A transconductance amplifier is driven by a difference voltage which is coupled through a first input terminal to a first voltage-to-current converter and through a second input terminal to a second voltage-to-current converter of identical design. An identical third voltage-to-current converter has its input terminal connected to the tap of a resistive voltage divider which is connected between the first and second input terminals. The output signal of the transconductance amplifier can be taken from the current outputs of the first and second voltage-to-current converters. The current outputs have a difference current which is proportional to the value of the difference voltage.