Comparing FET Technologies in Terms of Their Suitability for Broadband Envelope Tracking Power Amplifiers

Envelope tracking power amplifiers (ET PAs) are one of the main contenders for future communication systems. Their operation over a wide range of supply voltages ($V_{\text{D}})$ leads to an extended set of requirements, this paper investigates a number of technologies, including Si LDMOS, GaAs pHEMT and GaN HEMT and how they behave in broadband ET PAs. The increased impact of the feedback capacitance $C_{\text{GD}}$ on broadband PA design in HEMT devices caused by a reduced $V_{\text{D}}$ is described for the first time. Analysis of the $V_{\text{D}}$ dependent characteristics show that GaN HEMTs can exhibit the most promising behaviour for broadband ET PAs due to their favourable output voltage dependent gain variationand their low capacitance to power ratio, so long as field plate designs are used to guarantee a favourable knee walkout.