RF waveform investigation of VSWR sweeps on GaN HFETs

Solid state amplifiers are often fitted with an isolator component on the output to protect them from impedance mismatch. GaN based HFET's could offer the potential to remove the isolator due to their high breakdown voltages and high channel temperature operation. However the absence of an isolator would mean that the transistor would have to be able to withstand any load impedance that could be presented to it. The usual method to test for impedance mismatch is to select a fixed VSWR ratio and then sweep the load phase through 360°. In this paper a range of VSWR sweeps are investigated. The measurements are performed in a system that provides the RF voltage and current waveforms, as a consequence novel impedance contour plots can be generated. These plots can then aid in identifying potential failure mechanisms and load conditions to avoid.