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Survey of Transistor Development
November 1952 Radio & Television News

November 1952 Radio & Television News
November 1952 Radio & Television News Cover - RF Cafe[Table of Contents]

Wax nostalgic about and learn from the history of early electronics. See articles from Radio & Television News, published 1919-1959. All copyrights hereby acknowledged.

Mr. B. N. Slade, of the Tube Department of Radio Corporation of America, wrote a series of articles on transistor development for three 1952 issues of Radio & Television News magazine. Consider that it was only four years earlier, a few days before Christmas, that Messrs. Bardeen, Brattain, and Shockley announced their game-changing invention of the point contact transistor. Already a plethora of commercial transistors were on the market for incorporation into new electronic products. At the time, germanium was still the semiconductor of choice, although silicon was gaining ground in laboratories. This article covers the three basic transistor circuit topologies of common emitter, common base, and common collector, which are analogous to vacuum tube circuits using common cathode, common grid, and common plate topologies, respectively. Operation up to around 200 MHz was obtainable under certain conditions, but such frequencies were well outside the realm of capability for most transistors.

Unfortunately, I do not yet have the September issue of Radio & Television News that ran Part 1, but here is Part 2.

Survey of Transistor Development - Part 3

Survey of Transistor Development, November 1952 Radio News - RF CafeBy B. N. Slade

Tube Dept., Radio Corporation of America

Harrison, New Jersey

Part 3. Concluding article covers simple transistor amplifier circuits and designs for other applications.

Two views of an RCA transistor. The unit at the left is complete with components embedded in plastic. Unit at right is still under construction.

In this, the concluding article in this series, we will consider some simple transistor amplifier circuits, other transistor circuit applications, and several other types of germanium devices.

Transistor Amplifier Circuits

It is interesting to compare the amplifier circuit properties of the point-contact transistor and the junction transistor. A number of amplifier circuit connections are possible to obtain several combinations of input and out-put impedances. In the case of the point-contact transistor, however, special consideration must be given to the circuitry. If the internal feedback resistance is too large, and if the current amplification factor is greater than unity, the circuit may become unstable and oscillations will occur. It can be seen in the curves in Fig. 3, Part 2 (September issue, page 64) that the internal feedback resistance varies with the operating point. The current amplification factor may also vary somewhat with collector voltage, thus making the circuit stability dependent upon the d.c. biases. Resistance placed in series with the emitter and collector leads helps to suppress these oscillations, but may decrease the power gain of the circuit. For example, the input impedance to the grounded-base amplifier circuit shown in Fig. 1 is approximately 500 ohms and the output impedance is approximately 10,000 ohms. If the internal feedback resistance is too large, additional resistance necessary to stabilize the circuit will exceed these impedance values and, therefore, reduce the gain of the circuit. Point-contact transistors which have a very low value of internal feedback resistance, less than 100 ohms, for example, usually have such low feedback that amplifier circuits require no special stabilization. It is desirable in some r.f. circuits, particularly, that the transistor be stable under low impedance conditions such as off-resonance of a parallel-tuned circuit.

In the case of the simple junction transistor, the current amplification factor is always less than unity, and oscillations cannot occur. Ryder and Kircher1 have pointed out that the grounded-base circuit is analogous to an electron-tube grounded-grid circuit if the emitter, base, and collector of the transistor are compared to the cathode, grid, and plate of the electron tube, respectively. The grounded-grid electron-tube circuit also has a low input and high output impedance. The comparison is particularly appropriate in the case of the junction transistor, which, like the tube circuit, is stable even under extreme short-circuit conditions.

Transistor is used in grounded-base amplifier circuit - RF Cafe

Fig. 1 - Layout whereby the transistor is used in grounded-base amplifier circuit.

Transistor grounded-emitter amplifier circuit - RF Cafe

Fig. 2 - A transistor grounded-emitter amplifier circuit, as discussed in the text.

Transistor grounded-collector amplifier circuit - RF Cafe

Fig. 3 - The transistor grounded-collector amplifier circuit. See text for details.

If the emitter is grounded, as in Fig. 2, higher input impedances and lower output impedances may be obtained. Higher power gains may be obtained with this circuit configuration than with the grounded-base circuit, but in point-contact transistors the feedback may become large and lead to instability. If junction transistors are used, this type of circuit is similar to an electron-tube grounded-cathode circuit.

Higher input impedances and lower output impedances may also be obtained if the collector is grounded, as in Fig. 3. This circuit can become unstable if a point-contact transistor is used, and the power gain which may be obtained is low. However, the junction transistor can be used to good advantage in this circuit, because power gains ranging from 10 to 20 db may be obtained with input impedances and output impedances on the order of 200,000 and 50,000 ohms, respectively. In fact, appreciable gain may be obtained using equal input and output matching impedance, thus making cascading of several stages of amplification feasible. This circuit is similar to the electron-tube grounded-plate or conventional cathode-follower circuit.

