Load Modulation Measurements of X-Band Outphasing Power Amplifiers
An in-depth investigation of both isolated and non-isolated outphasing power amplifiers, with and without supply modulation is presented. X-band GaN MMIC power amplifiers with 70% power-added efficiency and 2.7 W output power at 10.1 GHz are configured in hybrid outphasing circuits with several combiners that include bi-directional couplers, enabling calibrated measurements of internal load modulation. It is experimentally demonstrated that the load modulation critically depends on the power balance of the two internal MMIC PAs. Despite the additional loss in the combiner, peak total efficiencies greater than 47% are achieved by full outphasing PAs with more than 3.7 W of output power. A comparison between several outphasing configurations quantifies the improvement in efficiency for both isolated and non-isolated outphasing PAs with supply modulation. This knowledge is then used to design a GaN MMIC outphasing PA at X-band (9.7 GHz) with an overall efficiency of 60% at 5W output power. Further, the MMIC PA is integrated with a discrete GaN MMIC supply modulator and shows close to 50% 6-dB backoff efficiency with good linearity for both radar and communications signals.