High-efficiency Doherty power amplifiers

This chapter discusses the power amplifier technique for amplitude-modulated (AM) radio-frequency signals was invented by William H. Doherty in broadcasting in the mid-1930s as a more efficient alternative to both conventional amplitude-modulation techniques and Chireix outphasing [4]. The amplifier was configured in a grounded-cathode structure where two vacuum tubes were connected in parallel, one as a Class B carrier tube and the other as a Class C peaking tube, and the tubes were split and combined through þ90° and -90° phase shifting networks. This novel power amplifier architecture removed limitation of low efficiency inherent in a conventional power amplifier circuit, permitting efficiencies of 60% to 65% to be realized, while retaining the principal advantages associated with low-level modulations systems and linear power amplifiers.