A waveform engineering approach for class F operation in a class C biased peaking branch of GaN MMIC Doherty power amplifiers

This paper presents a novel design strategy to enhance the performance of a Doherty Power Amplifier using a nonlinear driver stage in the Peaking branch. To implement a class F harmonic termination for the peaking final stage, a third harmonic voltage component is injected at the input by a driver stage, allowing the phase of the third harmonic current at the output of the final stage to be reversed compared to its normal evolution. This enables a class F design strategy for a class C biased device. The paper details the theoretical foundation of the proposed approach together with a thoroughly experimental verification of its applicability at both device and circuit level. The prototype for X-Band applications is implemented on the 120 nm gate-length GaN-on-SiC technology from WIN Semiconductors. The MMIC delivers over 36 dBm of output power and 40% efficiency at 10 GHz. Additionally, when tested with modulated signals having 10 MHz channel bandwidth and 6 dB Peak-to-Average Power Ratio, the linearity threshold of Adjacent Channel Power Ratio of -30 dBc is achieved with an average output power exceeding 32 dBm and efficiency greater than 32%.