Lumped-element output networks for high-efficiency power amplifiers

At UHF and higher frequencies, the output networks for class-E and -F power amplifiers are generally implemented by transmission-line networks. Typically, quarter-wavelength stubs for the second and third harmonics are placed at selected distances from the drain in order to create the required harmonic impedances inside the RF-power transistor. This paper presents an analogous technique using lumped-elements networks. This technique not only allows the networks to be adjusted for the frequency of operation, but also allows operation at lower frequencies (e.g., VHF and HF) where transmission-line networks are impractical. The new technique is verified by experiment with a second-harmonic class-E power amplifier. This amplifier operates at 900-MHz achieving 10-W output power and 73% drain efficiency with either type of network.