Unit 3: Introduction to Op- amps and Diodes

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1 Unit 3: Introduction to Op- amps and Diodes Differential gain Operational amplifiers are powerful building blocks conceptually simple, easy to use, versatile, and inexpensive. A great deal of analog electronic design can and is accomplished by the use of op- amps. There are many model circuits available that use op- amps in clever ways. You can refer to model circuits or invent your own applications. We re going to introduce op- amps in several consecutive chapters, at increasing levels of sophistication. We ll also use op- amps extensively throughout the rest of this book as we learn about other components. They are similarly ubiquitous in real- world engineering applications. Figure 3-1. The symbol for an op- amp is a triangle with two inputs and one output. The two inputs to an op- amp, as indicated in Figure 3-1, are called the non- inverting and inverting inputs, and are denoted on the op- amp symbol by + and. The most common nomenclature is that the voltages at the two inputs are named V + and V. These are unfortunate names because they are easily confused with the positive and negative power supply rails, which are often called +V and V. At the level of abstraction that we will use in this chapter, the function of an op- amp is to compare V + and V and to produce at V out a voltage that is positive or negative, depending on the comparison: V OUT = G(V + V ) (3-1) For the moment we will take G, which is called the open loop gain, to be an arbitrarily large positive number. Equation (3-1) means that if V + exceeds V, even by a microvolt, then V out will be as positive as possible, and otherwise V out will be as negative as possible. As possible reflects the fact that the op- amp cannot produce voltages outside the bounds of its power supply rails. If the op- amp is provided power by ±5 volt rails, then V out will always be ±5 volts depending on which input exceeds the other. Using only what we know so far we can build a comparator circuit that turns on a red or green LED, depending on light intensity falling on a phototransistor, as compared to a threshold set by the slider of a potentiometer.

2 Circuit 3-2. Light intensity comparator. In Circuit 3-2 the phototransistor (in the top right part of the circuit schematic) and fixed 22KΩ resistor form a voltage divider so that the voltage at the non- inverting (+) input of the 741 op- amp depends on the light intensity falling on the phototransistor. Depending on light intensity V + may range from 5 to +5 volts. The 10k potentiometer applies a voltage in the range ±5 volts to the inverting ( ) input of the 741 op- amp. If the voltage at the non- inverting (+) input exceeds the voltage at the inverting ( ) input the output of the 741 op- amp is +5V and the green LED illuminates, and otherwise the output is 5V and the red LED illuminates. LEDs are diodes, and can conduct a current only in their forward direction (down for LED1, up for LED2;). We ll talk more about diodes in the next section. Now let s fill in some detail. In the schematic shown in Circuit 3-2 we ve followed the usual convention of showing the two inputs and the output of the op- amp, but not its other connections. In fact an op- amp also needs connection to the positive and negative supply rails, which are called V S+ and V S or +V and V, or sometimes V CC and V EE. A lot of op- amps come in an eight- pin package such as shown in Figure 3-3. The datasheet for each type of chip (integrated circuit) will specify the assignment of functions to the numbered pins of the chip. This assignment list is called the pinout for the chip. The

package type shown is called DIP (dual inline package) referring to its two rows of pins on 0.1 inch centers, and DIP8 because it has eight pins. Figure 3-3.

3 package type shown is called DIP (dual inline package) referring to its two rows of pins on 0.1 inch centers, and DIP8 because it has eight pins. Figure 3-3. A LM741 op- amp in a DIP8 package, and its pinout. Pin 1 is shown in the upper left in Figure 3-3. There is always some kind of indication on the chip near pin 1, as seen in the photo and in the outline drawing as well. Almost all drawings in datasheets are top- view, although one finds an occasional nasty surprise. Some especially helpful schematics (circuit diagrams) will show the pin numbers (pinout) explicitly, but most will show only the inputs and outputs with no pin numbers. Power is provided at pins 7 and 4, for virtually all op- amps in DIP8 packages. It s a great convenience that almost all op- amps share a common pinout, so that you can easily substitute one type for another without rewiring. For now the differing characteristics of one type of op- amp relative to another are not important to us, so we ll stay with the famous and inexpensive 741. Many manufacturers make the 741, and the part number usually has a 741 in it somewhere. The 741 also has offset null adjustments on pins 1 and 5, which we will discuss later but we won t use, and pin 8 is not connected at all (NC). Other eight- pin op- amps may use pins 1, 5, and 8 in other ways. There are restrictions on an op- amp s operating conditions. Some of these are limits beyond which the chip may be damaged, which is a datasheet s euphemism for burned out. For instance there is an absolute maximum supply voltage of ±18 volts. The pinout, performance specifications, and absolute maximum ratings are all part of a chip s datasheet, which is an essential document to have in hand when working with the chip. Other restrictions specify limits beyond which the op- amp won t function ideally: limits on its performance. We already mentioned one of these: the output voltage swing cannot exceed the power supply rails. With ±5 volt supply rails, you can t expect the op- amp s output to be outside the range 5 < V out < +5. In fact an op- amp s output voltage swing cannot even approach the rails. For the 741, the output can get to within about 1.6 volts of each power supply rail, but no further. With power supply rails of ±5V, we find V out restricted to the range 3.4V < V out < +3.4V. At higher supply voltages, the output voltage swing will be about 1.6V short of the rails.

