The instantaneous value of an intermediate waveform I is the instantaneous value of a unipolar waveform U multiplied through amplification by an upscaling factor UF of 1.5. A plateau value P is subtracted from the intermediate value I, and the result of this subtraction is multiplied by a multiplication factor MF of 0.6. The result of the multiplication is added to the plateau value P, which sum becomes an auxiliary waveform A. During the ‘fall-below’ periods F, the value of a combined waveform C is arranged to follow whichever is the highest of the auxiliary value A and the plateau value P. Outside the fall-below periods, the value of the combined waveform C follows whichever is the highest of the unipolar value U and the plateau value P. This combined waveform C has, for a given plateau level P, a narrower fall-below window. Phase-chopping thus has an effect on the power of the output signal over a greater range of the cycle than can be provided by the corresponding prior art arrangement.