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We present a 2.4-GHz asymmetric multilevel outphasing (AMO) power amplifier (PA) with class-E branch amplifiers and discrete supply modulators integrated in a 65-nm CMOS process. AMO PAs achieve improved modulation bandwidth and efficiency over envelope tracking (ET) PAs by replacing the continuous supply modulator with a discrete supply modulator implemented with a fast digital switching network. Outphasing modulation is used to provide the required fine output envelope control. The AMO PA delivers 27.7-dBm peak output power with 45% system efficiency at 2.4 GHz. For a 20-MHz WLAN OFDM signal with 7.5-dB PAPR, the AMO PA achieves a drain efficiency of 31.9% and a system efficiency of 27.6% with an EVM of 2.7% rms.