Carrier subscriber telephone off-hook detector

Abstract

A resistor and diode means are electrically connected in series between nodes A and B on the base electrodes of first and second transistors of a differential amplifier. These nodes A & B are also connected to a source of temperature-compensated load-sensitive reference voltage and to a tap point, respectively, that senses current in the loop of a carrier-serviced subscriber's telephone set. The base and emitter of a control transistor are also connected to associated nodes A and B. Conduction of the diode means and control transistor prebiases the latter and reduces the reference voltage during an on-hook condition. When the handset goes off-hook, the node B voltage rises faster than that on node A for decreasing the conduction rate of the control transistor to indicate an off-hook condition, and eventually cutting off the control transistor prior to the diode means. This operation of the diode means extends the loop length over which the circuit will detect an off-hook condition. With the diode means cut off, the second transistors collector voltage biases an FET in the loop for varying the resistance thereof and making the loop current a prescribed value for loop lengths that are less than a limiting value.