| 摘要 |
An electric power circuit for telephone set use utilizing a power source drawn from a telephone circuit line. Alternating current components superimposed on a supply voltage, which is taken out by means of a by-pass capacitor, are delivered to a downstream-side of the common-base of an input-side transistor comprised in a current mirror circuit. A constant value of direct-current, which is the same magnitude as a potential difference between the supply voltage and a potential of the common-base of the current mirror circuit, is obtained at the input side transistor by varying the potential of the common base in accordance with fluctuations due to a.c. components superimposed on the supply voltage. An amplified direct-current is output from the output-side transistor capable of producing a magnitude of current in proportion to that of the input-side current. To secure the stable magnitude of the amplified currents, a transistor is disposed at the downstream of the output-side transistor because it is necessary to vary a potential at the output terminal of the output-side transistor in accordance with fluctuations due to a.c. components on the supply voltage in the same manner as to vary the base potential of the input-side transistor, thereby ensuring potential differences between the downstream of the output-side transistor and the emitter thereof at a predetermined value or more. Concurrently, presence of high impedance to voice signals superimposed on the supply voltage is secured by a pair of voltage dividing resistors disposed at the downstream of the input-side transistor.
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