| 摘要 |
PROBLEM TO BE SOLVED: To solve the problem that the S/N ratio of output voice signals has been degraded by thermal noise which occurs due to a feedback resister, which sets a D.C. bias voltage of an inverting input terminal of an operational amplifier for detecting changes in the capacitance of a capacitance-type sensor. SOLUTION: A D.C. bias setting circuit 26 is parallelly provided for a feedback capacitor C<SB>F</SB>of an operational amplifier 24. The D.C. bias setting circuit 26 includes a switched capacitor circuit 28 and controls ON/OFF of its MOS transistors Q<SB>1</SB>and Q<SB>2</SB>to function them as an equivalent resistor r<SB>SC</SB>. Since the on-resistance of the MOS transistors Q<SB>1</SB>and Q<SB>2</SB>is small, they generate only small thermal noise. Their off-resistance is extremely large to lower a cut-off frequency f<SB>0S</SB>of an LPF comprising the resistor f<SB>SC</SB>and the feedback capacitor C<SB>F</SB>below a voice band. As a result, a voice band component in thermal noise passed through the LPF and outputted to a terminal OUT is lowered. COPYRIGHT: (C)2008,JPO&INPIT
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