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A semi-lumped output transformer for fully-integrated RF CMOS power amplifier is proposed in this paper. We analyze and design the transformer with the even mode and odd mode method. To demonstrate this transformer, a 2.5 GHz CMOS power amplifier is implemented with 0.18 mum and 2.5/3.5 GHz dual band 0.13 mum RF CMOS process used for WiMax application are fabricated. The power amplifier can achieve 39% PAE (power added efficiency) at P1dB (1 dB compression point) output power of 30 dBm. The linearity can satisfy the spectrum mask of WiMax signal requirement basically for 2.5 GHz CMOS power amplifier. And the gain and PAE for dual band power amplifier can also achieve 26.5 dB and 24.8 dB and 24.5% and 27.5%, respectively.