Transistors 91

SCHEMATIC FEPFESENTATION

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TUBE. GERMANIUM, SILICON AND SELENIUM DIODES

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observed in metallic conductors. There exist in semiconductors both negatively charged electrons and positively charged particles, called holes, which behave as though they had a positive electrical charge equal in magnitude to the negative electrical charge on the electron. These holes and electrons drift in an electrical field with a velocity which is proportional to the field itself:

Vdh - flbE

where Vnh = drift velocity of hole

E = magnitude of electric field fih = mobility of hole

In an electric field the holes will drift in a direction opposite to that of the electron and with about one-half the velocity, since the hole mobility is about one-half the electron mobility. A sample of a semiconductor, such as germanium or silicon, which is both chemically pure and mechanically perfect will contain in it approximately equal numbers of holes and electrons and is called an intrinsic semiconductor. The intrinsic resistivity of the semiconductor depends strongly upon the temperature, being about 50 ohm/cm, for germanium at room temperature. The intrinsic resistivity of silicon is about 65,000 ohm/cm. at the same temperature.

If, in the growing of the semiconductor crystal, a small amount of an impurity, such as phosphorous, arsenic or antimony is included in the crystal, each atom of the impurity contributes one free electron. This electron is available for conduction. The crystal is said to be doped and has become electron-conduct-

PLAST(C CASE

N-TYPE GERMANIUM CRYSTAL LAYER

P-TYPE GERMANIUM CRYSTAL LAYER

COLLECTOR EMITTER

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Figure 2A CUT-AWAY VIEW OF JUNCTION TRANSISTOR, SHOWING PHYSICAL ARRANGEMENT

Figure 2B PICTORIAL EQUIVALENT OF P-N-P JUNCTION TRANSISTOR

EMITTER BASE CONNECTlOl

EMITTER

Figure 3 CONSTRUCTION DETAIL OF A POINT CONTACT TRANSISTOR

ing in nature and is called N (negative) type germanium. The impurities which contribute electrons are called donors. N-type germanium has better conductivity than pure germanium in one direction, and a continuous stream of electrons will flow through the crystal in this direction as long as an external potential of the correct polarity is applied across the crystal.

Other impurities, such as atuminum, gallium or indium add one hole to the semiconducting crystal by accepting one electron for each atom of impurity, thus creating additional holes in the semiconducting crystal. The material is now said to be hole-conducting, or P (positive} type germanium. The impurities which create holes are called acceptors. P-type germanium has better conductivity than pure germanium in one direction. This direction is opposite to that of the N-type material. Either the N-type or the P-type germanium is called extrinsic conducting type. The doped materials have lower resistivities than the pure materials, and doped semiconductors in the resistivity range of .01 to 10 ohm/cm. are normally used in the production of transistors.

The Transistor

In the past few years an entire new technology has been developed for the application of certain semiconducting materials in production of devices having gain properties. These gain properties were previously found only in vacuum tubes. The elements germanium and silicon are the principal materials which exhibit the proper semiconducting properties permitting their application in the new amplifying devices called transistors. However, other semiconducting materials, including the compounds indium antimonide and lead sulfide have been used experimentally in the production of transistors.

EMITTER. .COLLECTOR

P-n-P transistor or POINTCOnTaCt transistor

BASE-

N-P-N TRANSISTOR

Figure 4 ELECTRICAL SYMBOLS FOR TRANSISTORS

Types of Transistors There are two basic types of transistors, the point-contact type and the junction type (figure 2). Typical construction detail of a point-contact transistor is shown in Figure 3, and the electrical symbol is shown in Figure 4. The emitter and collector electrodes make contact with a small block of germanium, called the base. The base may be either N-type or P-type germanium, and is approximately .05 long and .03 thick. The emitter and collector electrodes are fine wires, and are spaced about .005 apart on the germanium base. The complete assembly is usually encapsulated in a small, plastic case to provide ruggedness and to avoid contaminating effects of the atmosphere. The polarity of emitter and collector voltages depends upon the type of germanium employed in the base, as illustrated in figure 4.

