412 



THE BELL SYSTEM TECHNICAL JOURNAL, APRIL 1951 



denser will remain charged approximately to the instantaneous peak value 

 of the input wave, which is the envelope of the input. The transistor circuit 

 works in a similar way. When the input current is positive, the circuit be- 

 haves like a cathode follower, and a current is sent through the inductor L 

 which is approximately equal to the input current. When the input current 

 falls, the emitter current rises, and the collector presents a low impedance. 



■/TOT^— ^ 



RF 



I ! S J 



I 



AF RF I 

 I 



I 



fHh- 



Zu' 



AF 



(a) (b) 



Fig. 22 — Plate modulator and dual. 



RFC 



Fig. 23— Constant current modulator and dual. 



The current through the inductor then decays slowly through the load and 

 the low collector impedance. Each time the input signal has a positive peak, 

 the effect is to draw a large current through the inductor, which persists dur- 

 ing the rest of the cycle. 



The dual of the infinite impedance detector is not a very attractive circuit, 

 because the transformer must act for the carrier frequency as well as for 

 the envelope frequencies. 



Figures 22, 23, 24, 25, 26 and 27 show various amplitude modulator 



