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This paper presents a fully integrated transformer-based Doherty power amplifier in a standard 90 nm CMOS process. A novel asymmetrical series combining transformer is used to achieve uneven Doherty operation. The transformer-based uneven Doherty architecture is analyzed to further improve the back-off efficiency without linearity degradation. The fabricated two-stage uneven Doherty PA achieves a maximum output power of 26.3 dBm at 2.4 GHz with a peak power added efficiency (PAE) of 33% at 2 V supply voltage. The PAE at 6 dB back-off is still as high as 25.1%. The PA is tested with 54 Mbps WLAN 802.11g signal and it meets the stringent EVM and spectral mask requirements at 19.3 dBm average output power with a PAE of 22.9% with no need of predistortion. An open loop digital predistortion is applied to further improve the linearity. The PA satisfies WLAN requirements at 20.2 dBm average output power with a PAE of 24.7% with predistortion.