PA Efficiency and Linearity Enhancement Using External Harmonic Injection

This paper presents analysis and experimental demonstration of a high-efficiency linear power amplifier (PA) with second-harmonic injection at the output. In this circuit, the transistor is not driven hard into compression and does not produce significant harmonic content. External injection at the output enables voltage and current wave-shaping to achieve high efficiency. Theoretical analysis of the waveforms shows that the maximal drain efficiency is 89.9% with, at most, 0.13 dB of reduction in output power compared with the class-A case. The overall PA efficiency is derived in terms of the injector circuit efficiency. A harmonically injected prototype GaN HEMT 2.45-GHz PA demonstrates over 80% efficiency with linearity improved over the class-AB PA without harmonic injection. Two-tone measurements show a reduction of the third-order intermodulation by$-$30 dBc in the linear region and greater than$-$10 dBc in saturation.