The effect of baseband impedance termination on the linearity of GaN HEMTs

This paper demonstrates the significant effect of baseband impedance termination on the linearity performance of a 10W GaN HEMT device driven to deliver a peak envelope power of approximately 40dBm. The paper also proposes a further refinement to a state-of-art active IF load-pull measurement system to allow the precise independent control of all significant baseband components generated as a result of the multi-tone excitation used. The presentation of specific baseband impedances has delivered a 20dBc and 17dBc improvement in IM3 and IM5 inter-modulation products respectively, relative to the case of a classical, ideal short circuit. As expected for this device, this was achieved by emulating appropriate negative impedances lying outside of the Smith chart, and when this observation is considered alongside the Envelope Tracking PA architecture, this raises the interesting possibility of significantly improving PA linearity using the very mechanisms that are employed to improve PA efficiency.