TRANSISTOR PULSE REGENERATIVE AMPLIFIERS 1091 



introduce an impedance in the feedback loop at a predetermined current 

 level. 



The degree of stability of the amplifier in the "on" state may be 

 thought of as the amount of power required to initiate the transition to 

 the "off" state. During the early portion of the output pulse duration 

 the degree of stability should be large, but near the end of the pulse 

 duration it should be relatively small to make turn-off easier. Also, the 

 degree of stability should not change over the range of output loading 

 expected for the amplifier and should be effected without excessive 

 wastage of pulse or supply power. These conditions are difficult to fulfill 

 when the range of output load current may be as large as 20 to 1 . 



Three methods of obtaining positive feedback in transistor circuits 

 will now be considered: (a) negative resistance feedback; (b) capacitor 

 coupled feedback; and (c) transformer coupled feedback. Of these, 

 transformer coupled feedback appears to be the best for most applica- 

 tions. It will be assumed that the type of feedback under discussion is 

 the dominant or only type present; circuits employing more than one 

 feedback mechanism generally violate the premise of simple circuitry 

 and will not be discussed. 



3.1 Negative Resistance Feedback 



With the advent of point contact transistors a novel form of negative 

 resistance was offered to circuit designers for use in positive feedback 

 applications.^ This negative resistance property occurs when the current 

 gain of a transistor is greater than unity and the emitter and base small 

 signal currents are in phase.* At first sight this property appears to 

 lead to attractively simple regenerative amplifiers. However, as systems 

 become more complex and, consequently, amplifier requirements more 

 severe, the original simplicity often is lost due to the additional circuitry 

 required to control the negative resistance. An example, shown in Fig. 

 4, is similar to a regenerative amplifier described by J. H. Felker.^ The 

 functional circuits are indicated by dashed outlines. 



This amplifier operates at a one megacycle pulse repetition rate with 

 one-half microsecond, three volt pulses. It is capable of driving from one 

 to six similar amplifiers. The output pulse rise time is 0.05 microsecond, 

 the average dc standby power is 33 milliwatts, only a few components 

 operate at as much as half of maximum ratings, and the supply voltage 

 marginsf are greater than ±15 per cent. Seven hundred of these ampli- 



* Although point contact transistors are noted for this property, certain types 

 of junction transistors also exhibit it. For example, see Reference 7. 



t Supply voltage margins, the amount by which the supply voltage may be 



