10 Semiconductors - Transistors

Transcription

1 10 Semiconductors - Transistors The transistor was invented in the late 1940s. Credit for its invention is given to three Bell Laboratories scientists, John Bardeen, Walter Brattain, and William Shockley. The term transistor is really a contraction of "transfer" and "resistor." Types of Transistors The bipolar junction transistor (BJT, commonly called transistor) is constructed with three doped semiconductor regions. 1

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3 We will discuss two types of transistors: bi-polar junction (BJT) and metal-oxide field-effect (MOSFET). We concentrate on the BJT. There are two types of BJT: NPN and PNP. Again, we concentrate on only one, the NPN. Symbols, Pins, and Construction Transistors are three-terminal devices. On a bi-polar junction transistor (BJT), pins are labeled collector (C), base (B), and emitter (E). Circuit symbols for the NPN and PNP BJT are shown below: 3

4 We remember the NPN with the saying: NPN: Not Pointing in Operation Modes Unlike resistors, which enforce a linear relationship between voltage and current, transistors are nonlinear devices. They have four distinct modes of operation, which describe the current flowing through them. (When we talk about current flow through a transistor, we usually mean current flowing from collector to emitter of an NPN.) The four transistor operation modes are: Saturation -- The transistor acts like a short circuit. Current freely flows from collector to emitter. Cut-off -- The transistor acts like an open circuit. No current flows from collector to emitter. Active -- The current from collector to emitter is proportional to the current flowing into the base. Reverse-Active -- Like active mode, the current is proportional to the base current, but it flows in reverse. Current flows from emitter to collector (not, exactly, the purpose transistors were designed for). To determine which mode a transistor is in, we need to look at the voltages on each of the three pins, and how they relate to each other. The voltages from base to emitter (V BE), and the from base to collector (V BC) set the transistor's mode: PREVIEW he transistor was invented in the late 1940s. Credit for its invention is given to three Bell Laboratories scientists, John Bardeen, Walter Brattain, and William Shockley. The term transistor is really a contraction of "transfer" and "resistor." This name suggests that transistors can be thought of as making use of an input current to control an output voltage. In this chapter, you will learn more about the basics of transistors. You will find out exactly what transistors are, you will see some of the things they can do, and you will find out how they work. You will also learn about the transistor specifications that are found in transistor data sheets. Basic Types of Transistors The bipolar junction transistor (BJT, commonly called transistor) is constructed with three doped semiconductor regions. The physical representation and the schematic symbol for each device are shown in Figure The NPN transistor consists of two N regions separated by a P region (Figure 25-1a). The PNP transistor consists of two P regions separated by an N region (Figure 25-1b). Terminals are connected to each region. The terminals are called the collector (C), the base (B), and the emitter (E). The operational characteristics of the transistor are determined by the structure of the transistor. Both NPN and PNP structures are valid and useful transistor types. The schematic symbols differ only by the direction the arrow points in the emitter part of the symbol. The arrow direction correlates with the P-N junction (remember P = anode and N = cathode). For an NPN transistor, the arrow points outward and away from the base (remember that NPN is "Not-Pointing-iN"). For a PNP transistor, the emitter arrow points in toward the base (remember that PNP is "Pointing-iN"). Some Common BJT Packages In Figure 25-2 you see some common transistor lead basing layouts (i.e., bottom view of transistor package). The topology outline numbers (such as TO-3 and SOT-23) identify each specific packaging configuration used by manufacturers. Figure 25-3 shows BJTs mounted on heat sinks. The reason that some transistors are mounted on heat sinks is discussed later in this chapter. 4

