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THE BELL SYSTEM TECHNICAL JOURNAL, APRIL 1951 



to increasing the power output. First, it delivers power directly to the load 

 and, secondly, it behaves as a negative resistance in series with the load, 

 thereby decreasing the impedance into which the vacuum tube works and 

 permitting it to deliver more power without increasing its plate voltage 

 swing. 



LOAD 



Fig. 11 — A Doherty-type amplifier in which low level signals are amplified by the vac 

 uum tube alone. 



LOAD 



Fig. 12 — A high efficiency untuned amplifier in which small signals are amplified by the 

 vacuum tubes alone. 



The Doherty amplifier is limited to narrow band operation only because 

 the networks Ni and A^2 will behave as gyrators only over a narrow range 

 of frequencies. Apart from a phase change of 180°, however, the transistor 

 behaves as a vacuum tube plus two ideal gyrators and is therefore capable of 

 acting as the dual of a vacuum tube over a wide band of frequencies. This 



