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This paper presents a Class-D outphasing RF Power Amplifier (PA) which can operate at a 5.5V supply and deliver +29.7dBm with 26.6% PAE at 1.95 GHz in a standard 65nm CMOS technology. The PA utilizes two on-chip transformers to combine the outputs of four Class-D stages. The Class-D stages utilize a cascode configuration, driven by an AC-coupled low-voltage driver, to allow a 5.5V supply without excessive device voltage stress. The measured 3dB bandwidth was 1.6 GHz (1.2-2.8 GHz). The PA was continuously operated for 168 hours (1 week) without any performance degradation. To evaluate the linearity of the outphasing PA, a WCDMA and an LTE signal (20 MHz, 16-QAM) were used. At +26.0 dBm channel power for the WCDMA signal, the measured ACLR at 5MHz and 10MHz offset were -35.6 dBc and -48.4 dBc, respectively. At +22.9 dBm channel power for the LTE signal, the measured ACLR at 20MHz offset was -35.9 dBc.