| 摘要 |
<p>PURPOSE:To reduce a call current and protect a call current supply circuit from an excess current, by increasing a DC impedance of the call current supply circuit, in case when abnormal overvoltage is genersted in a telephone line. CONSTITUTION:The terminals 1, 2 are connected to one pair of telephone circuits, and the terminals 3, 4 are connected to the electric power source. For instance, in the event of ground-fault of the terminal 2, the voltage between the terminal 2 and 4 rises abnormally, by which the transistor 20 conducts. As a result, the voltage between the noninversion input terminal of the optional amplifier 18 and the terminal 4 becomes small enough comparing with the voltage between the noninversion input terminal of the operational amplifier 18 and the terminal 2, therefore, the DC impedance between the terminal 2 and 4 incereases, and a current flowing into the first transistor 12 and the resistor 14 can be reduced. The circuit between the terminal 1 and 3 is operated in the same way, ane the first transistor 11 and the resistor 13 are protected.</p> |
