DUALITY AS GUIDE IN TRANSISTOR CIRCUIT DESIGN 415 



transistor, connected to a source of constant voltage approximated here by 

 a constant current supply in parallel with a large capacitor. The voltage 

 available for the operation of the class C amplifier is the difference between 

 the supply voltage and the collector voltage of the modulator transistor. 

 Inasmuch as the output of the class C amplifier is proportional to its supply 

 voltage, it is clear that the output will vary directly with the emitter current 

 of the modulator transistor. 



Both the vacuum tube and transistor circuit of Fig. 23 suffer because the 

 modulator element can never take all of the available power supply voltage 

 or current, and hence 100% modulation cannot be attained. The circuits of 

 Fig. 24 correct this defect with a transformer, which amphfies slightly the 

 variations in current or voltage in the modulator element. In both circuits 

 W2 > «i , and the total supply current or voltage is no longer constant, but 

 it is nearly so. 



Figure 25 shows a grid modulator and its dual. Here the non-linearity of 

 the transfer characteristics in the neighborhood of the cutoff point is made 

 use of directly to produce modulation products. The tube is biased approxi- 

 mately to plate current cutoff, and the transistor approximately to collector 

 voltage cutoff. The desired modulation products are selected by a tuned 

 circuit. 



Figure 26 shows a cathode modulator and its dual. These circuits combine 

 some of the features of the grid modulator and of the plate modulator. Un- 

 fortunately the phase relationships are such in the transistor circuit that 

 a transformer is required, and this transformer must be able to pass modula- 

 tion frequencies as well as carrier frequencies. 



The circuits of Fig. 25 and Fig. 26 can be operated as large-signal devices, 

 using the gross non-linearities of the circuits to produce modulation products, 

 or as small-signal devices, when they operate as 'square law' devices. They 

 can, moreover, be combined to form various push-pull or balanced modula- 

 tors. An example of such a circuit is shown in Fig. 27(a). This circuit has 

 two inputs and two outputs. If it is operated as a square-law device the rela- 

 tions between the input and output frequencies will be as follows: 



The same relations hold for the dual circuit, Fig. 27(b). The action of the 

 dual circuit is analogous to that of the vacuum tube circuit. It is a two- 

 transistor circuit operated at the same time as a push-pull circuit and as two 

 transistors in series, in phase. At various points in the circuit certain com- 



