| 摘要 |
PURPOSE:To decrease the capacitance of a capacitor in a filter circuit and to eliminate the need for a relay, by inserting a voltage dividing circuit including a high resistance between a line A and ground and connecting an active FIL (active low pass filter) at a voltage dividing point and detecting a DC output of the FIL with a comparator. CONSTITUTION:An amplifier AMP, resistors R6 and R7, capacitors C1 and C2 constitute a secondary active low pass filter FIL, which blocks a call tone in 16Hz and passes through a DC voltage without attenuation. One input terminal 2 of a comparator COMP receives a constant negative reference voltage V2 voltage dividing a power supply EA with resistors R5 and R9, and the COMP generates a positive or negative output voltage depending whether a voltage V1 of another input terminal 1 is higher than a voltage V2 or lower. Thus, since an operational amplifier is used for the FIL, the degree of freedom of the input impedance selection is large and the input impedance can sufficiently be made higher. |
