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This paper presents a high-efficiency broadband envelope-tracking power amplifier with operation at carrier frequencies from 500 to 1750 MHz. The RF power amplifier (RFPA) is provided by a single-chip gallium-nitride (GaN) integrated circuit (IC) whose design included a broadband output match achieved by operating directly into a load resistance of 50 Ω and broadband input match achieved using a feedback network. Under single-tone excitation, the Class-AB GaN PA IC provides output power above 12 W with greater than 50% drain efficiency and more than 10-dB gain at 40-V drain bias. When placed in envelope tracking configuration, using a representative WCDMA modulated signal with 4-MHz bandwidth and 6.6-dB peak-to-average power ratio, the ET PA achieved 4 W of average output power at its peak average drain efficiency of 31% at 752 MHz (including the power dissipation of the envelope modulator). The RFPA individually was measured to have an average drain efficiency of 58.5% for the WCDMA signal. Across the 500-1750-MHz band, using the WCDMA signal, greater than 25% average drain efficiency with more than 10 dB of gain was measured.