TRANSISTOR PULSE REGENERATIVE AMPLIFIERS 



1099 



[and pulse inversion. In a system of logic circuits interspersed with ampli- 

 fiers at regular intervals, it is apparent that the dc level at similar points, 

 such as the outputs of the amplifiers, must be identical if the amplifiers 

 are to be interchangeable. Without some circuit or element to restore the 

 dc level, the levels along the transmission path will monotonically de- 

 crease* due to the dc voltage loss through the logic circuits and across 

 the transistor in the amplifier. The output circuit is one point where 

 restoration of the dc level may be readily combined with other functions.! 

 In the following two sections three methods of output coupling are 

 discussed and the interaction between the output and feedback circuits 

 is considered. 



5.1 Output Coupling Elements 



Three types of coupling circuits are RC, transformer, and diode cou- 

 pling. Each of these methods permits the dc level of the signal pulses to be 

 corrected to a predetermined level. However, the restoration,! efl&ciency, 

 and versatility characteristics of each circuit are quite different. 



Although RC coupling is common in linear amplifiers, it is seldom used 

 in transistor pulse amplifiers that operate at duty cycles near 50 per 

 cent. The reason is that the time constants encountered do not permit 

 both proper restoration of the capacitor and high efficiency of the output 

 circuit. As indicated in Fig. 8 (a), the transistor is a low impedance in 



TRANSISTOR 



TRANSISTOR 



ON 



R1 yOFF 



(SMALL) 



".RS 



-—\ 



I 



>(LARGE) 



■R3 



(a) RC COUPLING 



(b) 



TRANSFORMER COUPLING 



Fig. 8 — Reactive output coupling circuits. 



* Decrease for positive pulses; increase for negative pulses. 



t An exception, to be discussed, is diode output coupling where it is occasionally 

 more convenient to correct the dc level in the input of the logic circuits or the 

 amplifier. 



t This refers to restoration of a reactive element (i.e., the return to a quiescent 

 state) and is not to be confused with restoration of the dc level of a circuit. 



