Basic limitations on waveforms achievable in single-ended switching-mode tuned (Class E) power amplifiers

The class-E switching-mode tuned power RF amplifier achieves high efficiency (ideally 100%) even if the switching times of the active device are appreciable fractions of the period of the output waveform, by shaping the switch voltage and current waveforms so that the voltage and current transitions are displaced in time from each other. As a second-order factor, nonideal waveform shaping results in power dissipation. That dissipation could be minimized if the nonideal voltage and current waveforms could be made to have zero values at the times the switch is turned on and off. This paper shows that no configuration of linear circuit elements can provide zero voltage and zero current at both transitions of the switch, if nonzero output power is to be delivered to a load. Therefore, a jump of current and/or voltage must be tolerated at the switch turn-off and/or turn-on; the minimum jump magnitude is derived. That jump imposes a lower bound on the power dissipation, determined by the degree of nonideality of the waveform shaping. Also derived is a new general equation for the output power of a single-ended switching-mode tuned power amplifier.