Sensor circuit for voice/fax switch with cascaded amplifier circuit

A sensor circuit for a telephone line, comprising;

• line current detecting means for magnetically detecting a line current which flows in said telephone line to output a detection signal according to the line current;
• first amplifying means for amplifying said detection signal from said line current detecting means, said first amplifying means having dynamic range wherein no saturation occurs in a signal outputted from said first amplifying means;
• second amplifying means for amplifying the signal amplified by said first amplifying means, said second amplifying means having an output characteristic such that a ratio of a change in the signal amplified by said first amplifying means, after being amplified by said second amplifying means, to a corresponding change in the line current, is larger than a ratio of a change in the signal amplified by said first amplifying means, to a corresponding change in the line current;
• supervisor means for outputting a line current supervisory signal based on the signal amplified by said second amplifying means;
• and
• tone deriving means for deriving a tone signal from the signal amplified by said first amplifying means.