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Fig, 5. Characteristic curve of a typical transistor is similar to that of a tube. Each curve in this family represents the collector voltage-collector current relationship for a single value of base cur* rent, Diagonal line is "load line'' for 3000 ohm collector load resistor; Point labeled "operating" is bias point to operate as an amplifier with this toad resistor, "On* and '"Off points illustrate switching action explained in the text,

(4) both input and output open. Of these, the first and third are most applicable to most circuits. The second applies only in a few special circuits, while the fourth is almost never used and can even destroy many types of transistors.

The characteristics of any particular type of transistor are measured under these specific conditions, and published as char-asteristic curves similar to those available for tubes. The conditions for each curve are always noted, but sometimes the notations are in engineeringese. Fig. 5 shows a typical characteristic curve. These curves are used to predict transistor performance in the same way that tube curves are used to predict how a type of tube will perform in any planned circuit.

The curve in Fig. 5 shows how gain is

affected by the amount of base bias supplied to the unit; the wider apart the horizontal lines are, the greater the beta and the higher the gain. With either too much or too little bias, gain falls off rapidly; operating conditions are rather critical.

With almost no bias, the gain falls to zero and current is virtually cut off. With ¡excessive bias, the gain again falls to zero because the collect or-base junction's resistance is as low as it can get. These two points, labelled "off" and "on" respectively in Fig, 5, are widely used in digital circuits—at these points, a transistor is a better switch than most ordinary switches. The TO keyer is a typical ham application of this type of circuit.

What Are The Basic Circuits? The transistor, as we have seen, is a gadget which can amplify—and as such it must have an input circuit and an output circuit, he gadget itself consists of three parts; the emitter, the base, and the collector. In most applications both the input and the output circuits work a "hot" signal lead against a "ground" connection so that the entire amplifier has only three signal terminals, which are "input", "output", and "ground" or

This is not unique to transistors. With vacuum tubes we have the grounded-grid v t r e u i t s and the cathode-follower or grounded-plate* as well as the conventional grounded-cathode arrangeinent

Since the transistor has three internal components, we can choose any one of ihes? three as the 'common" or grounded element. This leads us to the three basic circuits for trail sis tors—the common-emitter, common-collector (or emitter follower), and common-base (grounded base) circuits.

The word "grounded" is sometimes used to replace the compound adjective "commonin the circuit names; the FCC study list uses 'grounded" but most present writings appear to prefer the "common" phrasing.

Fig. 6 illustrates these three circuits, stripped to their basics. You can see that in every case, input signals are applied to the base-emitter circuit and output is taken off in the collector-base circuit—but the three circuits differ drastically in their char* asteristics because of the different ways in which they relate input and output circuits to each other.

For instance, in the common-base circuit out

out

rn common base

m common emitter rh common collector

Fig. 6. The three basic transistor circuits are shown here. While all are related y characteristics oj each differ from the others.

shown in Fig. 7 the input and output circuits are about as isolated from each other as it is possible to get with a transistor. The only point in the entire circuit at which both input and output currents are present is the transistor base (including its leads back to ground). The input circuit consists of the base-emitter junction and bias arrangement. Fhe output circuit consists of the collector-base junction and its bias and load arrangements.

In this circuit, the input circuit sees a forward-biased junction and so exhibits very low impedance. The output circuit, on the other hand, contains a reverse-biased junction and so is of very high impedance. The circuit corresponds to the vacuum-tube grounded-grid amplifier, and offers the same advantages of high-frequency operation with good power gain.

The common-collector circuit shown in Fig, 8, o]i the other hand, makes the output circuit an integral part input circuit so that the output tends to completely "buck out" the input signal. The input circuit consists of the base-emitter junction, the load resistor in the emitter lead, and the power supply (with the common point passiiig through the power supply). The output circuit consists of the forward-biased base-emitter junction (which acts for it as a low-valued resistor), the base-collector junction, and the emitter load resistor.

Fig. 7. Common base circuit was earliest. It has voltage gain, but no current gain. Input impedance is low and output imped mice is high. Trans* former input shown here can be replaced with coupling capacitor as in subsequent figures. Similarly, output load resistor can be replaced by transformer primary, il'hnMttW

Fig. 7. Common base circuit was earliest. It has voltage gain, but no current gain. Input impedance is low and output imped mice is high. Trans* former input shown here can be replaced with coupling capacitor as in subsequent figures. Similarly, output load resistor can be replaced by transformer primary,

Since actual active input must be applied between base and emitter, while the output appears across the emitter resistor, the actual active input to the transistor in this circuit is much smaller than the applied input signal. In fact, the signal seen by the transistor is equal to the applied input signal minus the resulting output signal. If a L-volt ac signal is applied and a 0.9-volt ac signal is produced at the output, the effective input to the transistor is only 0.1 volt and the transistor itself is performing at a gain of 9 although the circuits gain is only 0.9.

