Guide to Transistors--Audio Amplifiers

A TRANSISTOR is to a signal what a lens is to light--a magnifier. But what is this bit of magic whereby a few minerals are able to take a varying current and amplify it?

Let's start our investigation with Fig. 501. We're going to put a signal voltage right across R1. This varying voltage will modify the forward bias supplied by B1. The result of all this will be a varying current through the transistor and also through R2. Where, then, does amplification come in?

Fig. 501. This is an elementary amplifier circuit. An input voltage appearing across R1 will vary the forward bias. This will change the current through the transistor, developing a changing voltage across R2.

To understand this, there are a few items we must first consider. Our input circuit consists of the emitter and base of the transistor, battery B1 and resistor R1. The current that flows in this circuit is small--usually a matter of microamperes. How does this compare with the current in the output circuit? Here the current is much larger and is measured in milliamperes in this particular case. The output circuit consists of the emitter and collector, R2 and both batteries--B1 and B2.

This, then, is the first part of the answer to our question--why amplification? We have a much larger current in the output than in the input. But what more do we know about the currents? Both are varying at a rate determined by the input signal, for it is the input (ac) signal that modulates or changes the forward bias (dc).

Now examine the two resistors, R1 and R2 in Fig. 501. The resistor (R1) in the input circuit is much smaller than the resistor (R2) in the output circuit. We get signal voltages developed across both of these resistors. What about the magnitude of these two voltages, both changing at the same rate? At the input, we have a small current going through a small value resistor com pared with a large current going through a large resistor, at the output. And, since voltage is the product of current and resistance, we can see that the output voltage is going to be much larger than the input. In a representative transistor, the voltage across R2 will be several hundred times that across R1.

This reasoning on why a transistor amplifies may seem a bit too .smug and too pat, so let's examine the problem somewhat further. To do so, suppose we ask a question: "Just what is the collector current?" Theoretically, if we had no base current, the current flowing to the collector in an n-p-n transistor would be the emitter current. But we do lose some current, however small, to the base, and so the collector current is less than the emitter current by that amount. Offhand it might seem that, since the collector current is smaller than the emitter current, we could not possibly obtain amplification. The answer here lies in a com parison between the input and output resistances. We are not dealing with currents alone, but with voltages as well, since our currents are going to be made to flow through resistive or inductive components. Thus our varying voltage drop in the output is going to be much larger than that same varying voltage in the input simply because our output or load is much larger in value than the input resistance.

The fact that collector current is smaller than emitter current might seem contrary to what we have learned in tube theory, but if you will think about it for a moment or two, you will see it is identical. Consider a pentode. Is the plate current equal to the cathode current? If there is no secondary emission to worry about and if the screen draws current, the plate current will be less than the cathode current but the tube doesn't know this and goes right on amplifying.

Does a transistor amplify? Not really. Not any more than a tube does. All that a transistor (or tube) can do is to control or vary the current supplied by a battery or power supply. A transistor or tube is just a link, permitting a small voltage (usually the signal) to control or manipulate or modulate the current furnished by a voltage source.

Fig. 502. Want to invent an amplifying device such as a tube or transistor? Solve the basic problem shown here and you are on your way to fame and fortune. All you need do is to make the varying current through R1 control the current through. What is it that we have really accomplished? All we have done is to take a battery and a resistor and have managed to secure control of its current. You can invent an amplifying device of your own, if you wish. Fig. 502 shows the basic problem. We have a resistor R1 connected across an ac source. This ac source could represent the signal supplied by a receiving antenna. A varying current will flow through R1. Nearby, we have R2 connected across a battery. Flowing through R2 is a direct current. Because R2 is much larger than R1 and because the current supplied by the battery is also much greater than that supplied by the signal, the voltage across R2 is several hundred times that across R1.

Now all you have to do is to manage, in some way, to get the direct current through R2 to vary exactly in step with the current through R1 and you will have an amplifier.

Amplifier arrangements

In all the circuits we have examined so far we have used one special type. You may have noticed that in each case we always specified one element--the emitter--as common to both the input and output circuits. For this reason it is called the common emitter circuit. Since ground is often a widely used reference point, the circuit is sometimes referred to as a grounded emitter.

Because the transistor has three elements--emitter, base and collector--we can use any of the three as the common element.

Fig. 503. Basic transistor amplifier arrangements.

Fig. 503 shows the three possible circuit arrangements. Each of the circuits has its own characteristics. However, the common or grounded emitter circuit is the most widely used.

The three basic circuits have certain similarities. The common emitter and common collector arrangements are identical except for the transposition of the output elements. Thus, you could easily change a common emitter circuit into the form we call common collector just by transposing a few leads. Similarly, the common emitter and common base could be changed, one to the other.

Fig. 504. A slight case of phase inversion.

When point A becomes more positive [is positive-going], point C will become more negative [is negative-going].

Phase

The word phase is used so often in electronics that we must understand just what it is trying to tell us. Phase is used to de scribe the relationship existing between a pair of currents, a pair of voltages, or a current and a voltage. Sometimes a voltage is compared to what it was when it started or what it will be at some time other than this moment.

Now that we have managed to confuse you, let's unravel this tangled web of words. The classic description of phase is to com pare it with a seesaw. If you sit on one end and we on the other there is no possible way in which we can both rise or fall at the same time. When you go up, we go down. The two ends of the seesaw are in opposite phase, or out of phase--call it what you like. If I climb up when you march down--we are out of phase.

If you save while we spend--we are out of phase. If you charge a battery while we discharge a battery--the battery voltages will be out of phase. One will be increasing, the other decreasing.

