A New High-Efficiency Power Amplifier 

 for Modulated Waves * 



By W. H. DOHERTY 



THE use of increasingly higher power levels in broadcasting in the 

 last few years has attached new importance to the matter of more 

 efficient operation of the high-power stages in radio transmitters. 

 The resulting reductions in cost of power, size of high-voltage trans- 

 formers and rectifier, and water cooling requirements, are of particular 

 importance in transmitters having outputs of 50 kilowatts or more. 



The linear radio-frequency power amplifier, in the form in which it 

 has been used extensively in broadcast transmission, may not be 

 operated at a plate efficiency higher than about 33 per cent, if it is to 

 supply the peak power required for amplifying a completely modulated 

 wave. With this efficiency the d-c power input to a 50-kilowatt 

 amplifier, for example, is 150 kilowatts, of which 100 kilowatts must 

 be dissipated at the anodes of the water-cooled tubes. 



This inherent weakness of the conventional linear amplifier has 

 occasioned the development of certain other systems of amplification 

 or modulation which permit a more economical use of power. Of 

 these the high-level Class B modulation system ^ and the ingenious 

 "outphasing modulation" scheme of Chireix ^ are most worthy of 

 note. 



A new form of linear power amplifier has been developed which 

 removes the limitation of low efficiency inherent in the conventional 

 circuit, permitting efficiencies of 60 to 65 per cent to be realized, while 

 retaining the principal advantages associated with low-level modula- 

 tion systems and linear amplifiers, namely, the absence of any high- 

 power audio equipment, the ease of adding linear amplifiers to existing 

 equipment to increase its power output, and the adaptability of such 

 systems to types of transmission other than the carrier-and-double- 

 side-band transmission most common at present. 



Linear radio-frequency power amplifiers are ordinarily biased 



* Digest of a paper presented before the Annual Convention of the Institute of 

 Radio Engineers, May 11-13, 1936, at Cleveland, Ohio, and to be published later in 

 the Proceedings. 



' Chambers, Jones, Fyler, Williamson, Leach, and Hutcheson, "The WLW 500- 

 Kilowatt Broadcast Transmitter," Proc. I. R. E., Vol. 22, p. 1151, October, 1934. 



2 Chireix, "High Power Outphasing Modulation," Proc. I. R. £., Vol. 23, p. 1370, 

 November, 1935. 



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