A Voltage-Mode I/Q Switched-Capacitor Doherty Power Amplifier with Dynamic Core Scaling for Enhanced Backoff Efficiency

This work presents a new backoff efficiency enhancement technique for switched-capacitor power amplifiers (SCPA) that employs dynamic core size resizing enabled by a high-impedance (Hi-Z) control state. The SCPA unit cells’ size is the dynamically adjusted according to the instantaneous output power, which broadens the backoff efficiency peaking across a larger power backoff range and extend the saturated output power. The Hi-Z logic is designed so that disconnected capacitors do not remain floating. This is essential for ensuring correct capacitance ratios and impedance matching, and to prevent distortion and reliability concerns due to voltage stresses and kickback effects. This concept was implemented in a voltage-mode Doherty SCPA prototype fabricated in 65 nm CMOS. The chip delivers 32% peak system efficiency (SE) at 25 dBm output power around 2.5 GHz and a 3 dB operational bandwidth of 1.7-2.8 GHz. Compared to a Class-B PA, SE improves SE by 1.3× and 1.83× at 3 dB and 7 dB backoff, respectively. It also achieves an error vector magnitude (EVM) of -37 dB at 14 dBm average output power for a 20 MHz OFDM Wi-Fi signal using piecewise digital predistortion.