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This paper proposes a new design technique for a reconfigurable CMOS Class E power amplifier (PA) by selectively activating the transistor cells in the array of power transistors. The activated transistors will be used for amplification and the non-activated transistors will be used for input/output matching and switching capacitance for Class E operation. The quad-band PA for 1.9/2.3/2.6/3.5 GHz has been implemented in a 0.18 μm CMOS technology using the technique without any switch or tunable circuit. This PA obtains output powers of 24.2/23.8/23.4/ 20.5 dBm and efficiency of 48.2/44.3/40.9/35.6% at 1.9/2.3/2.6/ 3.5 GHz. The total MMIC chip size is as small as 0.92 × 1.75 mm2 for the reconfigurable quad-band operation.