Abstract: A synchronous detector circuit provides a direct current output as a function of an alternating current input signal. Direct current and noise level in the input is removed in a circuit including filters and an operational amplifier and a parallel path feedback network, including in one branch a semiconductor and in a second branch a load and switching circuit synchronously operating with the alternating current input signal. The switching circuit may include a field-effect transistor switch made operative by a semiconductor switching off and on as a function of synchronously chopped light received by a photosemiconductor.