Telephone line circuit

Details Telephone line circuit Telephone line circuit

1979-08-02

1981-07-28

Robinson, Thomas A.

H04Q 318

Patent

active

042812194

ABSTRACT:
A telephone line circuit includes balanced tip and ring leads for connection to a telephone set and an unbalanced line output lead. A first differential amplifier has its input terminals connected through individual equal-valued resistors to the tip and ring leads, the values of each resistor being at least 10 times the off hook impedance of the telephone set. The output of the first differential amplifier is connected in a circuit path to a line output lead, for applying outgoing signals thereto. A second differential amplifier has its input connected in a circuit path to a line input lead for receiving incoming signals, and its output connected in a circuit path to one of the tip or ring leads, and connected by a further resistor to the input of the first differential amplifier which is connected to the other of the tip or ring leads. The value of the further resistor is selected to apply sufficient output signal from the second differential amplifier to the other input of the first differential amplifier so as to substantially cancel signals within the first differential amplifier appearing at its input terminals which were applied from the output of the second differential amplifier to the tip and ring leads.

REFERENCES:
patent: 4178485 (1979-12-01), Cowpland et al.

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