Filter apparatus including differential slave gm-C filter with frequency characteristics automatically tuned by master circuit

A filter apparatus comprising:

• 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, 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 so as to directly generate said control voltage, and having an output directly connected to said first and second gm-C filters to directly apply said control voltage thereto so that said difference-voltage is brought close to zero, said differential amplifier output also being directly connected to the slave gm-C filter to apply said control voltage thereto so that the operational transconductance amplifier of said slave gm-C filter is controlled by said control voltage to tune a cut-off frequency or center frequency of said slave gm-C filter,
• 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 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.

A filter apparatus comprising:

• a master circuit for receiving a reference frequency signal having a reference frequency and generating a control voltage, said master circuit comprising:
• a first gm-C filter;
• a second gm-C filter, said first and second gm-C filters comprising:
• at least one operational transconductance amplifier controlled by said control voltage, and
• at least one capacitor, said first and second gm-C filters receiving said reference frequency signal and having different respective cut-off frequencies and different frequency characteristics from each other;
• 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, and to amplify a difference-voltage between, said first and second amplitude detectors, to generate said control voltage and feed said control voltage back to said first and second gm-C filters such that said difference-voltage is brought close to zero;
• 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,
• 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.