TRANSISTOR PULSE REGENERATIVE AMPLIFIERS 



1093 



The use of an inductor, instead of a resistance, in the base lead does 

 not appear to mitigate the hmitations. * 



3.2 Capacitor Coupled Feedback 



A second method of obtaining positive feedback is by external coupling 

 through a capacitor or capacitor-resistor network. This method is sel- 

 dom used for the principal feedback for reasons to be mentioned. Oc- 

 casionally, in conjunction with some other type of feedback, it may be 

 used to provide additional feedback during the rise time of an amplifier. 



Since the voltage and current gain of a capacitor can not exceed 

 unity, the open circuit voltage gain and the short circuit current gain of 

 the rest of the loop (Fig. 3) must be greater than unity for instability. 

 This criterion indicates that capacitor feedback is limited to point con- 

 tact, or other transistors with an alpha greater than unity, or to a junc- 

 tion transistor in the common emitter configuration.* 



A circuit with capacitor feedback around a short-circuit stable point 

 contact transistor might take the form shown in Fig. 5. Although this 

 type of circuit has the merit of simplicity, it has the following limitations : 



1. The initial feedback current is highly dependent upon the incre- 

 mental output load impedance. This may result in a failure to trigger 

 when the load approximates a short circuit, as in the case of diode gates 

 or a large stray capacitance. 



2. The degree of stability in the ''on" state is critically dependent on 

 the load current and the collector supply voltage. Variations in either 

 may cause a foreshortened output pulse or require an excessive timing 

 signal current for turn-off. 



FEEDBACK CIRCUIT 

 R 



V, \AAr 



TRIGGER CURRENT 

 FROM 

 INPUT CIRCUIT ■ 



INPUT TRIGGER 

 CIRCUIT 



AAA- 



c 



SINE WAVE 



TIMING 



VOLTAGE 



SYNCHRO- 

 NIZING 

 CIRCUIT 



n:i 



.^^^UTPU T ^ 



TRANSISTOR I 



OUTPUT COUPLING 

 CIRCUIT 



Fig. 5 — RC feedback amplifier. 



An inverting transformer is necessary with the junction transistor. 