We can also have in-phase conditions too. If we both go up in an elevator, we are in phase. We are rising at the same time. If we applaud a show at exactly the same time, we are in phase.

Suppose we have two ac voltages of identical frequency, and let us imagine that they both start from zero at the same time. If they both move in a positive direction, they are in phase. If one voltage becomes more positive while the other becomes more negative, they are out of phase.

Examine the common emitter circuit shown in Fig. 504. When a varying signal appears across R1, we will have a varying signal across R2 as well. Suppose point A (resistor R1) becomes more positive with respect to point B. When this happens, point C (resistor R2) will become more negative with respect to point D. But we have the input voltage across R1 and the output voltage across R2. Thus, the two voltages are out of step or out of phase.

But since the voltage across R2 is a magnified replica of that across R1, we can say that the common emitter circuit in Fig. 504 has given us not only amplification, but phase inversion as well.

Now suppose we were to take the common emitter circuit of Fig. 504 and just transpose the leads to the emitter and base. We would then have a common base circuit. But what happens to the current through R1 when we do that? It just flows in the opposite direction. As a result we have changed the polarity of the voltage across R1 so that it is now in step or in phase with the voltage across R2. We can understand this a bit more easily if you will remember that in going from the common emitter to the common base circuit, we made no change in the collector or its load resistor or in the output battery.

Now what about the common collector circuit? It has no phase reversal either. And the reasoning is exactly the same as that used for the common base arrangement. To get a common collector, we take the common emitter and transpose emitter and collector leads. The effect of this is to transpose the collector load resistor.

But if we do this, we also transpose the polarity of the voltage across the collector load. This puts it in phase with the input voltage.

Fig. 505. The input circuit is not properly biased since the ac input will supply, alternately, forward and reverse bias.

The complementary symmetry principle

The three basic amplifier arrangements--common emitter, common base and common collector--were shown in Fig. 503 as p-n-p units. We could have used n-p-n transistors just as well, once more keeping in mind that n-p-n's and p-n-p's may require the same voltages but opposite polarities. And because the polarities are transposed, so are the currents. We might even say that the two types are mirror images or complementary. We are going to build circuits around this knowledge. The idea of complementary units is unique to transistors since we have nothing like it in vacuum tubes. Tubes require a positive voltage on the plate--transistors can have either positive or negative voltage on the collector de pending on whether we use a p-n-p or n-p-n type.

Fig. 506. For proper biasing, the signal volt age must be less than the forward biasing voltage. Note that the de bias is far above the zero reference line. The signal voltage varies above and below the reference. In the graph you can see that the combination of de bias and ac signal does not permit the resultant voltage to drop below zero.

Biasing the amplifier

In radio tube circuits we will sometimes come across a tube designed to operate with zero bias. This is a condition you will not find with transistor audio amplifiers since bias is an essential part of their operating conditions. Let us see why this is so.

In Fig. 505 we have a circuit that is normal in every respect, except that we have no battery for forward bias. If the input signal is symmetrical (such as a sine wave) then, during half the input cycle the base-emitter circuit will be forward biased, but during the other half of the cycle it will be reverse biased. During forward biasing we will get current flow through RL' the collector load resistor. But during reverse biasing of the input by the signal, only an extremely tiny current will flow through the collector load.

Since the output signal does not resemble the input, we have a severe case of distortion. Actually, the circuit in Fig. 505 behaves more like a rectifier, although there is amplification for part of the cycle. (In Section 6 this action is used for detection.) What should the relationship be between forward bias and signal voltage? Since the input signal alternately adds to and sub tracts from the forward bias, we must make sure that the bias voltage is always large enough to give the signal something to work with. Since you may be snowed under by this torrent of words, let's consider several possible situations. Suppose the bias is 2 volts and we have an input signal of I volt, peak-to-peak.

Assuming the same sort of waveform we had earlier, the net result of combining the ac signal plus the de bias would mean (as shown in Fig. 506) that half the time our forward bias would be:

2 + .5 = 2.5 volts

2--.5 = 1.5 volts

Our forward bias now ranges between 1.5 and 2.5 volts. Because our bias battery is larger than the input signal voltage, we always meet the condition of having forward bias in the circuit.

But what if the opposite took place? What if we had a bias of just 1 volt de, but a signal voltage of 3 volts, peak-to-peak.

What has happened in our input circuit? We now have: I + 1.5 = 2.5 volts 1--1.5 = -0.5 volt Fig. 507 shows graphically, the result of such operation. So, we always want to make sure that our bias will be larger than the highest peak-to-peak signal voltage we are going to have.

Now that we have convinced you that an improperly biased transistor results in a distorted or a rectified output signal, it is our sad duty to inform you that what appears to be a vice can, if we so wish, be virtue triumphant. And, if you are about to leap forward to the conclusion that what we have in mind is some sort of rectifier or detector, control your reflexes, for this is a section on audio amplifiers. But, while you are still tantalized about this forthcoming tidbit of information, let's take a small side path. Don't fret about this detour, since it is really just a shortcut to the main highway.

Classes of amplifiers

Fig. 507. When the signal is larger than the bias voltage, the transistor input will be both forward and reverse biased.

Fig. 508. The class of amplifier operation is set by the operating point Q on the load line. Changing the base bias will move this point, resulting in a different class of operation.

There are many ways of classifying audio amplifiers. We can talk about voltage and power amplifiers, driver and output amplifiers, push-pull and single-ended. One other way is to arrange them in accordance with their bias requirements. For transistors, where we seldom stray far from the word "bias", this is a very attractive thought, indeed. And, with a strong penchant for original thinking, we reach right over into vacuum-tube theory, and borrow a classification, lock, stock and element.

