An electrical isolator circuit comprising: an input stage comprising a transmitter of an opto-isolator; and an output stage comprising: a receiver of the opto-isolator; said receiver comprising a photo-transistor an active circuit in series with the receiver between the receiver and a power rail of the output stage; a first resistor connected to an output of the active circuit, and an input for the application of a predetermined voltage (Ref) to the active circuit, wherein the active circuit comprises a differential amplifier having a first input that is electrically connected to an output of the receiver and a second input that is connected to the predetermined voltage (Ref), the first resistor being connected between an output and the first input of the differential amplifier; the output stage being configured such that a varying output voltage is developed across the first resistor in response to input of a high voltage AC signal to the input stage, characterized by a variable current source in parallel with the photo-transistor, which is operative to source current in an opposite direction to current flow in the photo-transistor, and a feedback circuit, which is operative to receive a voltage from the output of the differential amplifier and to control the current flowing through the variable current source in dependence on the received voltage. an input stage comprising a transmitter of an opto-isolator; and an output stage comprising: a receiver of the opto-isolator; said receiver comprising a photo-transistor an active circuit in series with the receiver between the receiver and a power rail of the output stage; a first resistor connected to an output of the active circuit, and an input for the application of a predetermined voltage (Ref) to the active circuit, wherein the active circuit comprises a differential amplifier having a first input that is electrically connected to an output of the receiver and a second input that is connected to the predetermined voltage (Ref), the first resistor being connected between an output and the first input of the differential amplifier; a receiver of the opto-isolator; said receiver comprising a photo-transistor an active circuit in series with the receiver between the receiver and a power rail of the output stage; a first resistor connected to an output of the active circuit, and an input for the application of a predetermined voltage (Ref) to the active circuit, wherein the active circuit comprises a differential amplifier having a first input that is electrically connected to an output of the receiver and a second input that is connected to the predetermined voltage (Ref), the first resistor being connected between an output and the first input of the differential amplifier; the output stage being configured such that a varying output voltage is developed across the first resistor in response to input of a high voltage AC signal to the input stage, characterized by a variable current source in parallel with the photo-transistor, which is operative to source current in an opposite direction to current flow in the photo-transistor, and a feedback circuit, which is operative to receive a voltage from the output of the differential amplifier and to control the current flowing through the variable current source in dependence on the received voltage.