| 摘要 |
In a filter apparatus including a master circuit for receiving a reference frequency signal having a reference frequency to generate a control voltage and a slave gm-C filter formed by at least one operational transconductance amplifier and at least one capacitor where the operational transconductance amplifier of the slave gm-C filter is controlled by the control voltage to tune a cut-off frequency or center frequency of the slave gm-C filter, the master circuit is constructed by first and second gm-C filters each formed by at least one operational transconductance amplifier and at least one capacitor and having different frequency characteristics from each other, the first and second gm-C filters being adapted to receive the reference frequency signal, first and second amplitude detectors having inputs connected to outputs of the first and second gm-C filters, respectively, and a differential amplifier having an input connected to outputs of the first and second amplitude detectors and being adapted to amplify a difference-voltage therebetween to generate the control voltage, so that the difference-voltage is brought close to zero.
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