Time-Domain Analysis of Optimum Bias Point in Inverse Class-F Power Amplifiers

The optimum bias point for an inverse class-F power amplifier is discussed in this paper from a time-domain point of view. It is shown that the inverse class-F power amplifier should be biased in shallow triode region where the bias current is slightly higher than DC current when driven into compression. Lower bias currents can cause significant efficiency degradation due to peaking in the drain-source voltage. Simulation results show that drain efficiency at 6dB over drive drops from 83.4% to 65.2% when bias current is reduced by 34% from optimum bias current. Moreover, it is shown that if the bias current is too high, then the efficiency under power back off is reduced.