| 摘要 |
A circuit for synthesizing an impedance across the tip and ring leads of a telephone line circuit is disclosed. The circuit includes tip drive and ring drive amplifiers connected to the tip and ring leads respectively, of a subscriber loop. The tip drive and ring drive amplifier circuits are arranged to convert feed voltage from a central office battery to feed current to drive the subscriber loop. A common-mode amplifier circuit connected between the tip and the ring leads senses the voltage dropped across the subscriber loop and outputs a control voltage to the tip and ring drive amplifiers, offsetting the feed currents applied to the subscriber loop. A differential amplifier circuit also connected to the tip and ring leads of the subscriber loop, detects a voltage difference between the tip lead and the ring lead and converts the detected differential voltage into a single ended output voltage of a specific gain. The output voltage from the differential amplifier circuit is coupled to a feedback circuit via a series connected resistor and capacitor network. The feedback circuit receives the differential amplifier output voltage that is modified by the values of the resistor and capacitor network and develops a pair of balanced output signals which are equal in amplitude but 180 degrees out of phase with each other. Each of the output signals is connected to a respective one of the tip drive and ring drive amplifier circuits where, an input impedance equal to the impedance of the subscriber loop is derived at the tip and ring leads.
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