Abstract: Efficiency in microwave power amplifiers (PAs) is limited by the active device parameters and operating conditions. High values can be obtained by a proper selection of bias point and harmonic terminations or, from a different point of view, by a proper output voltage and/or current waveform shaping. This work outlines the mathematical statements for the proper design of a second harmonic tuning (HT) PA. To validate the theory, the design of a highly efficient 3.5 (WiMAX applications) GHz GaN HEMT PA based on second HT is presented. The measured performances confirm the improvement obtained by means of harmonic manipulation over Class AB tuned load or Class J PAs. An output power of 35.3 dBm has been measured for a maximum of power-added efficiency of around 58% (drain efficiency 69.2%) with a IMD3 of 19.1 dBc.

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