| 摘要 |
A telephone interface circuit comprises: a first transistor which switch-controls a connection between a speech circuit and a pair of subscriber lines; a second transistor which controls an on/off state of the first transistor; a positive feedback circuit which connects a collector terminal of the first transistor to a base terminal of the second transistor; and an internal power source which supplies current for driving the second transistor. A circuit constant is set such that the first transistor is to operate in a saturated region when a voltage in a range of voltage for normal use is applied in between the pair of the subscriber lines, and such that the first transistor is to operate in an unsaturated region when an overvoltage exceeding the range of voltage for normal use is applied in between the pair of the subscriber lines.
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