Efficiency Enhancement of Doherty Amplifier Through Mitigation of the Knee Voltage Effect

Abstract

This paper presents an approach to maximize the efficiency of a Doherty power amplifier (PA) with the knee voltage effect. Since the carrier PA with 2R(opt), which is the usual matching impedance for a carrier PA at a low power region, does not reach to the saturated operation at the 6-dB back-off power level, the maximum efficiency could not be achieved. However, the carrier amplifier can be driven into the saturation using the load impedance larger than 2R(opt) and can deliver the maximum efficiency even under the knee voltage effect. The optimized design for the maximum efficiency at the back-off level is derived. The optimized amplifier is analyzed and simulated in terms of its load modulation behavior, efficiency, and output power, then compared with the conventional Doherty PA. The enhanced performance is demonstrated by the Doherty PA built using CREE's GaN HEMT CGH40045 devices at 2.655 GHz. For worldwide interoperability for microwave access applications with a 7.8-dB peak-to-average power ratio, the proposed PA delivers an efficiency of 49.3% at an output power of 42 dBm with an acceptable linearity of -23.1 dBc. The linearity is improved to -43 dBc by employing a digital feedback predistortion technique, satisfying the system linearity specification.