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

1. 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.

2. The filter apparatus as set forth in claim 1 , wherein said first gm-C filter comprises a second-order low pass filter, and said second gm-C filter comprises a first-order low pass filter.

3. The filter apparatus as set forth in claim 1 , wherein said first gm-C filter comprises a first-order high pass filter, and said second gm-C filter comprises a second-order high pass filter.

4. The filter apparatus as set forth in claim 1 , wherein said first gm-C filter comprises a first-order low pass filter, and said second gm-C filter comprises a first-order high pass filter.

5. The filter apparatus as set forth in claim 1 , wherein said first gm-C filter comprises a first-order low pass filter, and said second gm-C filter comprises a second-order band pass filter.

6. The filter apparatus as set forth in claim 1 , wherein said first gm-C filter comprises a second-order band pass filter, and said second gm-C filter comprises a first-order high pass filter.

7. The filter apparatus as set forth in claim 1 , wherein said first gm-C filter comprises a second-order band pass filter, and said second gm-C filter comprises another second-order band pass filter, said second-order band pass filters having different frequency characteristics from each other.

8. The filter apparatus as set forth in claim 1 , wherein said first and second amplitude detectors have the same structure.

9. 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.

10. The filter apparatus as set forth in claim 1 , wherein frequency characteristics of said single slave gm-C filter are substantially different from those of said first and second gm-C filters.

11. The filter apparatus as set forth in claim 9 , wherein frequency characteristics of said single slave gm-C filter are substantially different from those of said first and second gm-C filters.

12. 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.

13. The filter apparatus as set forth in claim 12 , wherein said first gm-C filter comprises a second-order low pass filter, and said second gm-C filter comprises a first-order low pass filter.

14. The filter apparatus as set forth in claim 12 , wherein said first gm-C filter comprises a first-order high pass filter, and said second gm-C filter comprises a second-order high pass filter.

15. The filter apparatus as set forth in claim 12 , wherein said first gm-C filter comprises a first-order low pass filter, and said second gm-C filter comprises a first-order high pass filter.

16. The filter apparatus as set forth in claim 12 , wherein said first gm-C filter comprises a first-order low pass filter, and said second gm-C filter comprises a second-order band pass filter.

17. The filter apparatus as set forth in claim 12 , wherein said first gm-C filter comprises a second-order band pass filter, and said second gm-C filter comprises a first-order high pass filter.

18. The filter apparatus as set forth in claim 12 , wherein said first gm-C filter comprises a second-order band pass filter, and said second gm-C filter comprises another second-order band pass filter.

19. The filter apparatus as set forth in claim 12 , wherein said first and second amplitude detectors have the same structure.

20. The filter apparatus as set forth in claim 12 , wherein respective orders of said first and second gm-C filters are lower than an order of said single slave gm-C filter.