The junction transistor consists of a piece of either N-type or P-type germanium between two wafers of germanium of the opposite type. Either N-P-N or P-N-P transistors may be made. In one construction called the grown crystal process, the original crystal, grown from molten germanium or silicon, is created in such a way as to have the two closely spaced junctions imbedded in it. In the other construction called the fusion process, the crystals are grown so as to make them a single conductivity type. The junctions are then produced by fusing small pellets of special metal alloys into minute plates cut from the original crystal. Typical construction detail of a junction transistor is shown in figure 2A.

The electrical schematic for the P-N-P junction transistor is the same as for the point-contact type, as is shown in figure 4.

Transistor Action Presently available types of transistors have three essential actions which collectively are called transistor action. These are: minority carrier injection, transport, and collection. Figure 2Б shows a simplified drawing of a P-N-P junction-type transistor, which can illustrate this

collective action. The P-N-P transistor consists of a piece of N-type germanium on opposite sides of which a layer of P-type material has been grown by the fusion process. Terminals are connected to the two P-sections and to the N-type base. The transistor may be considered as two P-N junction rectifiers placed in close juxaposition with a semi-conduction crystal coupling the two rectifiers together. The left-hand terminal is biased in the forward (or conducting) direction and is called the emitter. The right-hand terminal is biased in the back (or reverse) direction and is called the collector The operating potentials are chosen with respect to the base terminal, which may or may not be grounded. If an N-P-N transistor is used in place of the P-N-P, the operating potentials are reversed.

The Pe - Nb junction on the left is biased in the forward direction and holes from the P region are injected into the Nb region, producing therein a concentration of holes substantially greater than normally present in the material. These holes travel across the base region towards the collector, attracting neighboring electrons, finally increasing the available supply of conducting electrons in the collector loop. As a result, the collector loop possesses lower resistance whenever the emitter circuit is in operation. In junction transistors this charge transport is by means of diffusion wherein the charges move from a region of high concentration to a region of lower concentration at the collector. The collector, biased in the opposite direction, acts as a sink for these holes, and is said to collect them.

It is known that any rectifier biased in the forward direction has a very low internal impedance, whereas one biased in the back direction has a very high internal impedance. Thus, current flows into the transistor in a low impedance circuit, and appears at the output as current flowing in a high impedance circuit. The ratio of a change in collector current to a change in emitter current is called the current amplification, or alpha:

ic

where a = current amplification

ic = change in collector current ie = change in emitter current

Values of alpha up to 3 or so may be obtained in commercially available point-contact transistors, and values of alpha up to about 0.95 are obtainable in junction transistors.

Alpha Cutoff The alpha cutoff frequency of Frequency a transistor is that frequency

at which the grounded base current gain has decreased to 0.7 of the gain obtained at 1 kc. For audio transistors, the alpha cutoff frequency is in the region of 0.7 Mc. to 1.5 Mc. For r-f and switching transistors, the alpha cutoff frequency may be 5 Mc. or higher. The upper frequency limit of operation of the transistor is determined by the small but finite time it takes the majority carrier to move from one electrode to another.

Drift Transistors As previously noted, the signal current in a conventional transistor is transmitted across the base region by a diffusion process. The transit time of the carriers across this region is, therefore relatively long. RCA has developed a technique for the manufacture of transistors which does not depend upon diffusion for transmission of the signal across the base region. Transistors featuring this new process are known as drift transistors. Diffusion of charge carriers across the base region is eliminated and the carriers are propelled across the region by a built in electric field. The resulting reduction of transit time of the carrier permits drift transistors to be used at much higher frequencies than transistors of conventional design.

