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This work presents a modified LC matching transformer for a fully integrated CMOS power amplifier. The proposed structure transforms the impedance of a differential power cell pair into an unbalanced system impedance, and vice versa, with low insertion loss and a highly flexible impedance ratio. A simplified design procedure of the transformer that considers the output capacitance of power transistor is also presented. A class AB power amplifier in a standard 0.18-μm CMOS process is fabricated. Consequently, saturated output power of 27 dBm and linear power of 18.5 dBm with a power added efficiency of 12.5% are achieved.