Evaluation of a GaN HEMT transistor for load- and supply-modulation applications using intrinsic waveform measurements

In this paper, the efficiency of a GaN HEMT transistor and its intrinsic waveforms are measured at 0.9 GHz and investigated for load- and supply-modulation applications. The results show that both techniques perform equally well for back-off levels 6.5 dB. At higher back-off levels, the efficiency improvements achieved by supply modulation outperform load modulation. At 10 dB back-off, supply, and load modulation provide a power-added efficiency (PAE) of 68%, and 58%, respectively. Using measured intrinsic waveforms, it is shown that PAE degradations in load modulation can be mainly attributed to parallel losses rather than series losses, which are dominant in supply modulation. The harmonic contents of the intrinsic waveforms, in both techniques, are equally strong in back-off and peak power operations. There is, therefore, a great potential for further efficiency enhancement by circuit-level optimization of harmonic terminations for back-off.