The appearance of third-generation semiconductors represented by gallium nitride(GaN) material greatly improves the output power of a power amplifier(PA), but the efficiency of the PA needs to be further improved. The Class-F PA reduces the overlap of drain voltage and current by tuning harmonic impedance so that high efficiency is achieved. This paper begins with the principle of class-F PA, regards the third harmonic voltage as an independent variable, analyzes the influence of the third harmonic on fundamental, and points out how drain efficiency and output power vary with the third harmonic voltage with an Ⅰ–Ⅴ knee effect. Finally, the best third harmonic impedance is found mathematically. We compare our results with the Loadpull technique in advanced design system environment and conclude that an optimized third harmonic impedance is open in an ideal case, while it is not at an open point with the Ⅰ–Ⅴ knee effect, and the drain efficiency with optimized third harmonic impedance is 4% higher than that with the third harmonic open.
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