Abstract: Disclosed is a line interface circuit (103) having a negative feedback circuit (205) for maintaining the DC voltage on an electrically isolated communications line at a reference level such as ground for private branch exchanges using ground start supervisory signaling. The feedback circuit comprises a low-pass filter (301), a difference amplifier (302), and two voltage-to-current sources (303,304) each having a high output impedance. The low-pass filter passes only the voltage on the line below a predetermined frequency. The difference amplifier subtracts the passed voltage from a ground reference level to form a difference voltage. Depending on the polarity of the difference voltage, the two voltage-to-current sources source and sink current to and from the line to maintain the DC voltage on the line at the reference level.