| 摘要 |
PURPOSE:To attain efficient constant current feeding by providing a midpoint potential detecting circuit of a talking line so as to prevent a common mode noise, blocking a differential noise produced between an inverting input of an operational amplifier and a negative power supply and providing a circuit reducing the power consumption at on-hook. CONSTITUTION:A series circuit connecting resistors RC0, RC1 at a point M1 is inserted between communication lines B and A, its output is fed to a potential point M2 between a power supply voltage -VBB and ground potential from the point M1 via an output of an operational amplifier A2 so as to detect a midpoint voltage. Then resistors Rf0, Rf1 are inserted respectively between emitters of transistors (TRs) Tr0, Tr1 and the inverting input of the amplifiers A0, A1 and each inverting input is grounded by a series circuit comprising a resistor Rs0, a capacitor C0 and a resistor Rs1 and a capacitor C1. Further, resistors Rd0, Rd1 are inserted respectively between the output of the amplifiers A0, A1 and the base of the Tr0, Tr1, a resistor Rg is inserted between the output of the amplifier A2 and the point M2 and Zener diodes D0, D1 are provided respectively between the point M2 and ground and between the point M2 and the power supply -VBB. |
