| 摘要 |
PURPOSE:To incorporate a comparator to an LSI manufactured by a low- pressure process by lowering a voltage applied to an input section of the comparator and connecting the comparator and a talking line via a high resistor. CONSTITUTION:When a called party hooks off a handset, since a hook switch HS of a telephone set is closed and a bell circuit is short-circuit, the current includes a DC component. The hook-off is discriminated by inputting the output voltage to a comparator COMP having a threshold voltage Vref. The DC component of the input voltage to a filter circuit FIL and the output voltage of the filter circuit FIL is set optionally by the ratio of resistors RR0 and RR1, and the attenuation of the AC component of the output voltage of the filter FIL is decided by a product (time constant) between the resistor R and the capacitor C, then a high resistance is used for the resistor R.
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