A favorite vacuum-tube type of classification is to call amplifiers class A, B or C. This doesn't mean that A is better than B, or that B is slightly superior to C. In vacuum tube theory, a class A amplifier is one so biased that plate current flows no matter which part of the input cycle is tickling the grid at the moment. Thus, whether the input signal is crossing its zero axis, is at maximum positive or maximum negative, we always get more or less current flowing to the plate. A class B amplifier has plate current flowing for only fifty percent of the input cycle, while for class C, plate current exists for only a small fraction of the input cycle.

But how do we arrive at that happy condition in which we can make the plate current do our bidding? How can we take a single tube and make it work as class A, or class B, or class C, just as we wish? The clue to this--no, the whole answer to this--lies in the bias. Very high bias and you will probably have class C. Very low bias, class A. And class B somewhere in between.

If, from your studies of vacuum tube theory you know all this, be of good cheer for you also know it for transistors, and without effort on your part. In tubes, we talk about bias controlling plate current. In transistors, it is the amount of forward bias that determines collector current.

In Fig. 508 we have a drawing which demonstrates this for you graphically. What is the class of operation? All you need do is to select the correct amount of forward bias for the input circuit. However, we have been a little crafty about this since we haven't really told you the whole story. We start with what we fondly believe to be the correct amount of bias for the class of operation we want, and then (this is the sneaky part) we make sure that our input signal is never large enough to distort the output waveform. Take the case of class-A operation in Fig. 508.

We very carefully chose an amount of forward bias that put us as close to the center of the collector current curve as possible.

Now work along with us with the graph in Fig. 508. Locate the dashed line marked "base bias line". What is this line and just what does it represent? This line is our forward bias. If you will go back to Fig. 506 you will see that we have a straight line marked "dc". We have a similar line in Fig. 507. This is the type of line we are using in Fig. 508.

Now suppose that to the de represented by the base bias line we add an ac voltage. The result will be a variation around the base bias. To get the appearance of the output current and the output voltage, we can project to the left and also downward.

If you will look at the dashed lines in the drawing, you will see that they center equally around the base bias line. Also note that we are working along the straight portion of the collector current curves. As a result, our output waveform is a rather decent replica of our input waveform.

But what if we change our forward bias? We can either increase the bias or decrease it from the condition shown in Fig. 508. As a start, let us say that we increase the forward bias. The effect of this will be to move the base bias line down the slope of the load line. To see what will happen, just imagine the letter Q (our operating point) sliding down the load line and coming to rest somewhere near the bottom. We could still project our input current waveform onto the load line, but a projection of the output current and output voltage waveforms would show severe distortion. Similarly, if we decreased the forward bias, the base bias line would move up on the slope of the load line, once again producing a distorted output waveform--even though the input might be undistorted.

Now here is where the subtle part comes in. Note that in Fig. 508 our modulating wave never takes us into the saturation or cutoff regions of collector current. How do we know this? Look at the vertical dashed lines moving up from the output voltage waveform and note where they touch the collector current curve.

These two lines come close to the beginning and the end of the curve. But if our input signal were too large, our output waveform would be clipped top and bottom. And, since the output would no longer be an enlarged replica of the input, we would have distortion.

How do we get the other classes of operation--classes other than A? Simply move the operating point Q up or down on the load line. Class B works at the cutoff point--class C beyond that.

We also have one more class--AB--with an operating point on the collector current curve halfway between that of A and B. Class A vs class B There is just one class of operation that does not result in distortion. This is class A. All the others--AB, B and C--give us an output waveform that is an amplified, but amputated version, of the input. But before we rejoice about class A, before we decide that that's for us, consider its limitation. Make the input signal too strong and we will get an output waveform that will be clipped top and bottom--a distorted wave. This is the disadvantage.

With class A we actually cut the useful length of our collector current curve in half. We start our bias at its center point. It's just as though we had a street in which to practice running, but, instead of starting at one end and running the full length, we started at the center, with the option of running to either end.

How can we get around this seemingly dead-end situation? We can run our audio amplifier class B. But since class B gives us what is tantamount to rectifier operation, one transistor with half a waveform output won't be enough. We get right past this problem just as neatly as you please by using two transistors in class B pushpull. One transistor supplies the lower half of the output wave; the other supplies the upper half. But please don't get the idea that just class B works in pushpull. We have every class, A, AB, B and C using push-pull. Unfortunately, class C produces so much distortion that we can't use it for audio work, but it does nicely for rf.

Voltage amplifiers and power amplifiers Practically every circuit in a transistor radio receiver is an amplifier, starting at the antenna and going right on out to the speaker. And, if the detector is a transistor, we can make the statement a unanimous one.

In any radio set, the objective is quite simple. The problem is to take the signal off the antenna or loopstick and amplify it--and to keep right on amplifying it. Granted that various changes do take place. We get frequency conversion up at the front end and signal rectification somewhere along the middle of the chain of events. But as we move, stage by stage, in the direction of the speaker, we amplify the signal. The reason for this is that we must have a signal strong enough to dominate the activities of the output transistor.

With this idea in mind, we can roughly classify amplifiers as voltage or power types. Note the word "roughly", used deliberately, for this separation of amplifiers into voltage and power types is a crude designation, but a handy one, nevertheless. (In the case of transistors, it might be more accurate to call them current, rather than voltage amplifiers.) In the sense, though, that our signal is a voltage, and that our transistors are intended to augment this voltage, we have voltage amplifiers. And, because our audio output transistor must furnish the energy to move the voice coil, it is a power amplifier. The separation between voltage and power amplifiers is not sharp and clear-cut. Voltage amplifiers supply some power gain and our power amplifiers do deliver some voltage gain.

