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THE BELL SYSTEM TECHNICAL JOURNAL, APRIL 1951 



The part of the circuit of Fig. 8 shown inside the dotted box amounts to 

 a vacuum tube plus two gyrators, which is just the dual of a vacuum tube. 

 Apart from a phase shift of 180° this is equivalent to a transistor. This part 

 of the circuit can therefore be replaced by a transistor plus a phase reversing 

 transformer to obtain the basic transistor-vacuum-tube circuit of Fig. 9. 

 This results in a considerable simplification because the impedance inverting 

 networks are no longer needed. 



The operation of the circuit of Fig. 9 is exactly similar to that of Fig. 8 

 except that the transistor operates as the dual of Ti . This means that the 

 transistor is given a large forward emitter bias so that collector voltage is 



Fig. 8 — The basic arrangement of a Doherty amplifier. 



almost cut off. Under these circumstances, it is capable of operation as a 

 linear amplifier. The load resistance is just half that into which the transistor 

 could deliver maximum power. The transistor acts alone to amplify down- 

 ward swings of modulation {T2 being biased well below cutoff as before) 

 but as the input signal exceeds that of the unmodulated carrier the collector 

 current swing begins to approach the maximum value permitted by the 

 (current) supply and T2 begins to contribute to the output in just 'the ways 

 it did in the circuit of Fig. 8. First, it acts as a class C amplifier delivering 

 power directly to the load and second, it behaves as a negative resistance 

 bridged across the load and thereby increases the impedance into which the 

 transistor works. This permits the transistor to deliver more power without 

 increasing the collector current swing. 

 Just as the basic Doherty circuit of Fig. 8 needs tank circuits to suppress 



I 



