Impact of the Input Impedance on the Linearity of Supply Modulated GaN HEMT Power Amplifiers

Abstract

Envelope tracking power amplifiers (ET PAs) are one of the main contenders for high-efficiency PAs in future wireless communication systems. The importance of the input matching network (IMN) in GaN ET PAs is an under-explored topic. This paper explores the impact of the impedance presented to the gate of a transistor by the IMN on linearity, gain and efficiency. The impact is investigated by varying resistance and reactance of the source impedance with a fixed load over a wide range of supply voltages. Based on the results of this investigation, two IMNs are designed, one of which is optimised for maximum gain and the other for a good compromise between gain and linearity. Simulations show that the trade-off IMN achieves comparable power added efficiency (PAE) while the AM/PM distortion was reduced by more than 40%. The reduction in Q-factor of the input impedance is another advantage, possibly facilitating broadband matching. To verify these assumptions, a 2.9 GHz 10 W GaN RFPA was manufactured and measured. While measurements and simulations diverged to the degree expected from literature, they demonstrate that an input matching network can be used to optimise PAs for linearity in ET systems. A targeted IMN design can reduce phase and gain distortion effectively with little impact on efficiency, and additionally offers the possibility to further trade-off linearity and PAE.