Embodiments provide methods, apparatus and computer software for use in calibrating a transmitter in operative association with a variable supply voltage. A phase distortion of the power amplifier is determined for a given instantaneous power. On the basis of the determined phase distortion for the power amplifier at the given instantaneous power, the transmitter is calibrated. Examples of transmitters in respect of which embodiments may be practiced include envelope tracking transmitters, envelope elimination and restoration transmitters, and polar transmitters.