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A single-chip implementation of a variable supply voltage power amplifier (PA) is presented. The circuit is design in SiGe BiCMOS technology and is composed of a differential power amplifier designed for WCDMA operation and a "buck" DC-DC converter. The combined system varies the supply voltage of the PA from 2.7 V to 1.2 V and maintains system linearity while the output power is not at its maximum. The system achieves an average battery current reduction of 30% and an average efficiency improvement of 46% compared to a fixed supply voltage approach, while occupying a 2 mm × 2.5 mm die area.