| 摘要 |
PURPOSE:To realize a MODEM/telephone line interface satisfying the insulation resistance between power supply and a line specified by the technical standards with small size and light weight by using an optical isolation amplifier so as to couple the input/output of a speech network IC and a MODEM. CONSTITUTION:A DC resistance (50-200ohms) and an AC impedance (600ohms) viewed from a telephone line are kept as specified by a resistance hybrid and a speech network IC. Moreover, the input/output of the speech network IC and MODEM uses an optical isolation amplifier 24 (an amplifier comprising an optical coupler and an operational amplifier or a transistor whose input/ output is isolated electrically) and is coupled while keeping electric isolation. A constant current circuit 22 and a stabilizing circuit 23 supply power to elements of an optical isolation amplifier to the telephone line side. The operational amplifier, the transistor, the speech network IC or the like are easily of surface mount type, and no large sized component such as a transformer and a nonpolar large capacitor is not required and the small size/light weight of the circuit is attained.
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