Loop Analysis and Stability Considerations of Hybrid PA Supply Modulators

This brief provides a detailed loop gain and stability analysis of hybrid power amplifier supply modulators. Design considerations, including the relations between the voltage and current loops, the rationale of current loop compensation to ensure stability and improve bandwidth, and the impacts of current loop on the overall system stability and efficiency, are studied with combined system loop gain modeled mathematically. The theoretical analysis is proven by simulation results. Based on the study, a hybrid supply modulator has been designed and fabricated in 65-nm CMOS. In measurements, stable voltage and current closed-loop operations have been verified, with a peak efficiency of 85% when tracking a 20-MHz LTE envelope.