DUALITY AS GUIDE IN TRANSISTOR CIRCUIT DESIGN 



411 



of varying amplitudes. These are smoothed by the filter; and the output, 

 as before, is proportional to the envelope of the input. 



It is important for the proper operation of these circuits that the filter 

 in the plate detector have low input impedance outside of the pass band, and 

 that the filter in the dual circuit have low input admittance outside of the 

 pass band. The exact form of the filter is immaterial. 



Figure 20 shows a grid leak detector and its dual. The operation of the 

 grid leak detector depends on the same principles as that of the grid leak 

 bias circuit described before. The time constant of the bias circuit is chosen, 

 however, so that the bias will be able to vary fast enough to follow the en- 



fa) (b) 



Fig. 20 — Grid leak detector and dual. 



A — ^. 



(a) (b) 



Fig. 21 — Infinite impedance detector and dual. 



velope of the input wave. The overall grid voltage or emitter current is then 

 the input with a super-imposed wave proportional to the envelope of the 

 input. These are amplified together, and the undesired high-frequency com- 

 ponents are removed by a filter in the output circuit, leaving only the en- 

 velope wave. The filter must have approximately the same quahties as in 

 the previous case. 



Figure 21 shows an infinite impedance detector and its dual. This can be 

 thought of as a cathode follower with a large capacitor across the cathode 

 resistor. This capacitor charges through the comparatively low impedance 

 of the tube when the signal is positive and reaches approximately the peak 

 potential of the input wave. When the input voltage falls, the tube is cut 

 off and the condenser must discharge through a comparatively high im- 

 pedance. If the time constant of the discharge is properly chosen, the con- 