5 BJTs are listed according to device identification codes. Recall that JEDEC registered diodes are listed according to a code that begins with 1N (1N4001, for instance). JEDEC registered BJTs are listed according to a code that usually begins with 2N. Some actual NPN transistors are 2N697, 2N2222A, 2N4401, and BSR17A. Some actual PNPs are 2N2904, 2N4030, 2N5400, and BSR18A. There is no way to tell the difference between an NPN and PNP transistor just by looking at the part identification code. You should check the manufacturers' data sheets, a transistor data book, or a parts catalog to discover the specifications for a given transistor part identification number BJT Bias Voltages and Currents Figure 25-4 is a circuit that is intended to show the voltage and currents for an NPN BJT. It also introduces some labels you will find in nearly all discussions and descriptions of BJTs. First note that this is an NPN transistor. Observe the polarity of the power supplies and the direction of the cuitent flowing in the circuit. Now consider the following: Vee is the base-bias voltage source. Vcc is the collector-bias voltage source. VEE is the emitter-bias voltage source. Vce is the collector to base voltage drop. VcE is the collector to emitter voltage drop. VeE is the base to emitter voltage drop. Ie is the base current, the current at the base terminal of the transistor. Ic is the collector current, the current at the collector terminal of the transistor. IE is the emitter current, the cuitent at the emitter terminal of the transistor. Re is the resistor connected to the transistor base terminal. Rc is the resistor connected to the transistor collector terminal. RE is the resistor connected to the transistor emitter terminal. As labeled, all the voltages and currents will be positive for the normal operation of the NPN transistor. Figure 25-5 is a circuit that shows how currents flow through a PNP BJT. It also introduces some labels you will find in nearly all discussions and descriptions of BJTs. First note that this is a PNP transistor. Observe the polarity of the power supplies and the direction of the current flowing in the circuit. Notice also the change in the labels for the voltages around the transistor. VBC is the base to collector voltage drop. Vsc is the emitter to collector voltage drop. VsB is the emitter to base voltage drop. As labeled, all the voltages and currents will be positive for the normal operation of the PNP transistor. You will observe that not all of the voltage sources will be required to operate the transistors. BJT Operation Figure 25-6 is a demonstration circuit that is intended to show how currents flow through an NPN BJT. The base-emitter junction of a BJT behaves very much like the P-N junction of an ordinary diode. For instance, you have to forward bias the base-emitter junction in order to allow any current to flow through it. You can see from the diagram that the forward-bias current will indeed flow from the positive terminal of VB,, through the transistor base-emitter junction to the negative terminal of Vss. Increasing the value of VSB increases the value of Is much the same way that increasing the voltage applied to a forward-biased diode causes the diode current to increase. Base currents for BJTs, however, are usually limited to the microampere range. The forward-bias junction potential for the base-emitter junction of a BJT is identical to that of a diode, namely about 0.3 V for germanium transistors and 0.7 V for silicon transistors. You will confuse yourself, however, if you try to compare the collector-base junction of a BJT with the P-N junction of an ordinary diode. Under normal operating conditions, the 5

6 cotlector-base junction is reverse biased. Yet, the internal current flowing from the collector through the base region to the emitter actually is far more current than the forward-biased base-emitter junction. How can this be? Since the base region in a BJT is extremely thin and very lightly doped, the collector collects most of the electrons provided or emitted by the emitter. This accounts for why base currents are. usually quite small. When the base-emitter junction is forward biased, the base current controls the amount of collector current that is flowing within the transistor. Let's take a closer look at the emittercollector circuit. Note from the diagram that the dc source Vcc is connected in series with the collector resistor Ro The total voltage applied between the emitter and collector terminals (collector-emitter voltage, Vcÿ) of the BJT is equal to the dc voltage source minus the voltage drop across the collector resistor: Vcÿ = Vcc- IcRo If the base-emitter junction is forward biased and VcciS sufficiently high, there will be a complete path for current flow that begins from the positive terminal of Vcc through the collector resistor into the collector, and returns to the negative terminal of Vcc through the emitter. One of the vital characteristics of a BJT is that the collector current (Ic) is always much larger than the base current (IB) that controls it. The normal operating conditions for a BJT can be summarized this way: The base-emitter junction must be forward biased with a little bit of base current, Is. The collectorbase junction must be reverse biased. The following occur as a result of setting up these conditions: The emitter current, Iÿ, is equal to the sum of the base and collector currents. The collector current, Io is always much larger than the base current. Folÿula 25-1 expresses the fact that the emitter current is exactly equal to the sum of the collector and the base currents. FORMULA 25-1 Iÿ = Ic + Is The fact that Ic is always much greater than IB is expressed in terms of a ratio. Typically, Ic is between 40 and 300 times greater than Is. This ratio is expressed as the 13ÿc (de beta) or sometimes hfe. (We will use the [3do notation in our discussions, but you should expect to see the "h convention" often used in other places.) Formula 25-2 shows how you can calculate the value of dc beta when you know the values of the collector and base currents. FORMULA 25-2 ÿ3dc = Ic/I, Used in this way, [3de represents the current gain of the transistor An NPN Transistor Circuit Model Figure 25-9 shows a dc model of the NPN transistor. The model considers how the transistor operates in the three regions (cutoff, saturation, and finear). Figure 25-% models the collectoremitter junction of the transistor as a switch. If the voltage drop across the base-emitter junction is less than 0.7 V, then the switch is open (cutoff region). If the voltage drop across the baseemitter junction equals 0.7 V, then the switch is closed (saturation region). Figure 25-9b models the collector-emitter junction of the transistor 6