There are many ways in which we can classify amplifiers. We can pigeon-hole them quite neatly by bias (Class A, B, etc.) or by whether they are primarily voltage or power amplifiers. We can describe them as single-ended or pushpull. A single-ended amplifier uses a lone transistor to deliver power to the speaker.

Pushpull uses a pair. Or we can talk about amplifiers in terms of their functions--an output amplifier or a driver amplifier.

And, just as some people need four or five names to achieve complete identity, so too do we sometimes take all of these designations, and come up with a mouthful such as: class A, single-ended, power amplifier.

Fig. 509. Single-ended class-A power amplifier R-C-coupled to the preceding stage.

Class A, single-ended, power amplifier

This may seem like a rather elaborate name to hang on the inoffensive little circuit shown in Fig. 509, but we can extract a lot of information from that name. Class A gives us a clue about the forward bias and our location on the collector current characteristic. Single-ended tells us not to look for more than one transistor. Power amplifier is our clue to look for a speaker or output transformer as the load.

Let's examine the circuit of Fig. 509 to get acquainted just a little better. Our component parts are simple enough and few enough. A small assortment of resistors, capacitors, a transistor, transformer and a battery, and that is all there is to it. Starting with the capacitors we see that we have two of them. Both are electrolytics. C1 is a coupling capacitor from the collector of the preceding (driver) stage. C2 is the emitter bypass. With electrolytics we always have the problem of polarity. We've marked C1 and C2 carefully, but the question still remains as to why they are positioned in this way.

Let's start with Cl. One side of the capacitor connects to the collector of the p-n-p driver. But this collector is wired to the negative terminal of the battery. The plus side of C1 is soldered to the junction of R1 and R2. This is a plus point with respect to the top end of R1.

Now what about C2? Current flows through R3 from the emitter toward ground. Because of this direction of current flow, the top end of R3 is minus, the bottom end plus.

If you will look carefully, you will see that R1 and R2 are in series, but that this series combination is connected directly across the 4½ volt battery. It is true that the base is tied right on to a plus point on the voltage divider, but because of the voltage drop across R2, the base is less positive than ground. Ground is the maximum plus point since it is attached to the positive terminal of the battery.

How does the transistor get its forward bias? In two ways. Our voltage divider, R1 and R2, supplies fixed bias. R3, the emitter resistor, takes care of self bias. The values of R1 and R2 are chosen so as to put the operating point of the transistor at the center point of the collector current curve (with the help of R3). This gives us our class A operation.

The collector is transformer coupled to the speaker through an output transformer. The transformer is a stepdown type. L1 has a primary impedance of several hundred ohms. L2 is just a few ohms.

Fig. 510. Single-ended class-A power amplifier transformer-coupled to the preceding stage.

What other information can we get out of Fig. 509? The transistor is a p-n-p type. If we were to use an n-p-n we could still work with the same components, but rearranged. We would need to transpose the two electrolytics, and the battery. We probably would need to alter the values of R1 and R2 to meet the bias conditions of the new transistor.

Resistance coupling and transformer coupling The circuit of Fig. 509 is resistance-capacitance (R-C) coupled to the preceding driver stage, but a transformer could have served just as nicely. We see this new arrangement in Fig. 510. As you see, it is almost the same as the R-C stage. We've removed coupling capacitor C1 and put in transformer T1.

The change is an interesting one. T1 is a stepdown type since the collector of the preceding driver stage is a much higher impedance circuit than the base-emitter circuit of the power amplifier. As before, R1 and R2 form a voltage divider shunted directly across the battery. In series between the de voltage sup- plied at the junction of R1 and R2 we now have the ac signal voltage delivered by the secondary of T 1. This is no great development since we know that the purpose of the signal is to modulate or change the forward bias.

In vacuum-tube receivers, R-C coupling to the output stage is so customary that an occasional transformer coupled job causes some eyebrow lifting. In transistor receivers, though, transformer coupling is common.

Stability of operation

You would think that having gone to the trouble of determining the correct amount of forward bias to put on our transistor (thereby establishing the operating point on the collector current characteristic) this operating point would remain as fixed as a dab of glue. The transistor, though, is temperature sensitive. This means that as temperature goes up, so does collector current. Back in Section 4, earlier, we told you about feedback techniques for stabilizing the transistor. But there are other methods. One technique is to use a thermistor. This is a semiconductor which functions as a temperature-sensitive resistor having a negative temperature coefficient. Translated, this means a thermistor is a component whose resistance goes down when the temperature goes up, and vice versa.

How much change?

This doesn't tell the whole story. Metals have a positive temperature coefficient, meaning, of course, that their resistance tags along after temperature, rising when temperature goes up, de creasing when temperature goes down. But the amount of resistance change is small. Thermistors, however, have a large tempera ture coefficient, just another way of saying that they are very sensitive to temperature changes. To give you an example, in the range from zero to 200 degrees, a platinum wire would have an increase in resistance amounting to a fraction of an ohm. A similar volume of thermistor material, for the same temperature increase, would change from about 8,000 ohms down to about 11 ohms.

Thermistors are made in the shape of discs, washers and rods, of manganese, nickel and cobalt oxides. As in the case of resistors, suitable binder material is added, and then the combination of chemical and binder is extruded, pressed or molded into the de sired shape.

Fig. 511. Thermistor shunted across R2 prevents thermal runaway. Electrically, the thermistor is in parallel with R2. Physically, it is placed near the power transistor and is thus immediately affected by any temperature rise of this component.

