Adjustable load-modulation asymmetric Doherty amplifier design using nonlinear embedding

Abstract

We developed a new asymmetric Doherty load modulation matching network using identical transistors for the main and auxiliary amplifiers. Asymmetric Doherty power amplifiers (PA) require a larger size transistor for the auxiliary PA than for the main PA to provide the higher power and wider load modulation range. Additional impedance transformers are introduced to alleviate this requirement when using identical devices. The drain bias voltage of the main amplifier is also reduced to achieve a wider back-off. Furthermore, a large-signal model-based nonlinear embedding method is applied to predict the input and output harmonic terminations, removing the need for the multi-harmonic source/load pull characterization. An asymmetric Doherty amplifier was built using two 15 W peak power packaged GaN transistors of the same size. 71 % drain efficiency at the peak power of 41.8 dBm and 62.7 % at the second peak of 32.8 dBm (9 dB back-off) were observed. Above 50% drain efficiency was maintained over an 11 dB power range. 51.86 % average drain efficiency was observed after linearization maintaining -51.46 dBc adjacent channel power ratio excited by 10 MHz bandwidth long term evolution signals with 9.96 dB peak to average power ratio.