Investigation of a class-F−1 power amplifier with a nonlinear output capacitor

The practical operating conditions of class-F^-1 amplifier are investigated by analyzing the current and voltage waveform shapings. The bifurcated current of the amplifier is generated by the saturated operation at the knee region, resulting in quasi-rectangular current. The voltage shaping is studied for the active device with linear and nonlinear output capacitors (C_outs). In the linear C_out case, the half-sinusoidal voltage can be formed only at deeply saturated region, in which the proper second harmonic current is generated. The second harmonic voltage can be built using a large second harmonic load. When the nonlinear C_out is included, the harmonic load-pull simulation results show that the half-sinusoidal voltage is formed by the second harmonic generation of C_out rather than by the second harmonic current. This waveform shaping process is quite different from that of the conventional class-F^-1 PA, and we call the amplifier with the nonlinear C_out as the saturated amplifier. This saturated amplifier delivers better power performance than that of the amplifier with linear C_out since the former one can better shape the current and voltage waveforms. In the experiment, the saturated amplifier is designed and implemented using Cree GaN HEMT CGH40010 at 2.655 GHz. It provides a power-added efficiency of 73.9% at saturated output power of 41 dBm.