| 摘要 |
To provide an adequate dc supply to the electronics of a telephone, when on hook or off hook, there is a transistor (T2) with its emitter-collector path in one line wire, and a storage capacitor (Cpsu) across the input to the telephones's circuits. This capacitor is charged up and maintains dc supply when the line voltage (at A) falls below a preset level. The base of this transistor is connected via another transistor (T1) to the other line wire. The speech output (VIN) is connected via an operational amplifier (A) to the base of the second transistor (T2). Hence the dc supply and the speech output are in parallel.
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