The low cost power amplifiers (PAs) with power back-off (PBO) efficiency enhancement are dramatically demanded in modern wireless communication. The CMOS digital PAs (DPAs) and digital transmitters (TXs) [1]–[7] are developed due to the merits of high efficiency, high integration, and low cost. To achieve higher date rate, the quadrature digital TXs are preferred, which do not need the CORDIC and phase modulator in polar TXs. To improve deep PBO efficiency of DPA, the Class-G Doherty [3], hybrid Doherty with impedance boosting [4], load modulation [5], switched transformer [6], switched/floated capacitor power amplifier (SFCPA) [7], etc. are developed. To further enhance ultra-deep PBO efficiency, a watt-level triple-mode quadrature SFCPA with 56 efficiency peaks using IQ intrinsic impedance interaction and adaptive phase compensation is proposed, which achieves efficiency enhancement at 0/2/3/6/8/9/12/14/15/18/21dB PBOs.