An operational amplifier (21) of a first opto-coupler (28) is connected in-front of the transmit photodiode (25). The latter is series-connected to a transmit photodiode (26) of a second opto-coupler (29). The output signal is kept at a reception phototransistor (30) of the first opto-coupler, lying at a second operating voltage (UB12). A signal voltage is tapped at a reception phototransistor (31) of the second opto-coupler, at a first operating voltage (UB11) galvanically separated from the second voltage. The signal voltage is supplied to an input (34) of the operational amplifier. The transmit elements are pref. in series with a resistor (27) between the amplifier output (24) and a reference potential of the primary side (11). ADVANTAGE - Increased linearisation of LF transfer characteristic in transmitted frequency range. Distortion factor kept low, e.g. not more than 0.2 percent.
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