| 摘要 |
According to the invention, an interface circuit for quickly establishing a DC loop current in response to off-hook detection is provided by a circuit which uses a transistor, where its collector electrode is connected to the first one of the telephone lines; an emitter resistor connected between an emitter electrode of the transistor and the second one of the telephone lines; a bias resistor having a first terminal connected to the collector electrode; a Zener diode connected between a second terminal of the bias resistor and the second line; a first capacitor connected in parallel to the constant-voltage means; switching means connected between the second terminal of the bias resistor and a base electrode of the transistor for the switching means to be conductive in response to an off-hook signal; and charging means, typically formed of a series connection of a capacitor, a resistor and a Zener diode, connected in parallel to the bias resistor so as to allow a current to flow through the series connection to charge the first capacitor only during a transient of applying a Dc voltage from the telephone lines onto the transistor.
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