The built in electric field is in the base region of the drift transistor. This field is achieved by utilizing an impurity density which varies from one side of the base to the other. The impurity density is high next to the emitter and low next to the collector. Thus, there are more mobile electrons in the region near the emitter than in the region near the collector, and they will try to diffuse evenly throughout the base. However, any displacement of the negative charge leaves a positive charge in the region from which the electrons came, because every atom of the base material was originally electrically neutral. The displacement of the charge creates an electric field that tends to prevent further electron diffusion so that a condition of equilibrium is reached. The direction of this field is such as to prevent electron diffusion from the high density area near the emitter to the low density area near the collector. Therefore, holes entering the base will be accelerated from the emitter to the collector by the electric field. Thus the diffusion of charge carriers across the base region is augmented by the built-in electric field. A potential energy diagram for a drift transistor is shown in figure 5.

DECREASING POTENTIAL ENERCy OF MAJORITY CARRIER

COLLECTOR

DISTANCE

Figure 5

POTENTIAL ENERGY DIAGRAM FOR DRIFT TRANSISTOR (2N247)

5-4 Transistor

Characteristics

The transistor produces results that may be comparable to a vacuum tube, but there is a basic difference between the two devices. The vacuum tube is a voltage controlled device whereas the transistor is a current controlled device. A vacuum tube normally operates with its grid biased in the negative or high resistance direction, and its plate biased in the positive or low resistance direction. The tube conducts only by means of electrons, and has its conducting counterpart in the form of the N-P-N transistor, whose majority carriers are also electrons. There is no vacuum tube equivalent of the P-N-P transistor, whose majority carriers are holes.

The biasing conditions stated above provide the high input impedance and low output impedance of the vacuum tube. The transistor is biased in the positive or low resistance direction in the emitter circuit, and in the negative, or high resistance direction in the collector circuit resulting in a low input impedance and a high output impedance, contrary to and opposite from the vacuum tube. A comparison of point-contact transistor characteristics and vacuum tube characteristics is made in figure 6.

The resistance gain of a transistor is expressed as the ratio of output resistance to input resistance. The input resistance of a typical transistor is low, in the neighborhood of 300 ohms, while the output resistance is relatively high, usually over 20,000 ohms. For a point-contact transistor, the resistance gain is usually over 60.

The voltage gain of a transistor is the product of alpha times the resistance gain, and for a point-contact transistor is of the

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COMPARISON OF POINT-CONTACT TRANSISTOR AND VACUUM TUBE CHARACTERISTICS

order of 3 X 60 = 180. A junction transistor which has a value of alpha less than unity nevertheless has a resistance gain of the order of 2000 because of its extremely high output resistance, and the resulting voltage gain is about 1800 or so. For both types of transistors the power gain is the product of alpha squared times the resistance gain and is of the order of 400 to 500.

The output characteristics of the junction transistor are of great interest. A typical example is shown in figure 7. It is seen that the junction transistor has the characteristics of an ideal pentode vacuum tube. The collector current is practically independent of the collector voltage. The range of linear operation extends from a minimum voltage of about 0.2 volts up to the maximum rated collector voltage. A typical load line is shown, which illustrates the very high load impedance that would be required for maximum power transfer. A grounded emitter circuit is usually used, since the output impedance is not as high as when a grounded base circuit is used.

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Figure 7 OUTPUT CHARACTERISTICS OF TYPICAL JUNCTION TRANSISTOR

The output characteristics of a typical point-contact transistor are shown in figure 6. The pentode characteristics are less evident, und the output impedance is much lower, with the range of linear operation extending down to a collector voltage of 2 or 3. Of greater practical interest, however, is the input characteristic curve with short-circuited, or nearly short-circuited input, as shown in figure 8. It is this point-contact transistor characteristic of having a region of negative impedance that lends the unit to use in switching circuits. The transistor circuit may be made to have two, one or zero stable operating points, depending upon the bias voltages and the load impedance used.

