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A tunable power amplifier (PA) from 0.9 GHz to 3.0 GHz is presented. This paper proposes an output impedance tuning method by using resistive feedback and a parallel resonance consisting of an inductor and a tunable capacitor array. The proposed multi-band PA can adjust the output impedance to 50 Â¿ over a wide frequency range, so external isolators following PAs can be eliminated. The PA is implemented by using a 0.18 Â¿m CMOS process, and the supply voltage is 3.3 V. Over all of the frequency range, the PA realizes output return loss S22 of smaller than -10 dB, power gain of larger than 16 dB, output 1-dB compression point of larger than 17 dBm, and power added efficiency (PAE) at 1-dB compression point of larger than 10%.