An apparatus for detecting the amplitude and phase of an a.c. signal. The apparatus includes a signal detection circuit for detecting the a.c. signal, a signal splitting circuit for splitting the detected a.c. signal into first and second a.c. signals out of phase with each other by 90.degree.. The signal splitting circuit includes a first order lag circuit having a phase lag, a subtractor, and first and second amplifying circuits. The apparatus also includes a phase compensation circuit coupled to the signal splitting circuit for receiving the first and second a.c. signals from the signal splitting circuit and for advancing the phase of an output signal of the apparatus by the phase lag of the first order lag circuit. The phase compensation circuit also includes an amplitude and phase detection circuit for detecting the amplitude and phase of the detected a.c. signal by implementing a polar coordinate transformation of the first and second a.c. signals received from the signal splitting circuit.