these two diodes produces a three layer, two junction, three terminal device forming the basis of a Bipolar Transistor, or BJT for short. Transistors are three terminal active devices made from different semiconductor materials that can act as either an insulator or a conductor by the application of a small signal voltage. The transistor's ability to change between these two states enables it to have two basic functions: "switching" (digital electronics) or "amplification" (analogue electronics). Then bipolar transistors have the ability to operate within three different regions: 1. Active Region - the transistor operates as an amplifier and Ic = β.ib 2. Saturation - the transistor is "fully-on" operating as a switch and Ic = I(saturation) 3. Cut-off - the transistor is "fully-off" operating as a switch and Ic = 0 Typical Bipolar Transistor The word Transistor is an acronym, and is a combination of the words Transfer Varistor used to describe their mode of operation way back in their early days of development. There are two basic types of bipolar transistor construction, NPN and PNP, which basically describes the physical arrangement of the P-type and N-type semiconductor materials from which they are made. The Bipolar Transistor basic construction consists of two PN-junctions producing three connecting terminals with each terminal being given a name to identify it from the other two. These three terminals are known and labelled as the Emitter ( E ), the Base ( B ) and the Collector ( C ) respectively.

31 Bipolar Transistors are current regulating devices that control the amount of current flowing through them in proportion to the amount of biasing voltage applied to their base terminal acting like a current-controlled switch. The principle of operation of the two transistor types NPN and PNP, is exactly the same the only difference being in their biasing and the polarity of the power supply for each type. Bipolar Transistor Construction The construction and circuit symbols for both the NPN and PNP bipolar transistor are given above with the arrow in the circuit symbol always showing the direction of "conventional current flow" between the base terminal and its emitter terminal. The direction of the arrow always points from the positive P-type region to the negative N-type region for both transistor types, exactly the same as for the standard diode symbol. Bipolar Transistor Configurations

32 As the Bipolar Transistor is a three terminal device, there are basically three possible ways to connect it within an electronic circuit with one terminal being common to both the input and output. Each method of connection responding differently to its input signal within a circuit as the static characteristics of the transistor vary with each circuit arrangement. 1. Common Base Configuration - has Voltage Gain but no Current Gain. 2. Common Emitter Configuration - has both Current and Voltage Gain. 3. Common Collector Configuration - has Current Gain but no Voltage Gain. The Common Base (CB) Configuration As its name suggests, in the Common Base or grounded base configuration, the BASE connection is common to both the input signal AND the output signal with the input signal being applied between the base and the emitter terminals. The corresponding output signal is taken from between the base and the collector terminals as shown with the base terminal grounded or connected to a fixed reference voltage point. The input current flowing into the emitter is quite large as its the sum of both the base current and collector current respectively therefore, the collector current output is less than the emitter current input resulting in a current gain for this type of circuit of "1" (unity) or less, in other words the common base configuration "attenuates" the input signal. The Common Base Transistor Circuit This type of amplifier configuration is a non-inverting voltage amplifier circuit, in that the signal voltages Vin and Vout are in-phase. This type of transistor arrangement is not very common due to its unusually high voltage gain characteristics. Its output characteristics represent that of a forward biased diode while the input characteristics represent that of an illuminated photo-diode. Also this type of bipolar transistor configuration has a high ratio of output to input resistance or more importantly "load" resistance (RL) to "input" resistance (Rin) giving it a value of "Resistance Gain". Then the voltage gain (Av for a common base configuration is therefore given as: Common Base Voltage Gain

33 The common base circuit is generally only used in single stage amplifier circuits such as microphone pre-amplifier or radio frequency (Rf) amplifiers due to its very good high frequency response. The Common Emitter (CE) Configuration In the Common Emitter or grounded emitter configuration, the input signal is applied between the base, while the output is taken from between the collector and the emitter as shown. This type of configuration is the most commonly used circuit for transistor based amplifiers and which represents the "normal" method of bipolar transistor connection. The common emitter amplifier configuration produces the highest current and power gain of all the three bipolar transistor configurations. This is mainly because the input impedance is LOW as it is connected to a forward-biased PN-junction, while the output impedance is HIGH as it is taken from a reverse-biased PN-junction. The Common Emitter Amplifier Circuit In this type of configuration, the current flowing out of the transistor must be equal to the currents flowing into the transistor as the emitter current is given as Ie = Ic + Ib. Also, as the load resistance (RL) is connected in series with the collector, the current gain of the common emitter transistor configuration is quite large as it is the ratio of Ic/Ib and is given the Greek symbol of Beta, (β). As the emitter current for a common emitter configuration is defined as Ie = Ic + Ib, the ratio of Ic/Ie is called Alpha, given the Greek symbol of α. Note: that the value of Alpha will always be less than unity. Since the electrical relationship between these three currents, Ib, Ic and Ie is determined by the physical construction of the transistor itself, any small change in the base current (Ib), will result in a much larger change in the collector

