| 摘要 |
An active filter has a least an operational amplifier, a plurality of resistances and a plurality of capacitances, at least one of the resistances and the consequences is variable by controlling a controller connected to an output side of the active filter for receiving an output signal so that the controller counts a frequency of the output signal to generate a counted value so as to control the variable resistances and capacitances on the basis of the counted value.
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