A History of Switching-Mode Class-E Techniques: The Development of Switching-Mode Class-E Techniques for High-Efficiency Power Amplification

Improving the efficiency of vacuum-tube amplifiers by properly approximating the anode voltage and current waveforms was discussed and first demonstrated in the late 1910s. Researchers found that, if either the tube plate current or voltage is zero when the other is nonzero, then the loss must be zero and the efficiency equal to 100%. The required rectangular waveforms could be achieved by inserting harmonic resonators in the anode circuit and tuning them to the odd and/or even harmonics of the fundamental frequency (now called class-F). Three decades later, however, Model et al. demonstrated that this is not the only way to improve power amplifier efficiency: very high anode efficiency can be achieved without special tuning of the anode circuit at the particular harmonic, provided that the asymmetrical voltage and current waveforms are due to reactive impedances at the harmonics [1].