| 摘要 |
PROBLEM TO BE SOLVED: To provide a frequency-setting circuit comprising filter circuits, whose scale is reduced by employing a low-order gm-C filter in a control circuit. SOLUTION: The frequency-setting circuit disclosed is equipped with the gm-C filters 1, 2 composed of an OTA and capacitance, rectifiers 3, 4 for detecting output amplitudes of the gm-C filters 1, 2, and a comparator 5 for outputting a voltage resulting from the comparison result of rectifiers 3, 4 outputs. A drive current of the OTA of the gm-C filters 1, 2 is controlled by an output voltage of the comparator 5, at inputting of a predetermined frequency signal in parallel to the gm-C filters 1, 2, so that transconductance is changed. By using the output voltage of the comparator 5 when equalizing the output signal levels of the rectifiers 3, 4 with each other, the drive current of the OTA, whose configuration is the same as that of the gm-C filters 1, 2 constituting a controlled filter 10 composed of the OTA and capacitance, so that the transconductance, is changed. The transconductance thus changed causes the cut-off frequency of the controlled filter to be set at a predetermined frequency. COPYRIGHT: (C)2005,JPO&NCIPI
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