1090 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1956 



a diode or large resistor, these effects are enhanced and the shape, dura- 

 tion, and ampHtude of the output signal become independent of the 

 input signal. These results are possible because once the circuit has been 

 triggered and the feedback loop gain is greater than unity, the response 

 proceeds independently of input conditions and is determined solely by 

 the transistor and circuit parameters. 



By definition a regenerative amplifier must have positive feedback 

 sufficient to cause instability during the transition period between the 

 "off" and "on" states. When investigating various circuits, it is neces- 

 sary to eliminate circuits which are never unstable when a pulse is 

 applied to the input circuit. If the circuit is unstable under either of the 

 conditions shown in Fig. 3, sufficient instability is possible. However, 

 if the circuit is stable, linear, and either the small signal open circuit 

 voltage gain or the short circuit current gain is less than unity or nega- 

 tive at all frequencies, it is impossible to have instability. These latter 

 conditions for instability often can be easily checked by inspection with- 

 out tedious computation or experimentation. 



This use of positive feedback requires that attention be given to its 

 control. To be useful, the amplifier must be stable in one state and at 

 least quasi-stable in the other state. The change from instability in the 

 transition period to stability in the end states is accomplished by a non- 

 linear change in the gain or impedance of some element in the feedback 

 loop. Usually the "off" state is made stable by causing the voltage and 

 current conditions in the input circuit to reverse bias the transistor in- 

 put. The "on" state may be made stable (or quasi-stable when there are 

 reactive coupling elements in the loop) in several ways. For example, 

 the transistor may be permitted to saturate when the desired pulse 

 voltage is reached; a "catching" diode may be used to clip the pulse 

 voltage at an appropriate level; or a current switch may be used to 



FEEDBACK 



TRANSISTOR 



FEEDBACK 



TRANSISTOR 



Voc ef'v 



iSC 



(a) OPEN-CIRCUIT LOOP VOLTAGE (W SHORT-CIRCUIT LOOP CURRENT 



Fig. 3 — A check for instability. 



