I. Introduction
The ever-growing demand for battery powered wireless devices underscores the growing demand for energy efficient wireless transceivers. The power amplifier (PA) is usually the major power dissipating block in a transmitter wherein efficiency improvements are needed to save energy and increase battery lifetimes. Using non-constant envelope modulation standards with high peak-to-average power ratios (PAPR), the PA operates in the power back-off (PBO) region most of the time where energy efficiency decreases dramatically. Thus, techniques for higher efficiency at deep PBO levels are required to realize more efficient transceivers.