Signal distortion in polar architecture transmitters using class E RF power amplifiers

The trend towards increasing the energy efficiency of transmitters used in radio and telecommunications systems focuses the attention of equipment developers on finding optimal solutions when transitioning from linear power amplifiers (PAs) to amplifiers operating in switching mode. It is shown in the paper that signal distortion level along with high energy efficiency must be considered as an important parameter of PA. Reasons of signal distortions in class E switched-mode PA used in polar architecture transmitters are given. It is shown that most sufficient distortion is caused by phase shift of the load current, which results from the nonlinear change of transistors' output capacitance when amplifying signals with a non-constant envelope. The results of calculations of the error vector magnitude when using multilevel spectrally efficient modulation types, in particular quadrature amplitude modulation (QAM) and amplitude-phase shift keying (APSK), are presented. The results confirmed that for class E PA the most sufficient type of distortion is phase distortion. The results of analytical model are confirmed by simulation. It is shown that for 16-QAM at bit error rate = 10^-4, the energy loss is 0.5 dB, while for 16-APSK its value increases to 2.2 dB. The results presented in the paper can be used in the development of signal predistortion methods for switching class E PAs, ensuring a reduction in signal distortions.