The invention relates to optimizing efficiency of a power amplifier of a transmitter. In a solution according to the invention a detected envelope (306) of an input signal of the power amplifier is filtered with a non-linear filter (307) that substantially preserves a rise time of a peak in a waveform of the envelope but lengthens a temporal duration of the peak. A filtered envelope is used as an input quantity (308) for a control system that controls a supply voltage of the power amplifier. For the control system it is easier to react to peaks of the filtered envelope than to the peaks of the envelope because the temporal duration of the peaks of the filtered envelope is longer than that of the peaks of the envelope.