Subscriber line interface circuit with improved d.c. balance

A subscriber line interface circuit (SLIC) comprising:

• two terminals for coupling to a two-wire telephone subscriber line;
• two controlled current circuits (CCCs) for respectively supplying direct current to and sinking direct current from the line via the terminals;
• a feedback loop responsive to direct current on the line for controlling the CCCs;
• and
• d.c. balance control means responsive to a d.c. imbalance of potentials at the terminals for controlling the feedback loop to modify of the CCCs to reduce the d.c. imbalance;
• the d.c. balance control means comprising:
• a potential divider coupled betwen the terminals and having a tapping point;
• and
• a differential amplifier having a first input coupled to the tapping point, a second input coupled to a point of reference potential, and an output coupled to an output of the d.c. balance control means;
• and the feedback loop comprising:
• means for producing a sum current dependent upon a sum of direct currents flowing via the terminals;
• current splitting means for splitting the sum current into two control currents;
• and
• means for controlling the two CCCs each in dependence upon a respective one of the two control currents;
• the current splitting means being responsive to the output of the d.c. balance control means to control the relative proportions of the two control currents.