TRANSISTOR PULSE REGENERATIVE AMPLIFIERS 1105 



appreciable output current flows until the voltage across transformer Tl 

 is sufficient to forward bias diode D2. Then both the feedback and output 

 current build up simultaneously and rapidly since the turns ratio l:ni 

 of Tl is selected to give a feedback loop gain greater than unity. When 

 the sum of the voltages across the primaries of the feedback and output 

 transformers almost equals the collector supply voltage, the transistor 

 saturates and stabilizes the feedback loop. At the end of the pulse dura- 

 tion the timing signal voltage goes negative and robs current from the 

 feedback loop, thus forcing the transistor out of saturation and causing 

 the amphfier to turn off. 



Because the feedback current is proportional to the output current 

 during the rise time, the amplifier can deliver any value of load current 

 up to the current corresponding to the maximum allowable collector 

 current. Also, assuming that the leakage inductances of the transformers 

 are small, a large stray capacitance across the output terminal does not 

 appreciably degrade the rise time. Since a current switch is not neces- 

 sary, the standby power dissipation in the feedback loop is negligible. 

 These are the outstanding features of the series connection. 



Two important performance considerations of the series type amplifier 

 are the change in the degree of stability versus load current variations 

 and the action of the amplifier when the timing signal fails. Both of these 

 items may be controlled by the selection of suitable values for the turns 

 ratio and the primary inductance of the feedback and output transform- 

 ers.* In order to prevent burnout of the transistor in the event that the 

 timing signal fails, the amount of excess feedback current must decrease 

 during the pulse duration. Due to the low impedance of the feedback 

 loop, this condition may be approximately f stated in terms of the pri- 

 mary inductances as: 



Vi 

 niLi 



> 





a 



(3) 



where Vsat is the collector saturation voltage and Li and L2 are the 

 primary inductances of Ti and T2 respectively. 



The degree of stability in the series amplifier at the end of the pulse 

 duration is proportional to the output load current. This situation may 

 be seen more clearly if a "catching" diode (D6 in Fig. 12) is added to the 



* If the transistor is not short circuit stable, it is also usually necessary to 

 use a small resistance in series with the emitter. 



t The principal approximation is that alpha is constant versus collector current. 

 The value of alpha at the end of the pulse duration is a conservative value. 