Equivalent Circuit of a Transistor

As is known from network theory, the small signal performance of any device in any network can be represented by means of an equivalent circuit. The most

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EMITTER CHARACTERISTIC CURVE FOR TYPICAL POINT CONTACT TRANSISTOR

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Figure 9 LOW FREQUENCY EQUIVALENT (Common Base) CIRCUIT FOR POINT CONTACT AND JUNCTION TRANSISTOR

convenient equivalent circuit for the low frequency small signal performance of both point-contact and junction transistors is shown in figure 9. Гр, гь, and Гс, are dynamic resistances which can be associated with the emitter, base and collector regions of the transistor. The current generator а1ч, represents the transport of charge from emitter to collector. Typical values of the equivalent circuit are shown in figure 9-

Transistor There are three basic transis-

Configurations tor configurations: grounded base connection, grounded emitter connection, and grounded collector connection. These correspond roughly to grounded grid, grounded cathode, and grounded plate circuits in vacuum tube terminology (figure 10).

The grounded base circuit has a low input impedance and high output impedance, and no phase reversal of signal from input to output circuit. The grounded emitter circuit has a higher input impedance and a lower output impedance than the grounded base circuit, and a reversal of phase between the input and output signal occurs. This circuit usually provides maximum voltage gain from a transistor. The grounded collector circuit has relatively high input impedance, low output impedance, and no phase reversal of signal from input to output circuit. Power and voltage gain are both

low.

Figure 11 illustrates some practical vacuum tube circuits, as applied to transistors.

GROUNDED BASE CONNECTION

GROUNDED EMITTER CONNECTION

GROUNDED COLLECTOR CONNECTION

Figure 10

COMPARISON OF BASIC VACUUM TUBE AND TRANSISTOR CONFIGURATIONS

5-5 Transistor Circuitry

To establish the correct operating parameters of the transistor, a bias voltage must be established between the emitter and the base. Since transistors are temperature sensitive devices, and since some variation in characteristics usually exists between transistors of a given type, attention must be given to the bias system to

overcome these difficulties. The simple self-bias system is shown in figure 12A. The base is simply connected to the power supply through a large resistance which supplies a fixed value of base current to the transistor. This bias system is extremely sensitive to the current transfer ratio of the transistor, and must be adjusted for optimum results with each transistor. When the supply voltage is fairly high and

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Figure 11 TYPICAL TRANSISTOR CIRCUITS

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Figure 12

BIAS CONFIGURATIONS FOR TRANSISTORS.

T/ie vo/tcrge divider system of С is recommended for general transistor use. Ratio of Ri/Rs establishes base bias, and emitter bias is provided by voltage drop across Re. Battery Polarity is reversed for N-P-N transistors.

wide variations in ambient temperature do not occur, the bias system of figure 12B may be used, with the bias resistor connected from base to collector. When the collector voltage is high, the base current is increased, moving the operating point of the transistor down the load line. If the collector voltage is low, the operating point moves upwards along the load line, thus providing automatic control of the base bias voltage- This circuit is sensitive to changes in ambient temperature, and may permit transistor failure when the transistor is operated near maximum dissipation ratings.

A better bias system is shown in figure 12C, where the base bias is obtained from a voltage divider, (Rl, R2), and the emitter is forward biased. To prevent signal degeneration, the emitter bias resistor is bypassed with a large capacitance. A high degree of circuit stability is provided by this form of bias, providing the emitter capacitance is of the order of 50 /*fd. for audio frequency applications.

Audio Circuitry A simple voltage amplifier is shown in figure 13. Direct current stabilization is employed in the enitter circuit. Operating parameters for the

amplifier are given in the drawing. In this case, the input impedance of the amplifier is quite low. When used with a high impedance driving source such as a crystal microphone a step down input transformer should be employed as shown in figure 13B. The grounded collector circuit of figure 13C provides a high input impedance and a low output impedance, much as in the manner of a vacuum tube cathode follower.

