| 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 a phase shifter formed by at least one operational transconductance amplifier and at least one capacitor, the phase shifter being adapted to receive the reference frequency signal and change a phase of the reference frequency signal in accordance with the control voltage, a phase comparator adapted to compare a phase of an output signal of the phase shifter with a phase of the reference frequency signal to generate a phase error signal, and a loop filter adapted to exclude an AC component from the phase error signal to generate a DC component thereof as the control voltage. A phase locked loop of the master circuit is operated so that a loop gain thereof is maximum.
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