Table 1 shows typical values of input and output impedances and power gains for all three types of circuits for both junction-type and point-contact transistors. It will be noted that in the grounded-emitter and grounded-base circuits the input and output impedances of the point-contact transistor may actually become negative values, a condition which indicates that these circuits are potentially unstable. These characteristics of the point-contact types, which lead to potential instability in amplifiers, are of great advantage in oscillators and trigger devices.

Other Circuit Applications

When considering the possible circuit applications for the two types of transistors, one must be aware of the advantages and limitations of both types.

At the present time, the advantages of high gain, low noise, and greater stability of the simple junction transistor can be utilized at frequencies up to several megacycles in applications such as r.f. and i.f. amplifiers of standard broadcasting receivers. In addition, power outputs greater than one watt appear to be possible in oscillator and amplifier applications in the audio frequency and low frequency ranges. Another feature of the junction transistor is its ability to amplify and oscillate with microwatt power inputs.

The frequency response of the point-contact transistors, on the other hand, is somewhat higher than that of junction types. As with junction types, point-contact types which are currently available can be made to oscillate and amplify over the broadcast-frequency band. When used as an amplifier, point-contact transistors have a relatively flat response over the entire broadcast band and beyond. Types now under development will operate at considerably higher frequencies. Feedback in these units has been reduced to values which make stable operation at radio frequencies practical. The point-contact transistor, therefore, may also have considerable application in radio circuits and may be used in intermediate-frequency amplifiers, radio-frequency oscillators, and other circuits not associated with the high-power stages of r.f. systems. Point-contact transistors have been developed which are capable of oscillating at frequencies well over 100 mc. Oscillations at frequencies higher than 200 mc. have been obtained; one developmental unit has oscillated at a frequency over 300 mc.

One of the most important uses of the point-contact transistor probably will be in counter circuits. A number of recent publications2 describe some basic circuits which utilize the negative resistance properties of one or more transistors. These circuits generate pulses of various waveforms, store information for varying periods of time, add, subtract, multiply, and divide. Up to the present time these functions, and many others, have been performed in electronic computers by large numbers of electron tubes for which the heater-power supplies alone have been considerable. Use of the transistor would obviously alleviate this situation since no heater power is required. Furthermore, little d.c. power is necessary for operation. The adverse characteristics of transistors with regard to frequency response, noise, and power output are relatively unimportant factors in computer circuits. Computers which employ germanium devices would have the advantages of small size, ruggedness, and economy of operation and maintenance.

Other Germanium Devices

Input and output impedances and power gains for three circuit applications - RF Cafe

Table 1 - Input and output impedances and power gains for three circuit applications.

The progress in the field of germanium devices is not limited to the field of transistors. While the point-contact germanium diode has already attained commercial acceptance, new types of diodes utilizing the "p-n" junction rectification characteristics are being developed. One diode power rectifier which utilizes a p-type or acceptor impurity metal diffused onto a pellet of germanium has already been described.3 Peak inverse voltages of 400 volts are permissible with these devices which have very low resistances in the forward direction and current-carrying capabilities as high as 350 milliamperes. When the relative infancy of the germanium power rectifier is considered, it is difficult to estimate the ultimate importance of these devices. Because of improved efficiency, however, they appear to be suitable both as a replacement for the selenium rectifier and as an advantageous substitute for certain types of rectifier tubes.

Another germanium device of considerable significance is the phototransistor.4 This photocell is a photo-conductive device and operates on the principle that light absorbed by germanium changes its conductivity. In the phototransistor, a point contact acts as the collector and draws a small amount of current. Light in the vicinity of the collector increases the conductivity of the germanium and the current through the collector.

The first transistor was announced only three and one-half years ago. Great strides have been made in learning the fundamental theory of operation of transistor devices, and much progress has been made in the knowledge of the control of transistor characteristics and manufacturing processes. There appear to be a number of fields in which transistors will be used widely and to great advantage. Further improvements in their characteristics may be expected as research and development continue.

Acknowledgment

The author wishes to acknowledge the advice and contributions of Mr. E. W. Herold and Dr. J. Kurshan of the RCA Laboratories Division, Princeton, N. J., and of Mr. R. M. Cohen and Mr. H. Nelson of the RCA Tube Department, Harrison, N. J.

1. Ryder, R. M. and Kircher, R. J.; "Some Circuit Aspects of the Transistor" Bell System Technical Journal, Vol. XXVIII, pages 367-401, July, 1949

2. Eberhard, E., Endrey, R. O., and Moore, R. P.; "Counter Circuits Using Transistors," RCA Review, Vol. X, No.4, page 459, December, 1949.

3; Saby, J. S.; "Recent -Developments in Transistors and Related Devices," Tele-Tech, Vol. 10, No. 12, December, 1951.

4. Shive, J. N.; "The Phototransistors," Bell Laboratories Record;" Vol. XXVIII, No.8, pages 337-342, August, 1950.

 

 

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