A new harmonic termination that controls the waveform of the drain current to be rectangular is developed for high-efficiency power amplifier modules. Its harmonic termination is a short circuit at the third harmonic and a non-short circuit at the second harmonic. It is found experimentally and confirmed by simulation that the load-matching condition at the third-order harmonic improves the efficiency of a transistor by more than 13%. By using this tuning, 57.7% power-added efficiency of the module is achieved at the output power of 29.9 dBm with ACP of -50 dBc, NACP of -65 dBc at 925 MHz and Vdd of 3.5 V.

Microwave power transistors for high efficiency applications are surveyed briefly. Methodologies for microwave transistor power amplifier circuit design are discussed. Microwave transistor power amplifiers are categorized according to their operation into classes A, AB, B, C, and F, and some preliminary results on output power, power efficiency, and power gain for the amplifiers in various classes are provided by an analysis using an ideal transistor model. Circuit conditions controlling the voltage and current waveforms and device parameters such as the knee voltage in the device current-voltage characteristics are discussed for viewpoint of realizing high-efficiency power amplifier operation. A practical power amplifier design is considered with respect to the device characteristics and circuit conditions.