With the effective input reduced by an amount proportional to the transistor's current gain (beta), the current flow in the input circuit is reduced by that same factor, Since we did nothing outside the circuit to reduce the applied input signal, this has the effect: of multiplying the input impedance of the transistor by its beta and so the common-collector circuit has high input impedance.

Fig. 8. This is common collector circuit. Both input and output current flows through load resistor Rl and the result is high input impedance, low output impedance. Voltage gain is less than 1, but current gain can be high.

The bucking-out action we have just examined affects voltage in the output circuit but does nothing to hold down the current. The output current is as great as our load resistor and operating point will permit, but the voltage associated with this current is reduced by the buck-out. This causes output impedance of the circuit to be divided by transistor beta; the result is very low output impedance.

The common-collector circuit, then, corresponds to the vacuum-tube cathode follower, It has high input impedance together with low output impedance, and voltage gain is always less than 1. With high-beta transistors, though, voltage gain can be almost up to 1, It will be, to a first approximation, equal to the transistor's alpha factor —which can exceed 0.995.

The remaining basic circuit is the com

Fig. 9. Common-emitter circuit shown here is most widely used. It has both current and voltage gain, and so is only transistor circuit exhibiting appreciable power gain as well. Both input and output impedances are moderate.

Fig. 9. Common-emitter circuit shown here is most widely used. It has both current and voltage gain, and so is only transistor circuit exhibiting appreciable power gain as well. Both input and output impedances are moderate.

mon-emittcr arrangement shown in Fig, 9, This one acts in a manner midway between the other two. The input circuit—the baseemitter junction and its bias arrangement-is relatively unaffected by the output circuit, but in the output circuit the baseemitter junction is placed in series with the collector-base junction. This reduces output impedance by a bucking-out action, but the buck-out is much less than in the common-emitter since only a part of the output signal appears across the base-emitter junction while in the common-emitter the entire output signal appears in the input circuit.

The fraction of the output signal which does appear in the input circuit tends to increase the input impedance in much the same manner as in the common emitter. Again, the effect is much smaller.

Both the common base and the common collector circuits represent extremes—input /output isolation in one case and total interaction in the other. The common emitter circuit represents a compromise between these two extremes, and all its characteristics are intermediate between those of the other two. Voltage gain is moderate; not so high as in the common base but much higher than the common co+lector. Current gain, which is less than I in common base but almost equals beta in common collector, is also moderate.. Since power is the product of voltage times current and the common emitter circuit is the only one in which both voltage and current gain exceed ¡, it has the greatest power gain of the circuits.

Input/output isolation is only moderate in this circuit, which limits the frequency response and makes accidental feedback a possible problem. However, the common-emitter c i r c u i tj like the vacuum-tube grounded-cathodc circuit to which it corresponds, is the most widely employed in practical applications because of its power gain and preponderance of advantages over disadvantages.

You may have read elsewhere of the peculiar problems involved in adapting vacuum-tube circuits to transistors. Most of these problems are not inherent in transistors themselves, but are the product of the fact that the first transistor circuit used was the common base version. The same problems are present in grounded-grid vacuum-tube circuits—they'r e the problems of adapting one type of circuit io another type, not those of changing from one kind of component to another, '['he fabled ";ow impedance" of transistors, in particular, is apparent only in common base, With run-of-the-mill transistors, impedances equal to those of tubes can be obtained by proper blendings of the circuit types used, Common-collector circuits with high-gain transistors can have as much as 10 megohms input impedance; few vacuum tubes can stand that much grid resistance without developing "contact potential" bias problems I

What are the Transistor's Advantages? in the 20 years that transistors have been on the scene, they have virtually replaced vacuum tubes in many applications. Obviously, then, they must have some advantages over tubes. What are they?

The major advantages of the transistor as compared to the tube fall into three cate-gores—size, power requirements, and reliability.

The size advantage enjoyed by the transistor is obvious to anyone. Typical transistor sizes are much smaller than those of tubes with comparable characteristics, Che action ol the transistor occurs within the atoms which make up a single crystal of semiconductor material—that of the tube occurs in a stream of electrons flowing from one physical element past another to a third. Today's integrated circuits were made possible by !he transistor's capability of being reduced to truly microscopic size; the smallest tubes are still easily visible.

Power requirements for a transistor are much less than those for a tube of comparable abilities. The largest part of the power reduction comes about because the transistor needs no heater to make it work. Most tubes, also, require much higher operating voltages than do similar transistors, even though some tubes do operate at low voltages and some transistors are capable of operating at vacuum-tube voltage levels.

With no heaterf the transistor also runs much cooler than does a tube. Even a power transistor normally is cooler to the touch than is an ordinary low-power tube. This reduction of heat in the circuit adds to the size advantage by permitting transistors to be packed into more compact spaces, and is in itself an advantage since no special cooling is required in many applications.

