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This paper presents the design of a high efficiency, low THD, 5.7 GHz fully differential power amplifier for wireless communications in a standard 0.18 μm CMOS technology. The power amplifier employs a fully differential class-E topology to achieve high power efficiency by exploiting its soft-switching property. In order to achieve high operating frequency, an injection-locked oscillator is utilized, which makes the output voltage of the power amplifier tuned at the input signal frequency. A complementary CMOS cross-coupled pair topology is employed to realize the LC-tank oscillator because it has lower phase-noise, thereby giving lower THD than the single NMOS cross-coupled pair topology. The proposed power amplifier can deliver 25 dBm output power to a 50 Ω load at 5.7 GHz with 42.6% power-added efficiency (PAE) from a 1.8 V supply voltage.