Abstract: An apparatus for compensating the phase rotation in the feedback loop of a cartesian feedback power amplifier in a final transmitter stage for quadrature modulation of the complex difference signal between a complex input signal and the corresponding complex feedback signal with a complex modulation signal for forming a modulated real valued first signal and for quadrature modulation of the output signal from the power amplifier with a complex demodulation signal for forming the complex feedback signal. The phase shift between the first signal and the second signal and between the quadrature component of the first signal and the second signal is detected for determining the phase rotation of the feedback loop. Furthermore, the phase rotation of the complex modulation signal can be changed with a compensating phase rotation defined by the determined phase rotation.