A transmitter 2 comprises a power amplifier 3 and power controller 4. The power amplifier 3 is capable of both linear and non-linear operation, and is controlled by a controller 20 and first and second control loops 21,22. The second control loop 22 is used with the transmission of GMSK modulated signals, which have no amplitude modulation, operating the power amplifier 3 in a non-linear mode. The first control loop 21 is used for the transmission of EDGE modulated signals, which have a substantial amount of amplitude modulation, operating the power amplifier 3 in a linear mode. The first control loop 21 is also used with GMSK time slots if the output power to be produced by the power amplifier 3 is low and the power of an immediately preceding time slot is high. In this case, a sample and hold circuit 9, 10 of the first control loop 21 is used when the power detector voltage in the power amplifier would change over the time slot by cooling, avoiding the possibility of the power level varying over the time slot.