7 as a current-controlled current source (the base current controls the amount of collector current flowing in the circuit). Figure shows an ac model of the NPN transistor operating'in the linear region Basic Uses of the BJT Before we look at a few more details about the operation of BJTs, it is important to be able to describe basically how transistors are used. Transistors are controllers of current. The BJT uses a small amount of base current to control a larger amount of collector cun'ent. Later, you will learn that field-effect transistors use a voltage level to control the flow of current. BJTs are basically used in two ways: as switches and as amplifiers. Often, these two applications are combined in one circuit. The BJT as a Switch and Current Amplifier Figure shows a typical circuit where an NPN transistor is used as a switch and as an amplifier of current. The rectangular waveform at the input, switches between 0 V and +5 V. The LED has a forward voltage drop of 1,5 V when conducting. Resistor values are RB = 33 kÿ2 and Rc = 200 f2. The NPN transistor that is used in this circuit is a 2N3904. Let's examine how the BJT responds to the 0-V input and to the +5-V input. 1. When the input is at 0 V, the transistor is in cutoff. there is no base current flowing in the BJT (18 = 0 A) there is no collector current in the BJT (Ic = 0 A) the collector voltage is equal to the Vcc level (Vc = Vcÿ = Vcc = +5 V) (NOTE: If you measured the voltage at the collector, it may read less than 5 V since the digital multimeter provides a current conduction path and some voltage may be dropped across the LED.) the LED does not light The circuit is switched off. 2. When the input is at +5 V, the transistor is in saturation. base current is flowing in the BJT (IB > 0 A) 3. IB = (VBB -- VÿE)/Rÿ IB = (5 V- 0.7 V)/33 a IB = 130 ga 4. the collector voltage is equal to the saturation voltage of the BJT (Vc < +0.3 V) the collector current is flowing in the BJT ((Ic > 0 ma) 5. Ic = (Vcc - VD - Vcÿ(sÿ>)/gc Ic = (5 V V V)/200 :c= 16ma 6. the LED is turned on. The circuit is switched on. 7. In summary, the LED lights whenever the input waveform switches to +5 V, and the LED goes out whenever the input waveform switches to its 0 V level. The LED switches on and off at the frequency of the input waveform. At frequencies above 60 Hz, the LED visually appears to be on at all times. However, using the scope, you can observe that the LED is still turning on and off. But why do we go to the trouble of using a BJT? Why not connect the input waveform direct to the LED and simplify the circuit? Well, the reason is that the input waveform probably comes from a source that cannot provide the 16-mA current level that is required for operating most LEDs. Maybe the waveform source can provide a maximum of i ma. The BJT circuit draws only 130 ga to provide the 16-mA current for the LED. In this case, the BJT serves as a current amplifier as well as a switch. 8. The BJT as a Voltage Amplifier The circuit in Figure is a voltage amplifier. The input waveform in this example is a sinusoidal waveform that measures 2 V peak-to-peak. The values of the resistors at the base connection are selected so that the transistor is operating in the linear region (Vÿ = 2.2 V and Vc = 6 V). There is some base-bias current flowing, even when the input signal is at 0 V. Let's study how the BJT reacts to one complete cycle of the waveform at 1/4-cycle intervals At 0, the input is at 0 V. Due to the base-bias circuit, the base voltage is at 2.2 Vdo. The base current flowing is due to only the base-bias current. The BJT is conducting, so the collector 7

8 voltage is somewhere between 0 V and the +Vcc level of 12 V. Let's suppose that the base bias is adjusted so that the collector voltage is +6 V. 2. At 90, the input is at +1 V. The positive polarity of the input signal adds to the base voltage. The base voltage is now at 3.2 V. The increased base voltage causes a higher base current resulting in a larger collector current. This means the collector voltage (from collector to ground) drops to a lower level due to the I R drop across the collector resistor. Thus, less voltage is available to be dropped by the transistor. (Remember Kirchhoff's voltage law?) Let's say we find it drops down to +2 V. 3. At 180, the input is at 0 V. Again the base current flowing is due to only the base-bias current. There is less base current than while the input waveform was positive; there is less collector current, so there is a larger collector voltage. The collector voltage returns to +6 V At 270, the input is at -1 V. The negative polarity of the input signal subtracts from the basebias current set by resistors R1 and R2. The reduction in base current causes the base current IB to be lower. Subsequently, the collector current is lower resulting in an increase in the collector voltage. The collector voltage rises to a higher level, say, +10 V. 11. You can see from the resulting plot of the output waveforrn that it is a sinusoidal waveform that is 8 V peak-to-peak. This circuit is a voltage amplifier that increases the input signal from 2 Vp.p to 8 Vp.p. It is said that this amplifier has a voltage gain of 4 (8 Vp.p divided by 2 Vp.p). Also notice that this type of BJT amplifier inverts the input waveform or, in other words, shifts the waveform by 180. Even though the BJT is still a current amplifying device (IB is amplified to Ic), the surrounding circuit converted the input voltage into a current flow and used the output current (Ic) to create an output voltage. Thus making the system a voltage amplifier. After studying this chapter, you should be able to:!. Explain the derivation of the term operatlonal amplifier (op-amp) 2. Draw op-amp symbol(s) 3. Define the term differential amplifier 4. Draw a block diagram of typical circuits used in opamps 5. List the key characteristics of an ideal op-amp 6. Identify linear and nonlinear applications circuits for op-amps 7. Distinguish between inverting and noninverting op-amp circuits 8. Perform voltage gain and resistance calculations for standard inverting and noninverting op-amp circuits 9. 8

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