Behaviors of Class-F and Class-${\hbox{F}}^{-1}$ Amplifiers

Abstract

Operational behaviors of the class-F and class-F-1 amplifiers are investigated. For the half-sinusoidal voltage waveform of the class-F-1 amplifier, the amplifier should be operated in the highly saturated region, in which the phase relation between the fundamental and second harmonic currents are out-of-phase. The class-F amplifier can operate at the less saturated region to form a half sinewave current waveform. Therefore, the class-F-1 amplifier has a bifurcated current waveform from the hard saturated operation, but the class-F amplifier operates as a switch at the saturated region for a second harmonic tuned half-sine waveform. To get the hard saturated operation, the fundamental load is very large, more than √2 times larger than that of the tuned load amplifier. The operational behaviors of the amplifiers are explored with the nonlinear output capacitor. Since the capacitor generates a large second harmonic voltage with smaller higher order terms, the class- F-1 amplifier with the nonlinear capacitor can deliver the proper half-sinusoidal voltage waveforms at a lower power, but the effect of the nonlinear capacitor is small for the class-F amplifier. The class-F-1 amplifier delivers the superior performance at the highly saturated operation due to its larger fundamental current and voltage generation at the expense of the larger voltage swing. The simulation results lead to the conclusion that the class-F-1 amplifier with the nonlinear capacitor is suitable topology for high efficiency. However, in the strict sense, the class- F-1 amplifier with the nonlinear capacitor is not the classical class-F-1 amplifier because the voltage-shaping mechanisms and the fundamental load are quite different. We call it the saturated amplifier since the amplifier is the optimized structure of the power amplifier operation at the saturated mode.