Design of a 12 dB back-off asymmetric Doherty power amplifier using reactive output impedance

Abstract

This paper presents a method to extend the back-off range of an asymmetric Doherty power amplifier (DPA) to 12 dB using a reactive output impedance of the peaking amplifier. An analytical method is employed to determine the desired reactive output impedance for specific output power back-off (OPBO) range. Then, a peaking output matching network is designed to achieve the desired impedance to enlarge the efficiency of the carrier amplifier. When compared with conventional design, the OPBO range can be improved by about 2 dB using the proposed method. For verification, a 3.4-3.6 GHz asymmetric DPA with enhanced OPBO range was designed using 10 and 30 W GaN HEMT transistors. The measured efficiencies of 47%-49% at 12 dB back-off and 63%-66% at saturation are obtained over the whole frequency range. For a 40 MHz LTE signal at 3.5 GHz, the adjacent channel leakage ratio is -50 dBc after linearization with an average efficiency of higher than 50%.