34 current (Ic). Then, small changes in current flowing in the base will thus control the current in the emitter-collector circuit. Typically, Beta has a value between 20 and 200 for most general purpose transistors. By combining the expressions for both Alpha, α and Beta, β the mathematical relationship between these parameters and therefore the current gain of the transistor can be given as: Where: "Ic" is the current flowing into the collector terminal, "Ib" is the current flowing into the base terminal and "Ie" is the current flowing out of the emitter terminal. Then to summarise, this type of bipolar transistor configuration has a greater input impedance, current and power gain than that of the common base configuration but its voltage gain is much lower. The common emitter configuration is an inverting amplifier circuit resulting in the output signal being 180 o out-of-phase with the input voltage signal. The Common Collector (CC) Configuration In the Common Collector or grounded collector configuration, the collector is now common through the supply. The input signal is connected directly to the base, while the output is taken from the emitter load as shown. This type of configuration is commonly known as a Voltage Follower or Emitter Follower circuit. The emitter follower configuration is very useful for impedance matching applications because of the very high input impedance, in the region of hundreds of thousands of Ohms while having a relatively low output impedance. The Common Collector Transistor Circuit

35 The common emitter configuration has a current gain approximately equal to the β value of the transistor itself. In the common collector configuration the load resistance is situated in series with the emitter so its current is equal to that of the emitter current. As the emitter current is the combination of the collector AND the base current combined, the load resistance in this type of transistor configuration also has both the collector current and the input current of the base flowing through it. Then the current gain of the circuit is given as: The Common Collector Current Gain This type of bipolar transistor configuration is a non-inverting circuit in that the signal voltages of Vin and Vout are inphase. It has a voltage gain that is always less than "1" (unity). The load resistance of the common collector transistor receives both the base and collector currents giving a large current gain (as with the common emitter configuration) therefore, providing good current amplification with very little voltage gain.

36 Bipolar Transistor Summary Then to summarise, the behaviour of the bipolar transistor in each one of the above circuit configurations is very different and produces different circuit characteristics with regards to input impedance, output impedance and gain whether this is voltage gain, current gain or power gain and this is summarised in the table below. Bipolar Transistor Characteristics The static characteristics for a Bipolar Transistor can be divided into the following three main groups. Input Characteristics:- Common Base - ΔV EB / ΔI E Common Emitter - ΔV BE / ΔI B Output Characteristics:- Common Base - ΔV C / ΔI C Common Emitter - ΔV C / ΔI C Transfer Characteristics:- Common Base - Common Emitter - ΔI C / ΔI E ΔI C / ΔI B with the characteristics of the different transistor configurations given in the following table: Characteristic Common Base Common Emitter Common Collector Input Impedance Low Medium High Output Impedance Very High High Low Phase Angle 0 o 180 o 0 o Voltage Gain High Medium Low Current Gain Low Medium High Power Gain Low Very High Medium In the next tutorial about Bipolar Transistors, we will look at the NPN Transistor in more detail when used in the common emitter configuration as an amplifier as this is the most widely used configuration due to its flexibility and high gain. We will also plot the output characteristics curves commonly associated with amplifier circuits as a function of the collector current to the base current. The NPN Transistor

37 In the previous tutorial we saw that the standard Bipolar Transistor or BJT, comes in two basic forms. An NPN (Negative-Positive-Negative) type and a PNP (Positive-Negative-Positive) type, with the most commonly used transistor type being the NPN Transistor. We also learnt that the transistor junctions can be biased in one of three different ways - Common Base, Common Emitter and Common Collector. In this tutorial we will look more closely at the "Common Emitter" configuration using NPN Transistors with an example of the construction of a NPN transistor along with the transistors current flow characteristics is given below. An NPN Transistor Configuration Note: Conventional current flow. We know that the transistor is a "current" operated device (Beta model) and that a large current ( Ic ) flows freely through the device between the collector and the emitter terminals when the transistor is switched "fully-on". However, this only happens when a small biasing current ( Ib ) is flowing into the base terminal of the transistor at the same time thus allowing the Base to act as a sort of current control input. The transistor current in an NPN transistor is the ratio of these two currents ( Ic/Ib ), called the DC Current Gain of the device and is given the symbol of hfe or nowadays Beta, ( β ). The value of β can be large up to 200 for standard transistors, and it is this large ratio between Ic and Ib that makes the NPN transistor a useful amplifying device when used in its active region as Ib provides the input and Ic provides the output. Note that Beta has no units as it is a ratio. Also, the current gain of the transistor from the Collector terminal to the Emitter terminal, Ic/Ie, is called Alpha, ( α ), and is a function of the transistor itself (electrons diffusing across the junction). As the emitter current Ie is the product of a very small base current plus a very large collector current, the value of alpha α, is very close to unity, and for a typical low-power signal transistor this value ranges from about to α and β Relationship in a NPN Transistor

38 By combining the two parameters α and β we can produce two mathematical expressions that gives the relationship between the different currents flowing in the transistor. The values of Beta vary from about 20 for high current power transistors to well over 1000 for high frequency low power type bipolar transistors. The value of Beta for most standard NPN transistors can be found in the manufactures datasheets but generally range between The equation above for Beta can also be re-arranged to make Ic as the subject, and with a zero base current ( Ib = 0 ) the resultant collector current Ic will also be zero, ( β x 0 ). Also when the base current is high the corresponding

