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Presented is the linearity improvement technique of a power amplifier (PA) using a digitally-controlled tunable output matching network implemented in GaAs BiFET technology. The load impedance of the heterojunction bipolar transistor (HBT) power device in the last stage of the PA is adjustable in terms of output power levels by a metal-insulator-metal (MIM) capacitor array with pHEMT switches in the output matching network. A 2.4 GHz two-stage PA for IEEE 802.11g, Wireless Local Area Network (WLAN) application, is implemented to demonstrate the technique. Not only is the maximum linear output power of the PA increased by 2.5 dB, but the linearity is also improved by 4 dB at the output power of 15 dBm for the error vector magnitude specification of 28 dB.