A RF power amplifier was designed for WLAN 802.11 b/g 2.4 GHz based on 0.18 μm SiGe BiCMOS technology. The PA consists of three stage amplifiers working in Class AB. Adaptive bias circuit based on current mirror was used, with temperature compensation and linearization. The post-layout simulation results showed that the designed power amplifier had an output of 1 dB, a compression point of 27.73 dBm, a power gain of 25.67 dB, an S22 of less than -10 dB and an S12 of less than -60 dB in 2.4 GHz.