Now let's see how the thermistor does its job. In Fig. 511 we have a thermistor, quite properly shown as a variable resistor, shunted across R2. R1 and R2, as we know, supply forward bias to the base-emitter input circuit. And, because we have a p-n-p transistor, we want our emitter to be positive with respect to the base. The bottom end of R2 is our most positive point since it is connected directly to the plus terminal of the battery. As we move up on R2 we become less positive, since, if we continue long enough, we will go right on through R1 to the negative terminal of the battery.

Collector current, though, depends upon the proper forward biasing of the base-emitter circuit. If the forward bias should de crease. so would the collector current. The thermistor in Fig. 511 is quite able to go from a value of almost zero ohms to a value much higher than that of R2. As long as collector current is nor mal (by normal we mean the correct amount of collector current), the resistance of the thermistor will be so high that its paralleling effect on R2 will be negligible. Or, if not, then the combination value of thermistor and R2 can be selected so that the two in parallel supply the correct amount of resistance.

Now suppose that an increase in temperature causes the collector current to increase. This same temperature rise decreases the resistance of the thermistor--fast and substantially. Because of this, the total resistance between base and ground (R2 in parallel with the thermistor) becomes smaller. But this has the effect of lowering the amount of forward bias since the base now approaches ground. But ground is positive and the base needs to be negative. The reduction of forward bias cuts the collector current down to a smaller value.

Fig. 512. The purpose of the heat sink is to dispose of heat as quickly and as effectively as possible. Heat sinks are often made in a corrugated or ribbed style to provide as much surface area as possible. A metal chassis is also used as a heat sink. (Delco Radio Div., General Motors)

Heat sink

Since what we have said has given you the impression that heat and transistors had best be kept apart, it may not be surprising to learn that output transistors can get hot and do get hot. Power transistors, though, often have collector current ratings measured in amperes, and where we have amperes, plus resistance, we have heat. To dissipate the heat we do something that heating engineers and automobile engineers have been doing for a long time. We use a large metal surface to radiate the heat, and the more surface area, the better. Sometimes the chassis is used to conduct heat away from the transistor. In some cases the power transistor is mounted on what seems to be a series of metal fins. A unit of this sort is shown in Fig. 512.

Fig. 513. Push-pull amplifier. Class of operation is determined by the values of R1 and R2. Because of the way the circuit is drawn, it is difficult to see that R1 and R2 act as a voltage divider across the battery, to supply forward bias to the two transistors.

Efficiency-Class A

The power to operate a transistor radio comes from its battery.

The ac power or the signal power used to drive the voice coil of the speaker is derived from that battery. Basically, what happens in a transistor radio is that the power supplied by the battery is modified or altered by the signal, in cooperation with the transistors. But here, as in everything else, we must pay a price. Our battery delivers de watts. Our speaker needs ac watts. In the con version, the transistor will exact its little toll. The ratio of these two values, the output power divided by the input power, is always less than one. In the case of the single-ended class A power amplifier, the maximum efficiency is 0.5 or one half. Thus, if our battery supplies 6 de watts, and our audio power at the output is 3 watts, then 3/6 equals 0.5. We convert this to a percentage by multiplying our answer by 100. The maximum efficiency of our single-ended class A power amplifier is 50 percent, but usually less. And if you think this is a low value, some steam locomotives and machines clock in at 12 percent efficiencies. However, if efficiency is a consideration (and there are others) we can move on to pushpull amplifiers.

The pushpull amplifier

Our single-ended amplifier works class A because this is the only way in which it can work. We can't operate it class AB, B or C without running into some waveform distortion. However, this limitation does not cause trouble in some rf amplifiers.

The circuit of a class-B power amplifier is shown in Fig. 513.

We've drawn this circuit for you in a rather neat way, but un fortunately its very neatness hides the fact that the pushpull amplifier is nothing more than our single-ended job multiplied by two. To see that this is really so, we have separated the biasing system into two sections, as shown in Fig. 514. Since we have p-n-p units, we want our emitter to be maximum positive, the collector maximum negative, and the base somewhere in between. But isn't this the same arrangement we had for the single-ended amplifier? We've omitted the emitter resistor, but all you need do is to break open the line between the common emitter connection and the plus terminal of the battery and insert it. And, if we need to or want to, we can also add a thermistor.

Fig. 514. The circuit of Fig. 513 has been redrawn to show the voltage divider action of R1 and R2. The emitter is maximum positive. The voltage divider makes the base less positive than the emitter. Collector is maximum negative.

We haven't told you what class of operation we've been using in the pushpull circuit of Fig. 514. Actually, you can't tell without a score card. The class will depend on the relationship of R1 to R2. To understand this, imagine R1 and R2 as a single resistor (but with the value of R1 and R2 in series) and the fixed connecting point as a sliding arm on that single resistor. As we move the slide down toward the bottom end (the end connected to the plus terminal of the battery) we keep reducing the forward bias. This drives the transistor collector current down toward the cutoff point. If we move the slide arm up toward the top end of the resistor, we increase the forward bias, so that the collector current goes to saturation. But what are we really doing? We've actually been sliding up the operating point Q (see Fig. 508 once again)

from its point at cutoff to its point at saturation.

But if we can determine our operating point in this way, what we are really doing is establishing the class of operation we will use. Incidentally--we didn't have to go as far as we did. If we started at cutoff, and moved up to the center point of the collector current characteristic, that would have been far enough.

Efficiency--Class B and AB

Class B efficiencies run up to a little more than 75 percent while class AB comes somewhere between class A and class B. Class C is the most efficient, but as we told you earlier, it isn't used for audio work, because of the distortion it produces at audio frequencies.

