Analysis of Switched Capacitor Losses in Polar and Quadrature Switched Capacitor PAs

This brief derives the efficiency limitations of switched capacitor power amplifiers (SCPAs) due to switched capacitor (SC) losses during charging and discharging of their capacitor arrays. Polar modulation is covered, as well as quadrature modulation both with clock duty cycles of 50% (Q50) and 25% (Q25). Closed form expressions are derived for both the maximum output power and SC power loss for signals with arbitrary phase and amplitude. These expressions are verified by simulations using ideal switches and capacitors, and 22 nm CMOS transistor models. These results are used to analyze the SCPA efficiency for 64QAM signals using a statistical model. It is shown that for a given array capacitance and supply voltage, the polar architecture is the most power efficient. Compared to polar modulation, the SC loss for 64QAM is fundamentally 18% higher for Q50 and 46% higher for Q25 SCPAs, whereas the generated output powers are fundamentally 6 and 3dB lower, respectively.