The circuit of a two stage resistance coupled amplifier is shown in figure 14A. The input impedance is approximately 1100 ohms. Feedback may be placed around this amplifier from the emitter of the second stage to the base of the first stage, as shown in figure 14B. A direct coupled version of the r-c amplifier is shown in figure 14C. The input impedance is of the order of 15,000 ohms, and an overall voltage gain of 80 may be obtained with a supply potential of 12 volts.

It is possible to employ N-P-N and P-N-P transistors in complementary symmetry circuits which have no equivalent in vacuum tube design. Figure 15A illustrates such a circuit. A symmetrical push-pull circuit is shown in

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P-N-P TRANSISTOR VOLTAGE AMPLIFIERS

A resistance coupled amplifier employing an inexpensive CK-722 transistor is shown in A. For use with a high impedance crystal microphone, a step-down transformer matches the low input impedance of the transistor, as shown in B. The grounded collector configuration of С provides an input impedance of

about 300,000 ohms.

Figure 14

TWO STAGE TRANSISTOR AUDIO AMPLIFIERS

The feedback loop of В may be added to the r-c amplifier to reduce distortion, or to control the audio response. A direct coupled amplifier is shown in C.

figure 15B. Ttiis circuit may be used to directly drive a high impedance loudspeaker, eliminating the output transformer. A direct coupled three stage amplifier having a gain figure of 80 db is shown in figure 15C.

The transistor may also be used as a class A power amplifier, as shown in figure l6A. Commercial transistors are available that will provide five or six watts of audio power when operating from a 12 volt supply. The smaller units provide power levels of a few milliwatts. The correct operating point is chosen so that the output signal can swing equally in the positive and negative directions, as shown in the collector curves of figure 16B.

The proper primary impedance of the output transformer depends upon the amount of power to be delivered to the load:

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In a class A output stage, the maximum a-c

power output obtainable is limited to 0.5 the allowable dissipation of the transistor. The product IcEe determines the maximum collector dissipation, and a plot of these values is shown in figure 16B. The load line should always lie under the dissipation curve, and should encompass the maximum possible area between the axes of the graph for maximum output condition. In general, the load line is tangent to the dissipation curve and passes through the supply voltage point at zero collector current. The d-c operating point is thus approximately one-half the supply voltage.

typical push-pull class В is shown in figure 17A. is desirable for transistor even-order harmonics ate largely eliminated. This permits transistors to be driven into high collector current regions without distortion normally caused by non-linearity of the collector. Cross-over distortion is reduced to a minimum by providing a slight forward base bias in addition to the normal emitter bias. The base bias is usually less than 0.5 volt in most cases. Excessive base bias will boost the quiescent collector current and thereby lower the overall efficiency of the stage.

The circuit of a transistor amplifier Push-pull operation operation, since the

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Figure 15

COMPLEMENTARY SYMMETRY AMPLIFIERS.

N-P-N and P-N-P transistors may be combined in circuits which have no equiYahnt irt vacuum tube design. Direct coupling between cascaded stages using a single power supply source may be employed, as in C. Impedance of power supply should be extremely low.

Figure 16 TYPICAL CLASS-A AUDIO POWER TRANSISTOR CIRCUIT.

The correct operating point is chosen so that output signal can swing equally in a positive or negative direction, without exceeding maximum collector dissipation.

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The Operating point of the class В amplifier is set on the lc=0 axis at the point where the collector voltage equals the supply voltage. The collector to collector impedance of the output transformer is:

Rc-c -

In the class В circuit, the maximum a-c power input is approximately equal to five times the allowable collector dissipation of each transistor. Power transistors, such as the 2N301 have collector dissipation ratings of 5.5 watts and operate with class В efficiency of about 6l%. To achieve this level of operation the heavy duty transistor relies upon efficient heat transfer from the transistor case to the chassis, using the large thermal capacity of the chassis as a heat sink. An infinite heat sink may be approximated by mounting the transistor in the center of a 6 x 6 copper or aluminum sheet. This area may be part of a larger chassis.