39 collector current will also be high resulting in the base current controlling the collector current. One of the most important properties of the Bipolar Junction Transistor is that a small base current can control a much larger collector current. Consider the following example. Example No1 An NPN Transistor has a DC current gain, (Beta) value of 200. Calculate the base current Ib required to switch a resistive load of 4mA. Therefore, β = 200, Ic = 4mA and Ib = 20µA. One other point to remember about NPN Transistors. The collector voltage, ( Vc ) must be greater and positive with respect to the emitter voltage, ( Ve ) to allow current to flow through the transistor between the collector-emitter junctions. Also, there is a voltage drop between the Base and the Emitter terminal of about 0.7v (one diode volt drop) for silicon devices as the input characteristics of an NPN Transistor are of a forward biased diode. Then the base voltage, ( Vbe ) of a NPN transistor must be greater than this 0.7V otherwise the transistor will not conduct with the base current given as. Where: Ib is the base current, Vb is the base bias voltage, Vbe is the base-emitter volt drop (0.7v) and Rb is the base input resistor. Increasing Ib, Vbe slowly increases to 0.7V but Ic rises exponentially. Example No2 An NPN Transistor has a DC base bias voltage, Vb of 10v and an input base resistor, Rb of 100kΩ. What will be the value of the base current into the transistor. Therefore, Ib = 93µA.

40 The Common Emitter Configuration. As well as being used as a semiconductor switch to turn load currents "ON" or "OFF" by controlling the Base signal to the transistor in ether its saturation or cut-off regions, NPN Transistors can also be used in its active region to produce a circuit which will amplify any small AC signal applied to its Base terminal with the Emitter grounded. If a suitable DC "biasing" voltage is firstly applied to the transistors Base terminal thus allowing it to always operate within its linear active region, an inverting amplifier circuit called a single stage common emitter amplifier is produced. One such Common Emitter Amplifier configuration of an NPN transistor is called a Class A Amplifier. A "Class A Amplifier" operation is one where the transistors Base terminal is biased in such a way as to forward bias the Baseemitter junction. The result is that the transistor is always operating halfway between its cut-off and saturation regions, thereby allowing the transistor amplifier to accurately reproduce the positive and negative halves of any AC input signal superimposed upon this DC biasing voltage. Without this "Bias Voltage" only one half of the input waveform would be amplified. This common emitter amplifier configuration using an NPN transistor has many applications but is commonly used in audio circuits such as pre-amplifier and power amplifier stages. With reference to the common emitter configuration shown below, a family of curves known as the Output Characteristics Curves, relates the output collector current, (Ic) to the collector voltage, (Vce) when different values of Base current, (Ib) are applied to the transistor for transistors with the same β value. A DC "Load Line" can also be drawn onto the output characteristics curves to show all the possible operating points when different values of base current are applied. It is necessary to set the initial value of Vce correctly to allow the output voltage to vary both up and down when amplifying AC input signals and this is called setting the operating point or Quiescent Point, Q- point for short and this is shown below. Single Stage Common Emitter Amplifier Circuit Output Characteristics Curves for a Typical Bipolar Transistor

41 The most important factor to notice is the effect of Vce upon the collector current Ic when Vce is greater than about 1.0 volts. We can see that Ic is largely unaffected by changes in Vce above this value and instead it is almost entirely controlled by the base current, Ib. When this happens we can say then that the output circuit represents that of a "Constant Current Source". It can also be seen from the common emitter circuit above that the emitter current Ie is the sum of the collector current, Ic and the base current, Ib, added together so we can also say that " Ie = Ic + Ib " for the common emitter configuration. By using the output characteristics curves in our example above and also Ohm s Law, the current flowing through the load resistor, (RL), is equal to the collector current, Ic entering the transistor which inturn corresponds to the supply voltage, (Vcc) minus the voltage drop between the collector and the emitter terminals, (Vce) and is given as: Also, a straight line representing the Load Line of the transistor can be drawn directly onto the graph of curves above from the point of "Saturation" ( A ) when Vce = 0 to the point of "Cut-off" ( B ) when Ic = 0 thus giving us the "Operating" or Q-point of the transistor. These two points are joined together by a straight line and any position along

42 this straight line represents the "Active Region" of the transistor. The actual position of the load line on the characteristics curves can be calculated as follows: Then, the collector or output characteristics curves for Common Emitter NPN Transistors can be used to predict the Collector current, Ic, when given Vce and the Base current, Ib. A Load Line can also be constructed onto the curves to determine a suitable Operating or Q-point which can be set by adjustment of the base current. The slope of this load line is equal to the reciprocal of the load resistance which is given as: -1/R L In the next tutorial about Bipolar Transistors, we will look at the opposite or compliment form of the NPN Transistor called the PNP Transistor and show that the PNP Transistor has very similar characteristics to their NPN transistor except that the polarities (or biasing) of the current and voltage directions are reversed. The PNP Transistor The PNP Transistor is the exact opposite to the NPN Transistor device we looked at in the previous tutorial. Basically, in this type of transistor construction the two diodes are reversed with respect to the NPN type, with the arrow, which also defines the Emitter terminal this time pointing inwards in the transistor symbol. Also, all the polarities are reversed which means that PNP Transistors "sink" current as opposed to the NPN transistor which "sources" current. Then, PNP Transistors use a small output base current and a negative base voltage to control a much larger emitter-collector current. The construction of a PNP transistor consists of two P-type semiconductor materials either side of the N-type material as shown below. A PNP Transistor Configuration