In class B, when one transistor is hovering around the cutoff point, the other transistor is doing an honest day's work. No criticism here, though, since each transistor takes its turn in delivering collector current to the primary of the output trans former.

Aside from the greater efficiency of class B pushpull contrasted with class A single-ended, the pushpull arrangement has other attractive features. The core of the output transformer for push pull can be smaller than the core of a similar transformer for single-ended. The reason for this lies in the effect direct current has on the core of a transformer. All currents, whether ac or de, carry a magnetic field along with them. But in a transformer, only a varying magnetic field (such as that supplied by ac) is of any use in the transfer of energy from primary to secondary. The steady magnetic field of de not only does not transfer energy, but it actually pre-empts some of the iron core for itself. Thus, if we have both ac and de flowing through the primary of a trans former, we can get a greater transfer of energy if we simply remove the de. In pushpull, we have two direct currents flowing, one from each collector (or toward each collector). But these two currents have magnetic fields which oppose each other.

This leaves the core free to take care of the ac component--that is, the varying current due to the signal.

Power output

The power we can get out of a transistor depends on its class of operation, but this isn't the whole story. Power output is determined by the amount of heat the transistor can dissipate.

In turn, this depends on the cooperation we give the transistor in the way of a heat sink. And, to compound the confusion, not all heat sinks have the same thermal efficiency. In other words, some heat sinks get rid of heat better than others. If the chassis is used as the heat sink a heat-conducting insulating washer is inserted between the chassis and the power transistor. The washer is made of mica, anodized aluminum, Teflon, mylar or Fiberglas.

To improve heat conductivity, each side of the washer should be coated with a thin film of silicone lubricant. If we didn't use a lubricant, the space between the base of the transistor and the chassis might become a dead air region, and an air pocket of this sort could effectively block the transfer of heat to the chassis.

When you replace power transistors, coat the washer with silicone on both sides.

Unlike transistors used in stages preceding the audio output, power transistors usually have but two leads--base and emitter.

The collector is connected to the case of the transistor to help conduct heat away from the junction. There are some power transistors made with three leads, but these are not as common as the two-lead variety. Sockets can be used with power transistors but, when currents are measured in amperes, it doesn't take too much contact resistance to produce an unwanted (and, frequently, unexpected) voltage drop. In some cases it may be more practical to weld or to solder connections directly to the leads of the power transistor or to use machine screws and lugs.

Other pushpull arrangements

The pushpull circuits we've examined are common-emitter types. Common-base and common-collector arrangements could be used, but you will find the common emitter most often.

As in the case of tubes, transformers represent a natural application in transistor pushpull circuits. Transformers are not only highly convenient for impedance matching but also supply phase inversion. But, no sooner do we get a component to do a good job, than all sorts of schemes get underway for eliminating that component. Transistors lend themselves very nicely to this bit of skullduggery if we take advantage of the fact that in n-p-n and p-n-p transistors, currents move in opposite directions.

The pushpull circuit, known as complementary symmetry, is [...]

The pushpull circuit, known as complementary symmetry, is shown in Fig. 515. A careful looks shows that the two transistors are really connected in parallel.

Fig. 515. Complementary symmetry arrangement takes advantage of opposite current flows in n-p-n and p-n-p transistors, manages to eliminate the input pushpull transformer.

To understand how this circuit works, consider an input signal.

Depending on its polarity at the moment, it will either increase or decrease the forward bias. But if that signal is applied to the base-emitter input of two different transistor types, such as an n-p-n and a p-n-p, it will increase the forward bias of one and decrease the forward bias of the other during one half of the cycle and produce exactly the opposite effect during the other half of the cycle. But this is exactly what a transformer will do for us.

In the circuit of Fig. 515, our output is taken from the common emitter, joined by the primary of the output transformer. If you will trace R1 and R2 (for both transistors) you will see that once again we have our usual voltage dividers, R1 and R2 for the p-n-p being shunted across B1, while R1-a and R2-a for the n-p-n are in parallel with B2.

Fig. 516. Bias-adjust potentiometer permits small change in the forward bias of the push pull transistors.

Adjusting the bias

It doesn't take much of a change in forward bias to push us up or down on the load line. And, even using the bias voltage recommended by the manufacturer doesn't always ensure that we will get the class of operation we want. We can get out of this difficulty by making one of the biasing voltage divider resistors variable. Unfortunately, this makes biasing rather critical.

A better setup is shown in Fig. 516. Here we have our voltage divider, R1 and R2, furnishing forward bias to both p-n-p transistors. In series with R1 and R2, we have a third resistor R3.

The value of this resistor can range from practically zero, to almost full value, depending on the setting of the arm of the bias adjustment potentiometer. Note that the bias adjust pot is shunted across R3. With one position of the arm of the bias adjust pot, R3 is shorted. At the opposite position, R3 is almost full value, since the resistance of the bias adjust pot is so much greater than R3.

The driver stage Feeding the power output, whether this is single-ended or pushpull, is an audio amplifier known as a driver. Driver amplifiers are invariably class A since a single transistor is used. A driver circuit is shown in Fig. 517. The input is resistance-capacitance coupled to the preceding stage. In a transistor receiver this could be the detector. R1 and R2 form the usual voltage divider to supply forward bias. The interstage transformer, T, can be used to drive a pair of pushpull transistors.

Circuit-wise, how new or different is the driver compared to single-ended power output? Compare the two circuits, Figs. 509 and 517 and you will see the remarkable resemblance.

Fig. 517. Audio driver stage.

Fig. 518. Two types of tone control circuits.

