Ring trip circuit for subscriber telephone

Claim 1

A ring trip circuit for a subscriber telephone, the ring trip circuit operatively connected to a ringer sending resistor having first and second ends, the ringer sending resistor operatively connected between the subscriber telephone and a ringer source, said ring trip circuit comprising: voltage detecting means, operatively connected between the second and second ends of the ringer sending resistor, for detecting a voltage difference between the first and second ends of the ringer sending resistor and for converting the voltage difference to a current difference, the voltage difference varies in response to whether the subscriber telephone is in one of an on-hook state and an off-hook state, said voltage detecting means includes a plurality of current mirror circuits for the converting of the voltage difference to the current difference, each of the current mirror circuits formed by at least two transistors; and state detecting means, operatively connected to said voltage detecting means, for detecting one of the on-hook state and the off-hook state of the subscriber telephone based upon the voltage difference, said state detecting means including comparator means formed by a plurality of transistors, wherein said voltage detecting means includes at least a first current switching circuit operatively connected to the first end of the ringer sending resistor and having first to third ports, said first current switching circuit supplies a first current from the second port to the third port when a first voltage at the first end is positive, or supplies the first current from the first port to the first end through the third port when the first voltage at the first end is negative; and a second current switching circuit operatively connected to the second end of the ringer sending resistor and having first to third ports, said second current switching circuit supplies a second current from the second port to the third port when a second voltage at the second end is positive, or supplies the second current from the first port to the second voltage end through the third port when the second voltage at the second end is negative. wherein said plurality of current mirror circuits of said voltage detecting means includes at least a first current mirror circuit having a first terminal connected to the first port of said first current switching circuit and a second terminal; a second current mirror circuit having a first terminal Connected to the second port of said first current switching circuit and a second terminal connected to said second terminal of said first current mirror circuit at a common connection point; a third current mirror circuit having a first terminal connected to the first port of said second current switching circuit and a second terminal; a fourth current mirror circuit having a first terminal connected to the second port of said second current switching circuit and a second terminal; a fifth current mirror circuit having a first terminal connected to the second terminal of said fourth current mirror circuit and a second terminal connected to the common connection point; and a sixth current mirror circuit having a first terminal connected to the second terminal of said third current mirror circuit and a second terminal connected to the common connected point, wherein the common connection point of the second terminals of said first and second current mirror circuits and the second terminals of said fifth and sixth current mirror circuits is the current difference corresponding to the voltage difference between first and second ends of the ringer sending resistor, and wherein said ring trip circuit further comprises a voltage conversion circuit, operatively connected to the common connection point of the voltage detecting means, for converting the current difference to a reconstructed voltage difference and outputting the same.