The collector of most power transistors is electrically connected to the case. For applications where the collector is not grounded a thin sheet of mica may be used between the case of the transistor and the chassis.

Power transistors such as the Philco T-1041 may be used in the common collector class Б

configuration (figure 17C) to obtain high power output at very low distortions comparable with those found in quality vacuum tube circuits having heavy overall feedback. In addition, the transistor may be directly bolted to the chassis, assuming a negative grounded power supply Power output is of the order of 10 watts, with about 0.5% total distortion.

R-F Circuitry Transistors may be used for radio frequency work provided the alpha cutoff frequency of the units is sufficiently higher than the operating frequency. Shown in figure 18A is a typical i-f amplifier employing an N-P-N transistor. The collector current is determined by a voltage divider on the base circuit and by a bias resistor in the emitter leg. Input and output are coupled by means of tuned i-f transformers. Bypass capacitors are placed across the bias resistors to prevent signal frequency degener-arion. The base is conneaed to a low impedance untuned winding of the input transformer, and the collector is connected to a tap on the output transformer to provide proper matching, and also to make the performance of the stage relatively independent of variations between transistors of the same type. With a rate-grown N-P-N transistor such as the G.E. 2N293, it is unnecessary to use neutralization to obtain circuit stability. When P-N-P alloy

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Figure 17

CLASS-B AUDIO AMPLIFIER CIRCUITRY.

the common collector circuit of С permits the transistor to be bolte4 directly to the chassis for efficient

heat transfer from the transistor case to the chassis.

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TRANSISTORIZED I-F AMPLIFIERS.

Typical P-N-P transistor must be neutralized because of high collector capacitance. Rate grown N-P-N transistor does not usually require external neutralizing circuit.

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CONTROL CIRCUIT FOR TRANSISTORIZED I-F AMPLIFIER.

transistors are used, it is necessary to neutralize the circuit to obtain stability (figure 18B).

The gain of a transistor i-f amplifier will decrease as the emitter current is decreased. This transistor property can be used to control the gain of an i-f amplifier so that weak and strong signals will produce the same audio output. A typical i-f strip incorporating this automatic volume control action is shown in figure 19.

R-f transistors may be used as mixers or autodyne converters much in the same manner as vacuum tubes The autodyne circuit is shown in figure 20. Transformer Ti feeds back a

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THE AUTODYNE CONVERTER CIRCUIT USING A 2N168A AS A MIXER.

signal from the collector to the emitter causing oscillation. Capacitor Ci tunes the oscillator circuit to a frequency 455 kc. higher than that of the incoming signal. The local oscillator signal is inductively coupled into the emitter circuit of the transistor. The incoming signal is resonated in T2 and coupled via a low impedance winding to the base circuit. Notice that the base is biased by a voltage divider circuit much the same as is used in audio frequency operation. The two signals are mixed in this stage and the desired beat frequency of 455 kc. is selected by i-f transformer Тз and passed to the next stage. Collector currents of 0.6 ma. to 0.8 ma. are common, and the local oscillator injection voltage at the emitter is in the range of 0.15 to 0.25 volts, r.m.s.

A complete receiver front end capable of operation up to 23 Mc. is shown in figure 21. The RCA 2N247 drift transistor is used for the r-f amplifier (TRl), mixer (TR2), and high frequency oscillator (TR3). The 2N247 incorporates an interlead shield, cutting the interlead capacitance to .003 /a/j-fd. If proper shielding is employed between the tuned circuits of the r-f stage, no neutralization of the stage is required. The complete assembly obtains power from a 9-volt transistor battery. Note that input and output circuits of the transistors are tapped at low impedance points on the r-fcoils to achieve proper impedance match.