43 Note: Conventional current flow. The PNP Transistor has very similar characteristics to their NPN bipolar cousins, except that the polarities (or biasing) of the current and voltage directions are reversed for any one of the possible three configurations looked at in the first tutorial, Common Base, Common Emitter and Common Collector. Generally, PNP Transistors require a negative (-ve) voltage at their Collector terminal with the flow of current through the emitter-collector terminals being Holes as opposed to Electrons for the NPN types. Because the movement of holes across the depletion layer tends to be slower than for electrons, PNP transistors are generally more slower than their equivalent NPN counterparts when operating. To cause the Base current to flow in a PNP transistor the Base needs to be more negative than the Emitter (current must leave the base) by approx 0.7 volts for a silicon device or 0.3 volts for a germanium device with the formulas used to calculate the Base resistor, Base current or Collector current are the same as those used for an equivalent NPN transistor and is given as. Generally, the PNP transistor can replace NPN transistors in electronic circuits, the only difference is the polarities of the voltages, and the directions of the current flow. PNP Transistors can also be used as switching devices and an example of a PNP transistor switch is shown below. A PNP Transistor Circuit

44 The Output Characteristics Curves for a PNP transistor look very similar to those for an equivalent NPN transistor except that they are rotated by 180 o to take account of the reverse polarity voltages and currents, (the currents flowing out of the Base and Collector in a PNP transistor are negative). Transistor Matching You may think what is the point of having a PNP Transistor, when there are plenty of NPN Transistors available?. Well, having two different types of transistors PNP & NPN, can be an advantage when designing amplifier circuits such as Class B Amplifiers that use "Complementary" or "Matched Pair" transistors or for reversible H-Bridge motor control circuits. A pair of corresponding NPN and PNP transistors with near identical characteristics to each other are called Complementary Transistors for example, a TIP3055 (NPN), TIP2955 (PNP) are good examples of complementary or matched pair silicon power transistors. They have a DC current gain, Beta, (Ic / Ib) matched to within 10% and high Collector current of about 15A making them suitable for general motor control or robotic applications. Identifying the PNP Transistor We saw in the first tutorial of this Transistors section, that transistors are basically made up of two Diodes connected together back-to-back. We can use this analogy to determine whether a transistor is of the type PNP or NPN by testing its Resistance between the three different leads, Emitter, Base and Collector. By testing each pair of transistor leads in both directions will result in six tests in total with the expected resistance values in Ohm's given below. 1. Emitter-Base Terminals - The Emitter to Base should act like a normal diode and conduct one way only. 2. Collector-Base Terminals - The Collector-Base junction should act like a normal diode and conduct one way only.

45 3. Emitter-Collector Terminals - The Emitter-Collector should not conduct in either direction. Transistor Resistance Values for the PNP transistor and NPN transistor types Between Transistor Terminals PNP NPN Collector Emitter R HIGH R HIGH Collector Base R LOW R HIGH Emitter Collector R HIGH R HIGH Emitter Base R LOW R HIGH Base Collector R HIGH R LOW Base Emitter R HIGH R LOW The Transistor as a Switch When used as an AC signal amplifier, the transistors Base biasing voltage is applied so that it operates within its "Active" region and the linear part of the output characteristics curves are used. However, both the NPN & PNP type bipolar transistors can be made to operate as an "ON/OFF" type solid state switch for controlling high power devices such as motors, solenoids or lamps. If the circuit uses the Transistor as a Switch, then the biasing is arranged to operate in the output characteristics curves seen previously in the areas known as the "Saturation" and "Cut-off" regions as shown below. Transistor Curves

46 The pink shaded area at the bottom represents the "Cut-off" region. Here the operating conditions of the transistor are zero input base current (Ib), zero output collector current (Ic) and maximum collector voltage (Vce) which results in a large depletion layer and no current flows through the device. The transistor is switched "Fully-OFF". The lighter blue area to the left represents the "Saturation" region. Here the transistor will be biased so that the maximum amount of base current is applied, resulting in maximum collector current flow and minimum collector emitter voltage which results in the depletion layer being as small as possible and maximum current flows through the device. The transistor is switched "Fully-ON". Then we can summarize this as: 1. Cut-off Region - Both junctions are Reverse-biased, Base current is zero or very small resulting in zero Collector current flowing, the device is switched fully "OFF". 2. Saturation Region - Both junctions are Forward-biased, Base current is high enough to give a Collector-Emitter voltage of 0v resulting in maximum Collector current flowing, the device is switched fully "ON". An example of an NPN Transistor as a switch being used to operate a relay is given below. With inductive loads such as relays or solenoids a flywheel diode is placed across the load to dissipate the back EMF generated by the inductive load when the transistor switches "OFF" and so protect the transistor from damage. If the load is of a very high current or voltage nature, such as motors, heaters etc, then the load current can be controlled via a suitable relay as shown. Transistor Switching Circuit The circuit resembles that of the Common Emitter circuit we looked at in the previous tutorials. The difference this time is that to operate the transistor as a switch the transistor needs to be turned either fully "OFF" (Cut-off) or fully "ON" (Saturated). An ideal transistor switch would have an infinite resistance when turned "OFF" resulting in zero current flow and zero resistance when turned "ON", resulting in maximum current flow. In practice when turned "OFF", small leakage currents flow through the transistor and when fully "ON" the device has a low resistance value causing