4 There are many special kinds of op- amps, including ones for which V out can approach the rails more closely, sometimes as closely as a few millivolts (mv). These op- amps are advertised as having rail- to- rail output. An op- amp is supposed to be able to compare two input voltages and respond only to the difference. It can do this subtraction over quite a large range of input voltages, but not without limit. For most op- amps the range of input voltages for which a valid subtraction will be made is a few volts short of rail- to- rail. The 741 with ±5V rails can compare 2.2 vs. 2.3 volts, or 1.2 vs. 0.7 volts, but perhaps not 4.2 vs. 4.3 volts. Again there are special op- amps that can properly subtract voltages all the way up to the supply rails; these are called rail- to- rail input op- amps. However, if either input voltage goes outside the rails (beyond ±5V in our case), most op- amps will be damaged. Not only is the output voltage swing limited, so too is the maximum output current. The maximum current an op- amp can produce is called the output short circuit current. (The term short circuit does not necessarily imply anything harmful.) The LM741 datasheet specifies an output short circuit current of 25mA. LEDs are diodes You might wonder in our light- intensity comparator (Circuit 3-2) whether the LED indicators would operate properly with whatever output the op- amp was producing. We at first assumed that the output of the op- amp was ±5V depending upon which input was more positive, and you probably assumed that the LEDs were designed to run on 5V, something like a lightbulb. Both assumptions are wrong. Let s understand LEDs better. Let s figure out the voltage drop and current across each LED in the circuit shown above (Circuit 3-2). First measure the voltage across each LED, using your scope or multimeter. You ll see that when the LED is lit, the voltage drop will be approximately 1.6V (the voltage drop across the red LED will be negative when it is lit). Now measure the voltage drop across the LED and resistor together (note that this voltage will be the same for both LEDs, regardless of which one is lit). Using this voltage we can find the voltage drop across the resistor, by subtracting the voltage you measured across the LED: the voltage drop will be approximately 250mV. From this, you can calculate the current across the lit LED: approximately 25mA. LEDs are diodes; they are not lightbulbs. If an LED s anode (tail of the arrow symbol) is more positive than its cathode (head of the arrow symbol), the LED may conduct, but if it is the other way around it won t conduct. These conditions are called forward biased and reverse biased respectively.

5 Figure 3-4. Characteristic curve for an LED: forward current vs. forward voltage. When an LED is forward biased and does conduct, it shows an exponential relationship of forward current to forward voltage, as seen in the log- linear graph in Figure 3-4 (the vertical scale is logarithmic). At voltages below 1.6V the LED hardly conducts at all. Above 2.0V it conducts so much current that it burns out. As used in Circuit 3-2, the current through the LED is only ~25mA because that is the maximum that the op- amp can provide, and fortunately this is a healthy current for an LED. You can see from the LED s characteristic (Figure 3-4) that the voltage across it will not exceed about 1.82 volts, because that is the voltage that corresponds to 25mA. The op- amp's V out is not ±5V which is what we might predict given that the rail voltages are ±5V and that's all that is available to the op- amp. V out is not ±3.4V which is what we might expect for an op- amp that does not have a rail- to- rail output, and can only get to within about 1.6 volts of its supply rails. Instead, V out is only ~1.8V, because if V out were any higher the LED would conduct more current than 25mA, and 25mA is as much current as the output of this op- amp is capable of providing. Diodes LEDs are just one kind of diode. As a first level of approximation, we may say that a current passes through a diode only in the forward (anode- to- cathode) direction, and that a diode conducts when the anode- to- cathode voltage is positive (forward biased). Diodes are mostly used for their property of conducting only forward.

