An amplifier for linearly amplifying a signal using a pair of non-linear amplifiers 57 is described. A hard limiter 50 separates the input signal into a constant amplitude signal bearing the phase of the input and an amplitude related signal. Using the latter the function generator 62 develops two outputs (labelled cos (#) and sin (#), the sum of whose squared amplitudes is constant. These are each combined in 56 with a signal related to the constant amplitude signal to produce two constant amplitude signals for amplification by the pair of amplifiers. These produce an amplified output and a waste signal 61. Various other arrangements are described including: the use of a matrix of amplifiers responding to a plurality of inputs to increase the flexibility of use and the disposal (eg by radiation into a harmless direction) or recovery of energy of intermodulation products.