A high efficiency modulating RF amplifier (10) for amplitude modulating a signal defined by a phase information signal (1) and an envelope signal (2) comprises a power supply (30) arranged to provide an operating voltage under control of the envelope signal (2). The power supply (30) comprises a plurality of power supply stages (40) and a plurality of supply switches (50) coupled between the plurality of power supply stages (40) and the modulator (20). The power supply (30) is arranged to select one of the power supply stages (40) to provide the operating voltage under control of the envelope signal (2). The high efficiency modulator RF amplifier further comprises a modulator (20) for receiving the phase information signal (1), the envelope signal (2) and the operating voltage. The modulator (20) is arranged to provide an output signal of which an amplitude is modulated under control of the envelope signal (2).