Deliberate Source/Load Mismatch for Linearity and Efficiency of Discrete Supply Modulated GaN PAs

The effect of source (gate-side) and load (drain-side) impedance match on the linearity and efficiency of an envelope-tracked power amplifier (PA) is investigated. Conventionally designed GaN PAs exhibit an increasing gain magnitude and phase with increasing drain supply voltage. When this supply voltage is dynamically modulated in discrete levels for efficiency enhancement, the changing gain distorts the output signal and degrades linearity. The dependency of the PA gain on drain supply voltage can be minimized, however, through proper selection of the source and load impedances. This is illustrated through source/load pull simulations of a
$8\times 100$

$\mu$
m GaN HEMT, which reveal impedances that result in low gain magnitude and phase variation. Three PA design (matching) cases are then chosen and their performances are evaluated with an ideal dynamically changing supply. For the comparison, envelope simulations with a 67.5-MHz 64-QAM signal are performed and show that dynamic supply modulation of the mismatched PA design improves linearity and efficiency over a conventional PA designed for maximum efficiency/gain. The simulation results are validated through measurements of a deliberately mismatched, 6.2–12.6-GHz, 5-W GaN MMIC PA envelope-tracked with a GaN MMIC four-level supply modulator. Compared with a static supply, the dynamic results show improvement in average efficiency and a small penalty in linearity without using DPD.