47 a small saturation voltage (Vce) across it. In both the Cut-off and Saturation regions the power dissipated by the transistor is at its minimum. To make the Base current flow, the Base input terminal must be made more positive than the Emitter by increasing it above the 0.7 volts needed for a silicon device. By varying the Base-Emitter voltage Vbe, the Base current is altered and which in turn controls the amount of Collector current flowing through the transistor as previously discussed. When maximum Collector current flows the transistor is said to be Saturated. The value of the Base resistor determines how much input voltage is required and corresponding Base current to switch the transistor fully "ON". Example No1. For example, using the transistor values from the previous tutorials of: β = 200, Ic = 4mA and Ib = 20uA, find the value of the Base resistor (Rb) required to switch the load "ON" when the input terminal voltage exceeds 2.5v. Example No2. Again using the same values, find the minimum Base current required to turn the transistor fully "ON" (Saturated) for a load that requires 200mA of current. Transistor switches are used for a wide variety of applications such as interfacing large current or high voltage devices like motors, relays or lamps to low voltage digital logic IC's or gates like AND Gates or OR Gates. Here, the output from a digital logic gate is only +5v but the device to be controlled may require a 12 or even 24 volts supply. Or the load such as a DC Motor may need to have its speed controlled using a series of pulses (Pulse Width Modulation) and transistor switches will allow us to do this faster and more easily than with conventional mechanical switches. Digital Logic Transistor Switch

48 The base resistor, Rb is required to limit the output current of the logic gate. Darlington Transistors Sometimes the DC current gain of the bipolar transistor is too low to directly switch the load current or voltage, so multiple switching transistors are used. Here, one small input transistor is used to switch "ON" or "OFF" a much larger current handling output transistor. To maximise the signal gain the two transistors are connected in a "Complementary Gain Compounding Configuration" or what is generally called a "Darlington Configuration" where the amplification factor is the product of the two individual transistors. Darlington Transistors simply contain two individual bipolar NPN or PNP type transistors connected together so that the current gain of the first transistor is multiplied with that of the current gain of the second transistor to produce a device which acts like a single transistor with a very high current gain. The overall current gain Beta (β) or Hfe value of a Darlington device is the product of the two individual gains of the transistors and is given as: So Darlington Transistors with very high β values and high Collector currents are possible compared to a single transistor. An example of the two basic types of Darlington transistor are given below. Darlington Transistor Configurations

49 The above NPN Darlington transistor configuration shows the Collectors of the two transistors connected together with the Emitter of the first transistor connected to the Base of the second transistor therefore, the Emitter current of the first transistor becomes the Base current of the second transistor. The first or "input" transistor receives an input signal, amplifies it and uses it to drive the second or "output" transistors which amplifies it again resulting in a very high current gain. As well as its high increased current and voltage switching capabilities, another advantage of a Darlington transistor is in its high switching speeds making them ideal for use in Inverter circuits and DC motor or stepper motor control applications.

50 One difference to consider when using Darlington transistors over the conventional single bipolar transistor type is that the Base-Emitter input voltage Vbe needs to be higher at approx 1.4v for silicon devices, due to the series connection of the two PN junctions. Then to summarise when using a Transistor as a Switch. Transistor switches can be used to switch and control lamps, relays or even motors. When using bipolar transistors as switches they must be fully "OFF" or fully "ON". Transistors that are fully "ON" are said to be in their Saturation region. Transistors that are fully "OFF" are said to be in their Cut-off region. In a transistor switch a small Base current controls a much larger Collector current. When using transistors to switch inductive relay loads a "Flywheel Diode" is required. When large currents or voltages need to be controlled, Darlington Transistors are used. The Field Effect Transistor In the Bipolar Junction Transistor tutorials, we saw that the output Collector current is determined by the amount of current flowing into the Base terminal of the device and thereby making the Bipolar Transistor a CURRENT operated device. The Field Effect Transistor, or simply FET however, use the voltage that is applied to their input terminal to control the output current, since their operation relies on the electric field (hence the name field effect) generated by the input voltage. This then makes the Field Effect Transistor a VOLTAGE operated device. The Field Effect Transistor is a unipolar device that has very similar properties to those of the Bipolar Transistor ie, high efficiency, instant operation, robust and cheap, and they can be used in most circuit applications that use the equivalent Bipolar Junction Transistors, (BJT). They can be made much smaller than an equivalent BJT transistor and along with their low power consumption and dissipation make them ideal for use in integrated circuits such as the CMOS range of chips. We remember from the previous tutorials that there are two basic types of Bipolar Transistor construction, NPN and PNP, which basically describes the physical arrangement of the P-type and N-type semiconductor materials from which they are made. There are also two basic types of Field Effect Transistor, N-channel and P-channel. As their name implies, Bipolar Transistors are "Bipolar" devices because they operate with both types of charge carriers, Holes and Electrons. The Field Effect Transistor on the other hand is a "Unipolar" device that depends only on the conduction of Electrons (N-channel) or Holes (P-channel).

