| 摘要 |
A telephone interface circuit has a first transistor for opening and closing a connection between a speech circuit and a pair of subscriber lines, a second transistor for controlling ON/OFF states of the first transistor, and a positive feedback circuit for positively feeding back, to a base terminal of the second transistor, a part of collector current flowing through the first transistor. A circuit constant is set such that when normal use voltage is applied to the pair of subscriber lines, the second transistor operates in a saturation region, and when voltage exceeding the normal use voltage is applied to the pair of subscriber lines, the second transistor operates in a non-saturation region.
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