6 Figure 3-5. Forward current vs. forward voltage for a 1N4148 diode. If we look a little more carefully at diodes however, they have a current- voltage characteristic that is exponential, and varies from one kind of diode to another. Figure 3-5 shows the IV characteristic for the 1N4148 small signal diode, on log- linear axes. You can see from the straightness of this curve that a diode might be used as an excellent computer of exponentials or logarithms. In fact the curve continues just as straight down several more decades (down to na of current). You may have heard that diodes have a forward voltage of 0.6 volts. This is a useful approximation: over the two orders of magnitude of forward current shown in Figure 3-5, the forward voltage of the 1N4148 diode changes only from 500 to 740mV. Figure 3-6. A reverse biased diode. A diode is not supposed to conduct at all when it is reverse biased, and indeed reverse leakage currents are very small, in the na range or even the pa range 1. However, with too much reverse voltage the diode will break down catastrophically. For the 1N4148 the rated absolute maximum reverse voltage is 100V. Diodes are also rated for a maximum forward current that they can sustain without overheating. For the 1N4148 the continuous forward current limit is 200mA. Now we ve seen LEDs that have a forward voltage of about 1.6V, and silicon diodes like the 1N4148 with a forward voltage of about 0.6 volts. A few more things you might want to know about diodes: 1 na is nano, 10-9 amp, and pa is pico, 10-12

Diodes intended for high currents are often called rectifiers. They can sustain currents of amps or even hundreds of amps.

5V, as opposed to most silicon diodes which have a forward voltage of about 0.6V at 1mA current. SD103 is a typical Schottky diode.

12V. Figure 3-7. (a) A 35A rectifier, (b) 1N4001 (1 amp) rectifiers, (c) small- signal diodes, and (d) an LED.

7 Diodes intended for high currents are often called rectifiers. They can sustain currents of amps or even hundreds of amps. A good choice of 1A rectifier/diode is the 1N4003. Schottky diodes have a forward voltage of about 0.2V- 0.5V, as opposed to most silicon diodes which have a forward voltage of about 0.6V at 1mA current. SD103 is a typical Schottky diode. Zener diodes have a well- controlled reverse breakdown voltage, and are intended to be used in reverse bias and to break down without damage. For instance a zener diode with a reverse breakdown voltage of 12V can be used to control a voltage in a circuit, and limit it to 12V. Figure 3-7. (a) A 35A rectifier, (b) 1N4001 (1 amp) rectifiers, (c) small- signal diodes, and (d) an LED. Current flows anode- to- cathode, in the direction of the arrow in the diode s symbol. In these photos the anode is toward the upper right and the cathode toward the lower left í. Note the polarization stripes that indicate the cathode. The 35A rectifier also has the diode symbol painted on it. For LEDs the anode

8 is indicated by the longer lead. If the leads of an LED have been clipped, you can usually also identify the anode as the larger part of the support structure visible inside it. Using a diode as a thermometer We discussed the remarkable straightness (straight on a log- linear graph) of the characteristic curve of a silicon diode. Forward voltage is logarithmic in forward current, over many decades of current. We see from Figure 3-8 however that the forward voltage is also a function of temperature, decreasing at about 2 mv/k. A diode s temperature dependence can be used to make a thermometer. Figure 3-8. Diode IV curves for different temperatures. As you can see in the figure, the forward voltage of a diode decreases as its temperature increases: for the same current flowing across the diode, the corresponding voltage is lower. Because of this temperature dependence, we can use diodes to create thermometers, either using them simply to detect fluctuations in temperature, or using calibrated circuits to provide an accurate reading of temperature.

USER MANUAL FOR THE LM2901 QUAD VOLTAGE COMPARATOR FUNCTIONAL MODULE

USER MANUAL FOR THE LM2901 QUAD VOLTAGE COMPARATOR FUNCTIONAL MODULE LM2901 Quad Voltage Comparator 1 5/18/04 TABLE OF CONTENTS 1. Index of Figures....3 2. Index of Tables. 3 3. Introduction.. 4-5 4. Theory