A DC-to-DC converter as described herein is suitable for use as a variable voltage supply for an RF power amplifier. The DC-to-DC converter includes a buck converter for providing relatively low voltage and current output, and a linear voltage regulator for providing relatively high voltage and current output. When operating in a buck converter mode, the DC-to-DC converter disables the linear voltage regulator, thus providing efficient low-level operation. When operating in a linear regulator mode, the DC-to-DC converter enables the linear voltage regulator while maintaining at least some current flow through the buck converter; this is accomplished by keeping an output transistor of the buck converter continuously on during the linear regulator mode. The DC-to-DC converter can transition between operating modes in a smooth manner that does not result in discontinuous voltage drops.