FOREIGN EXCHANGE OFFICE CIRCUIT FOR INTERNET TELEPHONY GATEWAY SYSTEM

摘要

PURPOSE: An FXO(Foreign eXchange Office) circuit in an ITGS(Internet Telephony Gateway System) is provided to implement an FXO interface circuit in accordance with the loop status voltage-current characteristics of TBR21(Technical Basis for Regulation 21), an European PSTN-associated technical regulation. CONSTITUTION: An FXO circuit in an ITGS comprises an electronic inductor circuit part. The electronic inductor circuit part is composed of resistors(R1-R3), a Darlington transistor(Q1), a transistor(Q2), a capacitor(C1), a feedback diode(D2), and a constant voltage zener diode(D1). The resistors(R1,R2) divide the voltage supplied between a tip and a ring and supply bias voltage to the Darlington transistor(Q1). The Darlington transistor(Q1), driven by the bias voltage, generates transistor driving current for current limitation. The transistor(Q2), connected between the base and emitter of the Darlington transistor(Q1), generates low impedance between the tip and the ring. The feedback diode(D2) and the resistor(R3), connected to the base and emitter of the transistor(Q2) respectively, are provided for current limitation. The capacitor(C1) interrupts noise components from the signals inputted to the transistors(Q1,Q2). The feedback diode(D2). The constant voltage zener diode(D1) protects the Darlington transistor(Q1) against an instant transient voltage generated when the tip-ring state is changed into an off-hook state.