410 



THE BELL SYSTEM TECHNICAL JOURNAL, APRIL 1951 



and a low output impedance, and has a voltage gain nearly equal to one. 

 The dual circuit has a low input impedance and a high output impedance, 

 and a current gain nearly equal to one. This comes about as follows: Suppose 

 the collector circuit resistance and load are small. Then the collector is ap- 

 proximately at ground potential. Let the input current increase. This tends 

 to increase the voltage drop from base to collector, and therefore the base 

 potential tends to rise. This rise is transmitted to the emitter by the trans- 

 former, and therefore the emitter current rises. The rise in emitter current 

 causes a drop in collector resistance, and counteracts the tendency for the 

 collector-to-base voltage to rise. The result is that the input current passes 

 through the collector circuit into the load without any corresponding rise 

 in voltage between base and ground. This means that the input impedance 

 is low. Of course, not all the input current passes to the load; some is passed 

 to the emitter circuit. In a practical case tested, using a transistor whose 



^h — I.-F:: — r"^^^^^ 



c^—E 



(a) (b) 



Fig. 19 — Plate detector and dual. 



base and emitter resistances were of the order of a few hundred ohms, with 

 a load of 5000 ohms, the current gain was about .70 and the input impedance 

 was about 40 ohms. 



Figures 19, 20, and 21 show several ampUtude modulation detectors and 

 their duals. The purpose of all these circuits is to derive from an ampHtude 

 modulated wave a wave proportional to the envelope of the given wave. 



Figure 19 shows a plate detector and its dual. The plate detector looks like 

 a single-stage amplifier with a low-pass filter in its output circuit. It is 

 biased approximately to plate current cutoff. As an amplifier it amplifies 

 approximately the upper half of the input wave and does not pass the lower 

 half. The filter smooths the succession of current pulses in the plate circuit 

 and gives an output proportional to the average of the upper half of the 

 input wave. If the input is a true ampUtude modulated wave, this is also 

 proportional to the envelope. 



The dual circuit operates in the same way. The circuit looks like a one- 

 stage amplifier with a low-pass filter in the collector circuit. It is biased to 

 collector voltage cutoff. The negative part of the input signal is amplified, 

 and the positive part is not. The collector voltage is a succession of pulses 



