A baseband signal generator (102a) provides a polar signal (A, I p) to a processing sub-unit (704p). The processing sub-unit (704p) receives furthermore feedback signals from a down converting unit (704c) which feedback signals are used to determine the magnitude (B) of the amplified output signal and the actual error phase. The magnitude (A) of the polar signal and the determined magnitude (B) are applied to a comparator (710) having its output connected to the input of a predistortion unit (214, 216). The output of the predistortion unit (214, 216) is connected to the input of a pulse width modulating unit (210, 212) which comprises a mapping unit (210) outputting two constant magnitude signals. The actual error phase and the phase component of the polar signal are used to generate a corrected phase component which is applied to a further mapping unit (202) forming part of a phase modulating unit (202, 204). The output signals of this phase modulating unit (202, 204) are applied to the pulse width modulating unit (210, 212) providing finally two pulse width pulse position modulated signals. These signals are applied to the switching amplifier (110).