51 The Field Effect Transistor has one major advantage over its standard bipolar transistor cousins, in that their input impedance is very high, (Thousands of Ohms) making them very sensitive to input signals, but this high sensitivity also means that they can be easily damaged by static electricity. There are two main types of field effect transistor, the Junction Field Effect Transistor or JFET and the Insulated-gate Field Effect Transistor or IGFET), which is more commonly known as the standard Metal Oxide Semiconductor Field Effect Transistor or MOSFET for short. The Junction Field Effect Transistor We saw previously that a bipolar junction transistor is constructed using two PN junctions in the main current path between the Emitter and the Collector terminals. The Field Effect Transistor has no junctions but instead has a narrow "Channel" of N-type or P-type silicon with electrical connections at either end commonly called the DRAIN and the SOURCE respectively. Both P-channel and N-channel FET's are available. Within this channel there is a third connection which is called the GATE and this can also be a P or N-type material forming a PN junction and these connections are compared below. Bipolar Transistor Emitter - (E) Base - (B) Collector - (C) Field Effect Transistor Source - (S) Gate - (G) Drain - (D) The semiconductor "Channel" of the Junction Field Effect Transistor is a resistive path through which a voltage V ds causes a current I d to flow. A voltage gradient is thus formed down the length of the channel with this voltage becoming less positive as we go from the drain terminal to the source terminal. The PN junction therefore has a high reverse bias at the drain terminal and a lower reverse bias at the source terminal. This bias causes a "depletion layer" to be formed within the channel and whose width increases with the bias. FET's control the current flow through them between the drain and source terminals by controlling the voltage applied to the gate terminal. In an N-channel JFET this gate voltage is negative while for a P-channel JFET the gate voltage is positive. Bias arrangement for an N-channel JFET and corresponding circuit symbols.

52 The cross sectional diagram above shows an N-type semiconductor channel with a P-type region called the gate diffused into the N-type channel forming a reverse biased PN junction and its this junction which forms the depletion layer around the gate area. This depletion layer restricts the current flow through the channel by reducing its effective width and thus increasing the overall resistance of the channel. When the gate voltage V g is equal to 0V and a small external voltage (V ds ) is applied between the drain and the source maximum current (I d ) will flow through the channel slightly restricted by the small depletion layer. If a negative voltage (V gs ) is now applied to the gate the size of the depletion layer begins to increase reducing the overall effective area of the channel and thus reducing the current flowing through it, a sort of "squeezing" effect. As the gate voltage (V gs ) is made more negative, the width of the channel decreases until no more current flows between the drain and the source and the FET is said to be "pinched-off". In this pinch-off region the gate voltage, V gs controls the channel current and V ds has little or no effect. The result is that the FET acts more like a voltage controlled resistor which has zero resistance when V gs = 0 and maximum "ON" resistance (R ds ) when the gate voltage is very negative. Output characteristic voltage-current curves of a typical junction FET.

53 The voltage V gs applied to the gate controls the current flowing between the drain and the source terminals. V gs refers to the voltage applied between the gate and the source while V ds refers to the voltage applied between the drain and the source. Because a Field Effect Transistor is a VOLTAGE controlled device, "NO current flows into the gate!" then the source current (I s ) flowing out of the device equals the drain current flowing into it and therefore (I d = I s ). The characteristics curves example shown above, shows the four different regions of operation for a JFET and these are given as: Ohmic Region - The depletion layer of the channel is very small and the JFET acts like a variable resistor. Cut-off Region - The gate voltage is sufficient to cause the JFET to act as an open circuit as the channel resistance is at maximum. Saturation or Active Region - The JFET becomes a good conductor and is controlled by the gate-source voltage, (V gs) while the drain-source voltage, (V ds) has little or no effect.

54 Breakdown Region - The voltage between the drain and source, (V ds) is high enough to causes the JFET's resistive channel to break down and pass current. The control of the drain current by a negative gate potential makes the Junction Field Effect Transistor useful as a switch and it is essential that the gate voltage is never positive for an N-channel JFET as the channel current will flow to the gate and not the drain resulting in damage to the JFET. The principals of operation for a P-channel JFET are the same as for the N-channel JFET, except that the polarity of the voltages need to be reversed. The MOSFET As well as the Junction Field Effect Transistor, there is another type of Field Effect Transistor available whose Gate input is electrically insulated from the main current carrying channel and is therefore called an Insulated Gate Field Effect Transistor. The most common type of insulated gate FET or IGFET as it is sometimes called, is the Metal Oxide Semiconductor Field Effect Transistor or MOSFET for short. The MOSFET type of field effect transistor has a "Metal Oxide" gate (usually silicon dioxide commonly known as glass), which is electrically insulated from the main semiconductor N-channel or P-channel. This isolation of the controlling gate makes the input resistance of the MOSFET extremely high in the Mega-ohms region and almost infinite. As the gate terminal is isolated from the main current carrying channel ""NO current flows into the gate"" and like the JFET, the MOSFET also acts like a voltage controlled resistor. Also like the JFET, this very high input resistance can easily accumulate large static charges resulting in the MOSFET becoming easily damaged unless carefully handled or protected. Basic MOSFET Structure and Symbol