Control positions

There are three types of controls you will find associated with audio amplifiers. One of these, the bias control has already been mentioned, and is found in connection with power amplifiers. The other two, tone and volume (or gain) controls are part of driver circuitry or output stages.

Two tone control circuits are shown in Fig. 518. They both work in exactly the same way. They supply bass and treble boost, although actually, neither of the two circuits does anything of the sort. Both work because they bypass higher audio frequencies to ground, the amount of "highs" being shunted in this way depending on the position of the potentiometer in Fig. 518-A, or the setting of the rotary switch in Fig. 518-B. As more highs are bypassed, there is a seeming boost of the lower frequencies.

The tone control consists of a capacitor in series with a resistor.

The higher the value of resistance, the more "treble" in the sound since fewer higher audio frequencies are bypassed. The maximum bass "boost" is obtained when all, or almost all, the resistance is out of the circuit. In the case of Fig. 518-A, this would be when the slide arm of the potentiometer is at one of its end positions. In the case of Fig. 518-B, this same situation would prevail when the tone capacitor is connected directly to ground--when the switch is at the wire connection, rather than at R1, R2 or R3.

An alternate arrangement to the circuit shown in Fig. 518-B above would be to have fixed capacitors of various values in place of R1, R2 and R3. A variable resistor would then be used instead of capacitor C1.

Sometimes, just as in the case of vacuum-tube ac-dc sets, you will find a capacitor hanging across the primary of the output transformer. This is a tone control of sorts but tone-compensation capacitor might be more nearly correct. Its function is to cut down the highs, thus diminishing the somewhat tinny sound produced by the speaker.

Fig. 519. One possible position for the gain control is at the input to the driver stage.

Fig. 519 shows a gain control circuit. The secondary of the interstage transformer, T1, is shunted by a potentiometer. This picks off the amount of audio signal that will be used to modulate the de bias supplied by R1 and R2. Note C1, connected to the collector. This is a form of tone "control" but the word control here must be used tongue-in-cheek. C1 bypasses high tones to ground or chassis, giving apparent emphasis to lower-frequency tones.

The emitter follower (grounded collector)

In most audio amplifiers, the signal output is taken from the collector circuit. However, it is sometimes advantageous to take the output from a different element, such as the emitter. Such a circuit is known as an emitter follower or grounded collector.

The emitter follower has a high input resistance, a low output resistance, and no phase reversal. Because of these characteristics, the emitter follower is useful in coupling to low impedance loads.

The load can be another transistor, producing a rather nice circuit for us known as the direct-coupled amplifier. A circuit of this sort is shown in Fig. 520. Let's start with the collectors. The collector of the output transistor, V2, is transformer coupled to the speaker. The primary resistance of the output transformer is fairly low, so the collector of V2 gets almost the full negative voltage. The collector of V1 looks as though it has a load resistor, R3, but this resistor has such a very small value that practically no signal develops across it. If it did, we could always bypass it with a capacitor shunted right in parallel with R3.

Fig. 520. The emitter-follower circuit in this arrangement is used as a driver. VJ is direct-coupled to V2. V2 and its circuitry acts as the load on VJ.

We can see that R1 and R2 act as voltage divider for V1, but this isn't quite so clearly shown for V2. If you will examine V2 you will see that it forms the emitter load for V1. Thus, R1 and R2 really supply bias for both transistors.

How does current flow in a circuit of this sort? If you will start at the negative terminal of the battery, you will be able to move over to R3, through V1 and then back to the positive terminal of the battery through the base of V1 and R2. This base current is very small, so let's get back on to the main path.

We can now go from the collector of V1, over to the emitter of V1. From here, following along with the emitter current, we can go through RS and R6 or through the base-emitter circuit of V2.

But the emitter current of V1 is our amplified signal current.

Now let's get back to the negative terminal of the battery once more. This time, suppose we move down through the primary of the output transformer, through V2, and through R6 to ground (or the plus terminal of the battery). Is this the only current flowing through V2? Not at all, since we just described the passage of V1 's emitter current through V2. We now have two currents going through V2. One is the steady direct current supplied by the battery. The other is the amplified signal current out of the emitter of V1. This signal current will modulate the steady cur rent of V2. R6 is not only the stabilizing resistor for V2, but R6, together with R5, form the emitter return path to ground for V1. What about the input to the direct-coupled stage? There is nothing unusual here. It could be resistance-capacitance coupled to the preceding circuit or transformer coupling can be used.

Fig. 521. Direct-coupled amplifier uses complementary-symmetry principle.

Complementary symmetry in direct coupling

Transistors have one (some would claim more) great advantage over tubes. We got a glimpse of this advantage when we examined the complementary-symmetry pushpull amplifier. Complementary symmetry is just another way of saying that p-n-p and n-p-n transistors work with opposite polarities and opposite directions of current flow. We've put this principle to work for us again, in the direct-coupled amplifier circuit shown in Fig. 521. The fundamental idea is the same, though, as in the earlier direct-coupled circuit we described. The difference is that this time V2 forms the transistor load for V1. The advantage here is that loading the collector produces more gain than the emitter follower. And it isn't necessary for the p-n-p transistor to act as the driver for the n-p-n. The transistors can be transposed, but when we do, we must also remember to transpose the batteries.

Fig. 522. A transistor circuit can be substituted for one of the forward-biasing voltage divider resistors.

