Method and apparatus for calibrating an envelope tracking system

A method of calibrating an envelope tracking system for a supply voltage for a power amplifier module within a radio frequency (RF) transmitter of a wireless communication unit is described. The method comprising, within at least one signal processing module of the wireless communication unit, applying a training signal comprising an envelope that varies with time to an input of the RF transmitter, receiving at least an indication of instantaneous output signal values for the power amplifier module in response to the training signal, calculating instantaneous gain values based at least partly on the received output power values, and adjusting a mapping function between an instantaneous envelope of a waveform signal to be amplified by the power amplifier module and the power amplifier module supply voltage to achieve a power amplifier module gain, for example that is monotonically increasing as a function of power amplifier output power.