Optimum Biasing for Parallel Hybrid Switching-Linear Regulators

Hybrid combinations of switching and linear regulators have been proposed for both audio amplifiers and dynamic supply modulators for radio frequency (RF) power amplifiers (PAs). Such topologies may provide benefits in terms of efficiency, dynamic range, and speed of dynamic response compared to pure linear regulators or class-D switching amplifiers. This paper presents a framework for analyzing the bias constraints of switching and linear voltage regulators operated in a parallel-hybrid configuration. Particular emphasis is given to polar and envelope tracking RF power amplifier (RF PA) applications. Ideal expressions are derived for the optimum current contribution of the switching regulator under quasi-static operating conditions. In contrast to previous work, it is shown that the optimum mean current contribution of the switching regulator is not necessarily the dc current to the load. Explicit expressions for theoretical maximum efficiency are derived for envelope waveforms that result from two-tone and sinusoidal amplitude modulation of the RF carrier; IS-95 CDMA and IEEE 802.11a/g wireless LAN envelope waveforms are treated in simulation and experiment. Theoretical predictions are validated with measured results.