534 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1951 



be biased in such a way that current flows out of the emitter into a suitable 

 current supply. Furthermore, the collector current corresponding to any 

 given emitter current can be seen to be almost equal in magnitude to the 

 emitter current. Since these two currents are opposite in sign, this means 

 that most of the current which flows into the collector leaves by way of the 

 emitter with the result that the current in the base circuit is very small. 



Suppose that the collector is held at a constant positive voltage as, for 

 example, by connecting a battery between collector and base (with a trans- 

 former winding in series, perhaps). Now if a negative current is forced into 

 the emitter by a battery and resistance connected in series between emitter 

 and base, the collector current can be controlled by varying the emitter 

 current and will always be approximately equal in magnitude to the emitter 

 current. Suitable collector currents for this particular transistor range from 

 about 20 microamperes to about five milliamperes. 



The exact choice of collector current and voltage within the ranges men- 

 tioned above will be dictated largely by the amount of power output re- 

 quired. The more power output required, the more current and voltage will 

 be needed from the power supply. Since the collector circuit efficiency can- 

 not exceed the theoretical limit of 50% in Class A operation, the signal 

 power output cannot exceed half the power supplied by the battery. This 

 means, for example, that if the collector is worked at 20 volts and 2 milli- 

 amperes the Class A power output cannot exceed 20 milliwatts. 



From the lower plot of Fig. 4 it is possible to obtain information about 

 the bias voltage required for the emitter. Note, first, that the entire emitter 

 voltage plot corresponds to a very small range of emitter voltages near zero 

 and, furthermore, that the part of the characteristics corresponding to the 

 operating range covers only a few thousandths of a volt. This means that 

 if the collector voltage is held constant very small changes in emitter volt- 

 age will produce fairly large changes in collector current, or if the collector 

 current is held constant very small changes in emitter voltage will produce 

 relatively enormous changes in collector voltage. This at once suggests the 

 use of this transistor as a d-c. amplifier between a low impedance source 

 and a high impedance load. In this application, voltage stepup of the order 

 of 10,000 times is possible. 



The very great sensitivity of the collector circuit to emitter voltage sug- 

 gests, however, that for a-c. ampHfiers one should use a current source as 

 an emitter bias supply. This can be obtained from a battery and a large 

 resistance in series. Furthermore, since the emitter voltage is always nearly 

 zero, the emitter current can be calculated in advance by dividing the bat- 

 tery voltage by the value of the series resistance (provided, of course, that 



