Abstract: An amplifier for differentially driving a two wire line has two output terminals for connection to the line. The differential voltage on the line is monitored and a negative-feedback voltage derived therefrom is combined with the input signal voltage. This combination is applied to a reference impedance. The resulting current in this impedance controls the current fed to the line. In one embodiment the control is direct via controlled current sources for example precision current mirrors and in another embodiment a current mirror controls the current to one wire and the current to the other wire is controlled indirectly via circuitry arranged to maintain equal voltage excursions on the two wires of the line.