Highly efficient operation of the power amplifier can also be obtained by applying biharmonic or polyharmonic modes when an additional single-resonant or multi-resonant circuit tuned to the even harmonics of the fundamental frequency is added into the load network. An infinite number of even-harmonic resonators results in an idealized inverse Class-F mode with a half-sinusoidal voltage waveform and a square current waveform at the device output terminal. In inverse Class-F power amplifiers analyzed in frequency domain, the fundamental and harmonic load impedances are optimized by short-circuit termination and open-circuit peaking to control the voltage and current waveforms at the device output to obtain maximum efficiency. In this chapter, different inverse Class-F techniques using lumped and transmission-line elements including a quarter-wave transmission line are analyzed. Design examples and practical RF and microwave inverse Class-F power amplifiers are described and discussed.

4.1 Biharmonic Operation Mode

Biharmonic operation mode can be realized using a second harmonic peaking when an additional parallel resonant circuit tuned to the second harmonic of the fundamental frequency is included in series into the load network. Similarly to the load network with the third harmonic peaking, the additional resonator creates a high impedance at the second harmonic resulting in an efficiency improvement. In this case, by limiting to a biharmonic operation condition, the partial Fourier series of current i(t) and voltage v(t) in normalized form can be respectively written as

where Vo and /o are the dc voltage and current components, respectively. Note that an infinite...