| 摘要 |
PURPOSE:To secure the dynamic range of the sound signal, etc., and to execute the branch calling by supplying the constant electric current when the line electric current is the prescribed value or above the shorting the constant voltage element at the time of the prescribed value or below. CONSTITUTION:When the usual calling condition, namely, a line electric current IL is larger than a drain eletric current IDSS of the zero gate voltage of an FET 10, a drain electric current ID (=IL-ID>0) flows at a Zener diode 8, and a telephone set edge voltage VL is boosted for a Zener voltage VZ. Consequently, the dynamic range of the sound signal overlapped at the telephone set edge voltage VL is secured. On the other hand, at the time of the branch calling, namely, when the electric current IL is smaller than the electric current IDSS, the electric current ID is equal to the electric current IL, and the Zener electric current IZ will not flow at the Zener diode 8. At such a time, the FET 10 comes to be one condition.
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