Described are circuits and techniques to increase the efficiency of radio-frequency (rf) amplifiers including rf power amplifiers (PAs) through “supply modulation” (also referred to as “drain modulation” or “collector modulation”), in which supply voltages provided to rf amplifiers is adjusted dynamically (“modulated”) overtime depending upon the rf signal being synthesized. For the largest efficiency improvements, a supply voltage can be adjusted among discrete voltage levels or continuously on a short time scale. The supply voltages (or voltage levels) provided to an rf amplifier may also be adapted to accommodate longer-term changes in desired rf envelope such as associated with adapting transmitter output strength to minimize errors in data transfer, for rf “traffic” variations.