Method of providing d.c. feed to a telephone line

A method of providing d.c. feed to a two-wire telephone line, comprising the steps of:

• generating a supply voltage for a drive circuit including two amplifiers each having an output coupled to a respective wire of the line;
• supplying an a.c. transmission signal to the line via the amplifiers of the drive circuit;
• determining potentials at the amplifier outputs in dependence upon the supply voltage and headroom voltages required for operation of the amplifiers to supply the transmission signal to the line, a difference between the potentials at the outputs of the two amplifiers producing a loop current on the line;
• monitoring the loop current on the line;
• and
• digitally controlling the generated supply voltage in dependence upon the monitored loop current by the steps of, iteratively at each of a plurality of successive time instants:
• determining a target value corresponding to a target voltage inversely dependent upon an amount by which the monitored loop current exceeds a predetermined value;
• and
• adjusting the generated supply voltage towards the target voltage in dependence upon the target value;
• whereby the target voltage and the generated supply voltage converge over said plurality of successive time instants.