A few thoughts

What is it that we are trying to do when we couple one transistor stage to the next? All we want is to vary the forward bias so that it goes up and down exactly in step with every little wiggle and movement of the signal. If you will look back at the single-ended stage shown in Fig. 510, and if you could change R1 into a potentiometer that you could operate rapidly enough, you would get audio output. This gives us a clue to how we can

operate an audio stage, as shown in Fig. 522. Instead of having R1 and R2 as a voltage divider to supply forward bias, we have R2 and some kind of a transistor circuit substituted for R1. But is this so surprising? Earlier, in Fig. 519, didn't we put the secondary winding of an interstage transformer in series with R1 and R2? What we have in Fig. 522, then, is just another way of varying the forward bias.

What would a circuit of this sort look like? We've already had several, but may not have recognized them as such. As an example, let's go back to Fig. 520 in which we show an emitter-follower circuit used as a driver. If you will examine the base circuit of V2, you will see that we have R5 and R6 in series. One end of R6 connects to the plus terminal of the battery. And one end of R5 goes to the base of V2. These two series resistors, then, correspond to R2 in Fig. 522. Now what about the block marked "transistor circuit"? Still working with the emitter follower of Fig. 520, we know that we must work our way up from the base to the other end (the minus end) of the battery. To do this we must go through the collector-emitter circuit of V1 and R3. But the collector-emitter circuit of V1 and R3 just represent a substitute for R1. And since this substitute supplies a varying audio signal, we will get audio modulation of the base circuit of V2.

Diode compensation or stabilization

Fig. 523. The circuit shown in [A] uses R1 and R2 as a voltage divider for forward bias.

In [B] we have substituted a diode for R2.

All along we've been using a pair of series resistors, which we have marked R1 and R2, to supply forward bias for our transistors. But there is an assumption here that we haven't told you about. We have taken for granted that we will have a constant voltage drop across the resistors. Was this reasonable on our part? To answer this question we need to ask another one. What could possibly cause the voltage across R1 and across R2 to change? Presumably a weak battery could be one cause of this trouble, but we can't take this too seriously. If the battery is weak, its regulation will be poor, the forward bias will be incorrect and we will get distortion. The end result of this is that what we hear out of the speaker will be a constant reminder to get a fresh battery. Now assuming that we have a fresh battery, what other difficulties could we have? What about our resistors, R1 and R2 As resistors, they have a positive temperature coefficient. But isn't this the opposite of what the transistor has? If you will examine Fig. 528-A you will see that we have R2 in shunt with the base-emitter circuit of the transistor. This is like compounding a felony, since R2 and the base-emitter circuit have opposite temperature coefficients.

Fig. 524. Diode DJ, substituted for voltage-divider resistor R2, has the advantage of acting as a temperature-compensating de vice. Note that the diode is forward-biased. Its forward-biased resistance is the correct value so that the bases of both transistors are properly biased.

To get around this unhappy situation, we can substitute a diode for R2, as shown in Fig. 523-B. The theory here is that the diode has a negative temperature coefficient. But what about the base emitter part of our transistor? That's a diode, too.

Suppose temperature increases. The resistance between base and emitter decreases, causing an increased current flow between these two elements. At the same time, though, the resistance of the diode (substituting for R2) also decreases. Because the diode is in shunt with the base-emitter circuit, we have a bypass path for additional current.

-------p134 This all transistor stereo preamplifier features computer-like plug-in modules. A transformerless circuit gives a 1 cycle to 1 megacycle frequency response.

(Harman-Kardon Inc.)

Now what if we had a temperature increase and had R2 in place of the diode? With a rise in temperature, the resistance of R2 goes up and so this very action forces even more current to flow between base and emitter.

Diode compensation or stabilization is generally found with power output stages since it is in such stages that temperature has a chance to do its dirty work. In Fig. 524 we have a circuit using a diode in this way. Note also the capacitor connected from collector to collector. This bypass is a bass-boost unit. The higher the capacitance, the higher the boost. A typical value would be

.04 µf.

The circuit of Fig. 524 uses n-p-n transistors. P-n-p's could also have been used, but in this case both the diode, D1, and the 9-volt battery would need to be transposed.

A final word Amplifiers, like people, come in all sizes and shapes. A popular indoor sport among engineers is to n-y to see just how many different variations of audio amplifiers they can dream up -

and, considering the choice of audio transistor circuitry we now have, they haven't just been wool-gathering. We have pushpull drivers feeding pushpull output; amplifiers in which all trans formers (interstage and output) are eliminated; new arrangements of bias supplies; various feedback arrangements; temperature compensation, etc. This is like a huge plate of spaghetti. Dig around with sufficient seriousness, and you're bound to find yourself a few meatballs. The same is true of any audio circuit.

Keep in mind just what it is the transistor is supposed to do, rearrange the circuit so that the biasing becomes obvious, and you'll be able to brush aside the clutter of extra components with ease, so that the basic amplifier circuits just shine through at you.

This doesn't mean we're finished with amplifiers. Those we've examined cover audio frequencies. We now have to climb the frequency range up to if and rf. We start our ascent in the next section.

QUESTIONS

1. What is base current? Emitter current? Collector current?

2. What is a common-emitter circuit? Common-collector? Common-base?

3. What is meant by in phase? Out of phase?

4. What is phase inversion?

5. Describe biasing of a transistor amplifier. What is the effect of forward biasing? Reverse biasing?

6. Describe class-A, -AB, -B and -C amplifier operation.

7. What are the differences between voltage and power amplifiers?

8. How does a single-ended amplifier differ from a pushpull amplifier?

9. What are the advantages and disadvantages of R-C coupling?

10. What are the advantages and disadvantages of transformer coupling?

11. What is meant by temperature coefficient?

12. What is positive temperature coefficient of resistance? Negative temperature coefficient? Zero temperature coefficient?

13. What is a thermistor? How is it used?

14. How does temperature affect a transistor?

15. What is a heat sink? How is it used?