DUALITY AS GUIDE IN TRANSISTOR CIRCUIT DESIGN 



403 



leads to the possibility of an entirely new, wide band, high efficiency ampli- 

 fier which operates on the same principles as the Doherty circuit. 



Both the transistor part and the vacuum tube part of the amplifier must 

 be made push-pull in order that both halves of the input wave be amplified 

 equally. In the circuit of Fig. 12 the vacuum tubes are biased for class B 

 operation, while the transistors are given a large forward emitter current 

 bias so that they are operated well below collector voltage cutoff. For small 

 input signals the transistors are inactive, serving simply as short circuit 

 elements in series with the load. As the input signal reaches half the peak 



LOAD 



Fig. 13 — A high efficiency untuned amplifier in which small signals are amplified by the 

 transistors alone. 



permissible value the vacuum tubes begin to distort because their voltage 

 swings approach the supply voltage. At this point the transistors begin to 

 operate in two separate ways, just as in the Doherty amplifier. First, they 

 work as class C amplifiers delivering power directly to the load and second 

 they behave as a negative resistance in series with the load thereby serving 

 to reduce the impedance into which the vacuum tubes work. This permits 

 the vacuum tubes to deliver more power without increasing their plate 

 voltage swing. 



Just as there are two forms of the Doherty amplifier, there are also two 

 forms of this wide band arrangement. In the second form shown in Fig. 13 

 the transistors are biased for class B operation (near collector voltage cutoff) 

 while the vacuum tubes are biased well below cutoff. For small signals the 

 transistors act alone as class B amphfiers and the vacuum tubes act simply 



