A method includes amplifying a non-constant-envelope input signal using a main amplifier having a first gain that varies depending on a supply voltage of the main amplifier. A constant-envelope signal is amplified using a reference amplifier having a second gain that varies depending on the supply voltage of the reference amplifier. Both the main amplifier and the reference amplifier are operated using a same variable supply voltage whose amplitude varies over time. A gain control signal is produced so as to compensate for changes in the second gain of the reference amplifier that are caused by the variable supply voltage. The gain control signal is applied in compensating for variations in the first gain of the main amplifier.