CIRCUIT FOR MAINTAINING THE DC VOLTAGE ON AN ELECTRICALLY ISOLATED TELECOMMUNICATION LINE AT A REFERENCE LEVEL

摘要

CIRCUIT FOR MAINTAINING THE DC VOLTAGE ON AN ELECTRICALLY ISOLATED TELECOMMUNICATION LINE AT A REFERENCE LEVEL A DC reference voltage circuit in a line interface circuit 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 DC reference voltage circuit comprises a low-pass filter, a difference amplifier and two voltage-to-current sources 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 provide source and sink currents to and from the line to maintain the DC voltage on the line at the reference level. A load resistor also included in the feedback circuit keeps the circuit from oscillating where the phase shift of any signal through the circuit is an integer multiple of 360 degrees. The high output impedance of the two voltage-to-current sources does not effect the impedance balance on the line.