DUALITY AS GUIDE IN TRANSISTOR CIRCUIT DESIGN 



413 



circuits and their duals. These have as their object the production of a carrier 

 frequency wave with a given envelope. 



Figure 22 shows a plate modulator and its dual. This circuit makes use 

 of the fact that the output of a class C amplifier is proportional approximately 

 to the plate supply voltage. In 22(a) the vacuum tube acts as a class C ampU- 

 fier amplifying the carrier frequency wave, and the supply voltage is varied 



Fig. 24 — Modified constant current modulator and dual. 



U 



K 



(b) 



Fig. 25 — Grid modulator and dual. 



by adding to it the modulating voltage. The peak output voltage thus varies 

 with the modulating voltage. The dual transistor circuit, 22(b), is also a 

 class C amplifier, with its collector supply current varied by the addition of 

 a modulating current. The output is proportional approximately to the total 

 supply current, and hence varies with the modulating current. 



Figure 23(a) shows a particular embodiment of the plate modulator which 

 is called a constant current modulator, because the total supply current to 

 the two tubes in the circuit is (approximately) constant. It is easier to ex- 



