In a telephone system a subscriber circuit is powered by a DC current that is transmitted through the telephone line. The audio communications between subscribers are also transmitted over the same lines as the DC current. The DC power which is transmitted through the telephone line may vary substantially in voltage depending upon a number of factors. The greater the length of the telephone line, the lower the voltage is which is available to the subscriber circuit. Further, in certain applications termed "a subscriber carrier system" an additional telephone line is tied onto an existing line and a minimal voltage is supplied through the additional line. In addition, tone dialing circuits utilize a substantial current to generate the tone signals. The current consumed by the tone generation circuit can pull the voltage at the subscriber circuit down to an unacceptable level.
In view of these problems in providing sufficient power to a telephone subscriber circuit through the telephone lines, there exists a need for a circuit which can detect when a regulated voltage produced by the circuit has become unacceptably low and then provide supplemental current into the regulated voltage terminal to boost the terminal to an acceptable voltage state.