Impedances Needed by High-Efficiency Power Amplifiers

The performance of any power amplifier depends upon its load impedance, and techniques such as Doherty and outphasing work best with specific load-impedance trajectories. This presentation addresses the requirements of power amplifiers and transmitters for high-efficiency operation. The transistor can be modeled as an ideal transistor with drain capacitance and series inductance due to bond wires or leads (Cripps' method). The requirements for ideal amplifiers operating in class B, F, and E are reviewed and then transformed into contours for real amplifiers. Next, the derivation of "continuous" modes for broadband operation is presented. The impedance trajectories needed by amplifiers used in Doherty and outphasing transmitters are shown. Finally, "Brute-force" techniques for synthesizing output networks using quarter-wavelength stubs and load-pull techniques are explained.