Peak-Current-Ratio-Enhanced Compact Symmetrical Doherty Amplifier Design by Using Active Harmonic Control

Abstract

This article proposes a new and compact symmetrical Doherty power amplifier (DPA) configuration that provides extended output-back-off (OBO) and enhanced load modulation. The concept of active harmonic control (AHC) is introduced to remedy the reduced-conduction-angle operation of the class-C auxiliary device, providing enhanced auxiliary current and power capability. Semianalytical study based on generalized load-modulation theory and large-signal simulation reveal that an optimal AHC can offer increased peak current ratio, improved load modulation, boosted output power, and power utilization factor (PUF). Besides, a new output combiner, with the inclusion of parasitic effect, is devised for ultracompactness and simplicity. For verification, a 11-dB-OBO 2-GHz symmetrical DPA based on GaN HEMT transistors is designed, fabricated, and measured. Excellent performance with drain efficiency of 74% (52%) is obtained at saturation (back-off of 11 dB). Moreover, a saturation power of 42.98 dBm is achieved, corresponding to an uplifted PUF of 1.25 (compared with conventional symmetrical DPA design). Under 20-MHz modulated signal (peak-to-average power ratio of 11 dB) excitation, the prototype exhibits average efficiency of 53.2% at an average output power of 32 dBm. The adjacent channel leakage power ratio (ACLR) is found to be −27.5 and −50.9 dBc before and after digital predistortion, respectively.