Active impedance transformer assisted line feed circuit with supervision filtering

A line circuit for terminating a two wire communciation line at tip and ring terminals, comprising:

• a transformer having tip and ring primary windings and a secondary winding;
• tip and ring feed resistors;
• tip and ring amplifiers for supplying energizing currents for operation of the communication line, the tip and ring amplifiers having outputs being series connected to the tip and ring terminals via the tip and ring primary windings and the tip and ring feed resistors respectively;
• a feed switch for conducting the energizing currents via the tip and ring feed resistors and the tip and ring primary windings, and being operable in response to a ring signal for conducting ring currents via the tip and ring feed resistors to the exclusion of the tip and ring windings;
• a resistance network including tip and ring voltage taps, the resistance network being connected in combination with the tip and ring feed resistors for supplying voltage signals being indicative of currents in the tip and ring feed resistors at tip and ring voltage taps;
• a control circuit being differentially responsive to the voltage signals at the tip and ring voltage taps and being responsive to a.c. signals from the secondary winding being coupled via a resistive capacitive network to an amplifier in the control circuit, for providing a supervisory signal for indicating the state of the communication line as being one of ON HOOK and OFF HOOK, and during the OFF HOOK state for providing a control signal for operation of the tip and ring amplifiers;
• and
• control switch means being responsive to the ring signal for open circuiting the secondary winding such that the amplifier in the control circuit in combination with the resistive capacitive network functions as a low pass filter to reduce a.c. signal components in the supervisory signal.