55 We also saw previously that the gate of a JFET must be biased in such a way as to forward-bias the PN junction but in a MOSFET device no such limitations applies so it is possible to bias the gate in either polarity. This makes MOSFET's specially valuable as electronic switches or to make logic gates because with no bias they are normally non-conducting and the high gate resistance means that very little control current is needed. Both the P-channel and the N-channel MOSFET is available in two basic forms, the Enhancement type and the Depletion type. Depletion-mode MOSFET The Depletion-mode MOSFET, which is less common than the enhancement types is normally switched "ON" without a gate bias voltage but requires a gate to source voltage (V gs ) to switch the device "OFF". Similar to the JFET types. For N-channel MOSFET's a "Positive" gate voltage widens the channel, increasing the flow of the drain current and decreasing the drain current as the gate voltage goes more negative. The opposite is also true for the P- channel types. The depletion mode MOSFET is equivalent to a "Normally Closed" switch. Depletion-mode N-Channel MOSFET and circuit Symbols

56 Depletion-mode MOSFET's are constructed similar to their JFET transistor counterparts where the drain-source channel is inherently conductive with electrons and holes already present within the N-type or P-type channel. This doping of the channel produces a conducting path of low resistance between the drain and source with zero gate bias. Enhancement-mode MOSFET The more common Enhancement-mode MOSFET is the reverse of the depletion-mode type. Here the conducting channel is lightly doped or even undoped making it non-conductive. This results in the device being normally "OFF" when the gate bias voltage is equal to zero. A drain current will only flow when a gate voltage (V gs ) is applied to the gate terminal. This positive voltage creates an electrical field within the channel attracting electrons towards the oxide layer and thereby reducing the overall resistance of the channel allowing current to flow. Increasing this positive gate voltage will cause an increase in the drain current, I d through the channel. Then, the Enhancement-mode device is equivalent to a "Normally Open" switch.

57 Enhancement-mode N-Channel MOSFET and circuit Symbols Enhancement-mode MOSFET's make excellent electronics switches due to their low "ON" resistance and extremely high "OFF" resistance and extremely high gate resistance. Enhancement-mode MOSFET's are used in integrated circuits to produce CMOS type Logic Gates and power switching circuits as they can be driven by digital logic levels. MOSFET Summary The MOSFET has an extremely high input gate resistance and as such a easily damaged by static electricity if not carefully protected. MOSFET's are ideal for use as electronic switches or common-source amplifiers as their power consumption is very small. Typical applications for MOSFET's are in Microprocessors, Memories, Calculators and Logic Gates etc. Also, notice that the broken lines within the symbol indicates a normally "OFF" Enhancement type showing that "NO" current can flow through the channel when zero gate voltage is applied and a continuous line within the symbol indicates a normally "ON" Depletion type showing that current "CAN" flow through the channel with

58 zero gate voltage. For P-Channel types the symbols are exactly the same for both types except that the arrow points outwards. This can be summarised in the following switching table. MOSFET type Vgs = +ve Vgs = 0 Vgs = -ve N-Channel Depletion ON ON OFF N-Channel Enhancement ON OFF OFF P-Channel Depletion OFF ON ON P-Channel Enhancement OFF OFF ON The MOSFET as a Switch We saw previously, that the N-channel, Enhancement-mode MOSFET operates using a positive input voltage and has an extremely high input resistance (almost infinite) making it possible to interface with nearly any logic gate or driver capable of producing a positive output. Also, due to this very high input (Gate) resistance we can parallel together many different MOSFET's until we achieve the current handling limit required. While connecting together various MOSFET's may enable us to switch high current or high voltage loads, doing so becomes expensive and impractical in both components and circuit board space. To overcome this problem Power Field Effect Transistors or Power FET's where developed. We now know that there are two main differences between FET's, Depletion-mode for JFET's and Enhancementmode for MOSFET's and on this page we will look at using the Enhancement-mode MOSFET as a Switch. By applying a suitable drive voltage to the Gate of an FET the resistance of the Drain-Source channel can be varied from an "OFF-resistance" of many hundreds of kω's, effectively an open circuit, to an "ON-resistance" of less than 1Ω, effectively a short circuit. We can also drive the MOSFET to turn "ON" fast or slow, or to pass high currents or low currents. This ability to turn the power MOSFET "ON" and "OFF" allows the device to be used as a very efficient switch with switching speeds much faster than standard bipolar junction transistors. An example of using the MOSFET as a switch

59 In this circuit arrangement an Enhancement-mode N- channel MOSFET is being used to switch a simple lamp "ON" and "OFF" (could also be an LED). The gate input voltage V GS is taken to an appropriate positive voltage level to turn the device and the lamp either fully "ON", (V GS = +ve) or a zero voltage level to turn the device fully "OFF", (V GS = 0). If the resistive load of the lamp was to be replaced by an inductive load such as a coil or solenoid, a "Flywheel" diode would be required in parallel with the load to protect the MOSFET from any back-emf. Above shows a very simple circuit for switching a resistive load such as a lamp or LED. But when using power MOSFET's to switch either inductive or capacitive loads some form of protection is required to prevent the MOSFET device from becoming damaged. Driving an inductive load has the opposite effect from driving a capacitive load. For example, a capacitor without an electrical charge is a short circuit, resulting in a high "inrush" of current and when we remove the voltage from an inductive load we have a large reverse voltage build up as the magnetic field collapses, resulting in an induced back-emf in the windings of the inductor. For the power MOSFET to operate as an analogue switching device, it needs to be switched between its "Cut-off Region" where V GS = 0 and its "Saturation Region" where V GS(on) = +ve. The power dissipated in the MOSFET (P D ) depends upon the current flowing through the channel I D at saturation and also the "ON-resistance" of the channel given as R DS(on). For example. Example No1 Lets assume that the lamp is rated at 6v, 24W and is fully "ON" and the standard MOSFET has a channel "ONresistance" ( R DS(on) ) value of 0.1ohms. Calculate the power dissipated in the MOSFET switch. The current flowing through the lamp is calculated as: Then the power dissipated in the MOSFET will be given as:

