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A quasi-four-pair structure of an RF CMOS power amplifier (PA) is proposed. The structure is applied to a 1.8-GHz class-E CMOS PA for a global system for mobile communications with a 0.18-mum RF CMOS process. This allows a simple design, as well as a high output power. The transistor size issues of the cascode PA are also studied. An integrated passive device transformer is used as both a power combiner and a matching circuit of the power stage. The results show an output power of 33.4-33.8 dBm and a power-added efficiency of 47.4%-50% with a supply voltage of 3.3 V at a frequency range of 1.71-1.91 GHz.