TRANSISTOR PULSE REGENERATIVE AMPLIFIERS 1089 



Again, this represents wasted power, but also important is the fact that 

 it places an upper limit on the output power available from the transistor. 

 Hence, it is desirable to have as low a saturation voltage as possible. 

 Alloy junction transistors are especially good in this respect. 



The speed with which a transistor changes state is principally a func- 

 tion of the alpha cut-off frequency (which should be high), base re- 

 sistance, and collector capacitance (both of which should be low).*'* 

 Both the rise and fall times of the transistor response are greatly in- 

 fluenced by the associated circuitry; generally a blocking oscillator 

 circuit yields the fastest response. 



The amount of effective power gain available from a regenerative 

 amplifier is influenced by two transistor properties. One property is the 

 breakdown voltage, which may be the collector to base breakdown volt- 

 age or the collector to emitter punch through voltage (whichever is 

 lower). This limits the output power by limiting the collector supply 

 voltage. The other factor is the variation of alpha with emitter current, 

 especially at low emitter currents. The minimum average emitter current 

 required to initiate self-sustaining positive feedback determines the 

 minimum input power. Point contact units are especially good in this 

 respect in that alpha may approach ten at emitter currents as low as 

 five microamperes. Junction units are poor since alpha generally de- 

 creases rapidly at emitter currents below one hundred microamperes. 



Even though the attainable margins against false operation are largely 

 a matter of circuit design, two transistor properties occasionally become 

 important. In point contact units trouble with lock up in the "on" state 

 may occur due to internal base resistance. Although this property of base 

 resistance is exploited in negative resistance feedback circuits, it is un- 

 desirable in circuits where the feedback is obtained by external coupling. 

 In grown junction units the emitter to base reverse breakdown voltage 

 may limit the voltage margin against false triggering caused by noise or 

 crosstalk. Normally it is desirable to have a one or two volt margin. 



From the above discussion it can be seen that no one type of transistor 

 is outstanding in all features. The choice of which unit to use in a specific 

 amplifier depends upon the repetition rate, gain, and power requirements 

 desired of the amplifier. Although the point contact type has the best 

 overall performance of the types shown in Table I, it is quite possible 

 that new types (such as PNIP or diffused triodes^^) and improved de- 

 signs of the present types will change the picture. 



3. FEEDBACK CIRCUITS 



The use of positive feedback in an amplifier results in high gain and 

 short rise time. If the input circuit is isolated from the feedback loop by 