60 You may think, well so what!, but when using the MOSFET as a switch to control DC motors or high inrush current devices the "ON" channel resistance ( R DS(on) ) is very important. For example, MOSFET's that control DC motors, are subjected to a high in-rush current as the motor first begins to rotate. Then a high R DS(on) channel resistance value would simply result in large amounts of power being dissipated within the MOSFET itself resulting in an excessive temperature rise, and which in turn could result in the MOSFET becoming very hot and damaged due to a thermal overload. But a low R DS(on) value on the other hand is also desirable to help reduce the effective saturation voltage ( V DS(sat) = I D x R DS(on) ) across the MOSFET. When using MOSFET s or any type of Field Effect Transistor for that matter as a switching device, it is always advisable to select ones that have a very low R DS(on) value or at least mount them onto a suitable heatsink to help reduce any thermal runaway and damage. Power MOSFET Motor Control Because of the extremely high input or Gate resistance that the MOSFET has, its very fast switching speeds and the ease at which they can be driven makes them ideal to interface with op-amps or standard logic gates. However, care must be taken to ensure that the gate-source input voltage is correctly chosen because when using the MOSFET as a switch the device must obtain a low R DS(on) channel resistance in proportion to this input gate voltage. For example, do not apply a 12v signal if a 5v signal voltage is required. Power MOSFET s can be used to control the movement of DC motors or brushless stepper motors directly from computer logic or Pulse-width Modulation (PWM) type controllers. As a DC motor offers high starting torque and which is also proportional to the armature current, MOSFET switches along with a PWM can be used as a very good speed controller that would provide smooth and quiet motor operation. Simple Power MOSFET Motor Controller As the motor load is inductive, a simple "Free-wheeling" diode is connected across the load to dissipate any back emf generated by the motor when the MOSFET turns it "OFF". The Zener diode is used to prevent excessive gatesource input voltages. Summary of Bipolar Junction Transistors

61 The Bipolar Junction Transistor (BJT) is a three layer device constructed form two semiconductor diode junctions joined together, one forward biased and one reverse biased. There are two main types of bipolar junction transistors, the NPN and the PNP transistor. Transistors are "Current Operated Devices" where a much smaller Base current causes a larger Emitter to Collector current, which themselves are nearly equal, to flow. The most common transistor connection is the Common-emitter configuration. Requires a Biasing voltage for AC amplifier operation. The Collector or output characteristics curves can be used to find either Ib, Ic or β to which a load line can be constructed to determine a suitable operating point, Q with variations in base current determining the operating range. A transistor can also be used as an electronic switch to control devices such as lamps, motors and solenoids etc. Inductive loads such as DC motors, relays and solenoids require a reverse biased "Flywheel" diode placed across the load. This helps prevent any induced back emf's generated when the load is switched "OFF" from damaging the transistor. The NPN transistor requires the Base to be more positive than the Emitter while the PNP type requires that the Emitter is more positive than the Base. Summary of Field Effect Transistors Field Effect Transistors, or FET's are "Voltage Operated Devices" and can be divided into two main types: Junction-gate devices called JFET's and Insulated-gate devices called IGFET s or more commonly known as MOSFET's. Insulated-gate devices can also be sub-divided into Enhancement types and Depletion types. All forms are available in both N-channel and P-channel versions. FET's have very high input resistances so very little or no current (MOSFET types) flows into the input terminal making them ideal for use as electronic switches. The input impedance of the MOSFET is even higher than that of the JFET due to the insulating oxide layer and therefore static electricity can easily damage MOSFET devices so care needs to be taken when handling them. FET's have very large current gain compared to junction transistors. They can be used as ideal switches due to their very high channel "OFF" resistance, low "ON" resistance. The Field Effect Transistor Family-tree

62 Field Effect Transistors can be used to replace normal Bipolar Junction Transistors in electronic circuits and a simple comparison between FET's and transistors stating both their advantages and their disadvantages is given below. Field Effect Transistor (FET) Bipolar Junction Transistor (BJT) 1 Low voltage gain High voltage gain 2 High current gain Low current gain 3 Very input impedance Low input impedance 4 High output impedance Low output impedance 5 Low noise generation Medium noise generation 6 Fast switching time Medium switching time 7 Easily damaged by static Robust 8 Some require an input to turn it "OFF" Requires zero input to turn it "OFF" 9 Voltage controlled device Current controlled device 10 Exhibits the properties of a Resistor 11 More expensive than bipolar Cheap 12 Difficult to bias Easy to bias