Patent ID: 7459963

Claim:
A filter apparatus comprising: a master circuit for receiving a reference frequency signal having a reference frequency and generating a control voltage; and a slave circuit comprising a single slave gm-C filter and formed by at least one operational transconductance amplifier and at least one capacitor, the operational transconductance amplifier of said single slave gm-C filter being controlled by said control voltage to tune a cut-off frequency or center frequency of said single slave gm-C filter, said master circuit comprising: 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, said first and second gm-C filters having respective cut-off frequencies and being adapted to receive said reference frequency signal; first and second amplitude detectors having inputs connected to outputs of said first and second gm-C filters, respectively; and a differential amplifier having inputs connected to outputs of said first and second amplitude detectors to amplify a difference-voltage therebetween to generate said control voltage, so that said difference-voltage is brought close to zero, wherein at least one of said reference frequency and said respective cut-off frequencies of said first and second gm-C filters is different from the other two of said reference frequency and said respective cut-off frequencies of said first and second gm-C filters, and wherein a type of said single slave gm-C filter comprises one of a low pass filter, a high pass filter, and a band pass filter and is different from a type of said first and second gm-C filters.