The most spectaetular advantage, however, is in the area of reliability. A tube operates by boiling off electrons from its cathode. Eventually, the tube wears out and must be replaced. Even before this happens, the tube is likely to become gassy—or to be burned out or broken.

The transistor, on the other hand, operates by the injection of electrons into a crystal, No boil-off is involved, and there is nothing in the basic action to cause the device to ever wear out. The transistor can become contaminated, which corresponds to the tube going gassy, and it can be burned out—but if manufacture is controlled with enough care, contamination is not likely, and if the circuit is properly designed and operated, burnout is equally unlikely to occur. The result is that a transistorized circuit can be expected to perform properly for from 10 to 1000 times as long as a similar circuit using tubes.

So far as breakage is concerned, this is easy to demonstrate. Just put a tube and a transistor side by side at the edge of a table, and sweep both over the side onto the floor. After picking up the broken glass from the tube, test the transistor. It will probably be working perfectly.

The reliability of the transistor is what has made the digital computer industry, to cite but one example, possible. Premium tubes have a life expect an cv of around 50,000 hours. If a circuit requires 100,000 tubes, the law of averages tells us that we can expect a burnout on the average of once every half-hour. This means that the longest period of operation we can expect from this complicated gadget is 30 minutes.

Similar quality transistors, however, have an estimated life expectancy of over 8,001,000 hours (the reason that it's estimated is that nobody has been able to run a test long enough yet to be sure that the figure is accurate). If those 100,000 tubes are replaced by transistors, then failure can be expected only once in 80 hours. The reliability of the device has been improved by

160 times—and any device that complicated can do any job expected of it in less than SO hours.

While the figures may sound a bit extreme, many modern industrial devices contain tens of thousands of transistors. Such devices simply were not practical during days of tubes, ['his is, then, the major advantage enjoyed by the transistor.

What are the Disadvantages? We have seen that the transistor's advantages over the tube are primarily those of size, lower heat, less power required, and greater reliability. Why, then, have they not replaced all tubes?

Transistors do have some disadvantages. The greatest of these are in the area of high-frequency capability, and power handling. Bolli are being overcome, but it's a safe bet that in applications which involve both high power and high frequencies, tubes will be preferred for many years to come.

The first transistors were limited to low-frequency use. ¡n fact, they did not perform adequately even over the audio-frequency range, and use at rf was impossible. The limit was pushed gradually higher until broadcast-band operation became practical —and kept going higher. Today, inexpensive transistors which perform nicely at 50 and 141 MHz are available. In fact, they outperform tubes at VHF frequencies today, having noise figures which are much lower than those of even premium tubes.

The first transistors, also, were limited to very low power. Even in the af region, they were unable to develop enough power to drive a loudspeaker. Like the (juest for higher-frequency operation, the search for additional power moved forward. Audio power transistors became available, and some years later rf power transistors hit the scene. Today its possible to build a 100-watt 6-meter transmitter using nothing but transistors.

However, the disadvantages are still present. That 100-watt transistorized transmitter is going to cost you more than would a kilowatt with tubes—and if vou want a m full gallon the tube is still the only practical choice at 6 meters- At any higher frequencies, the power capabilities are much less.

So, while it is possible to use transistors instead of tubes for power applications at rf\ it's still much more expensive—and this is in itself a disadvantage.

There are some electrical disadvantages too. A transistor is, bv its verv nature, a

voltage-variable capacitor as well. In high-frequency circuits, this capacitance can hamper action ot the circuit. The transistor is a triode type of device, and so far it hasn't been practical to introduce elements which correspond to the tube's screen or suppressor grids. This prevents the transistor from being used in circuits which make use of these elements. Isolation between input and output circuits is much greater with tubes than with transistors. Tubes are operated by voltage changes while transistors operate by current variations— and this means that a transistor must impose some load upon its input and output circuits while a tube can be made essentially a 110n-loading amplifier.

The net result is that, while the transistor's advantages make it the preferred choice for many types of uses, its disadvantages rule it out for many special applications.

What are the Critical Factors When Using Transistors? The basic differences between tubes and transistors come out most clearly when a designer attempts to put a transistor into a circuit. Because of these basic differences, transistor circuits have a number of critical factors which can be almost ignored when using tubes.

The two major critical factors are voltages and heat, The active area of a transistor may be no more than a few atoms in thickness. Since it is so thin, any applied voltage which is too high can cause "punch-through" which is similar to the puncturing of a capacitor. When punch-through happens, the result is a dead short at that point. Transistor action ceases and the device turns into a low-value resistor.

Punch-through can be either temporary or permanent, depending upon the power available at the puncture region, A temporary punch-through is of little consequence—as soon as the over-voltage is removed the problem is over. If, however, enough power is available, the transistor material is melted at the punched-through spot and the damage becomcs permanent.

Since the material involved is so very small, it doesn't take much power to be "enough l\ A few milliwatts is more than adequate in most cases. For this reason, transistors should always be protected against over-voltage, The over-voltage may be in either the power supply or in the

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