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
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
Survey of Transistor Development - Part 3
By 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.
Fig. 1 - Layout whereby the transistor is used in grounded-base
Fig. 2 - A transistor grounded-emitter amplifier circuit, as
discussed in the text.
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
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
Table 1 - Input and output impedances and power gains for three
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
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.
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.
Posted November 14, 2022