Cutoff frequency adjusting method, GmC filter circuit and semiconductor device

1. A cutoff frequency adjusting method for adjusting a cutoff frequency of a GmC filter circuit which has a capacitor and an Operational Transconductance Amplifier (OTA) circuit with a controllable Gm value and a first switch provided between an output terminal of the OTA circuit and an electrode of the capacitor, comprising:

detecting a Gm value of the OTA circuit based on a voltage of the capacitor which is charged by an output of the OTA circuit; generating a control signal to the OTA circuit using a control circuit comprising a code generating circuit coupled to an adjustable current source in response to the detected Gm value;

setting a cutoff frequency of the GmC circuit to a certain value by controlling the Gm value of the OTA circuit constant based on the control signal; and,

wherein the capacitor is charged for a period when the first switch is in an ON state.

2. The cutoff frequency adjusting method as claimed in claim 1 , wherein said detecting includes:

inputting an input signal having a certain value to the OTA circuit;

charging the capacitor by supplying the output of the OTA circuit to the capacitor for a certain time;

comparing the voltage of the capacitor which is charged and a voltage of the capacitor when the Gm value having a certain value is obtained; and

increasing the Gm value of the OTA circuit when the voltage of the capacitor which is charged is lower than the voltage of the capacitor when the Gm value has the certain value.

3. The cutoff frequency adjusting method as claimed in claim 2 , further comprising:

carrying out an initializing process by short-circuiting the capacitor prior to detecting the Gm value in said detecting.

4. The cutoff frequency adjusting method as claimed in claim 3 , further comprising:

controlling, by the control circuit internal or external to the GmC filter circuit, a timing when the capacitor is short-circuited in said carrying out the initializing process, a timing when the capacitor is charged by supplying the output of the OTA circuit to the capacitor for the certain time in said detecting, and a timing when the voltage of the capacitor which is charged and the voltage of the capacitor when the Gm value having the certain value is obtained are compared in said detecting.

5. The cutoff frequency adjusting method as claimed in claim 1 , wherein said setting supplies the control signal for controlling the Gm value of the OTA circuit constant to said OTA circuit and to each OTA circuit of one or a plurality of GmC filter circuits each having an OTA circuit identical to said OTA circuit and a capacitor identical to said capacitor in order to adjust the cutoff frequency of two or more GmC filter circuits.

6. A GmC filter circuit comprising:

an Operational Transconductance Amplifier (OTA) circuit with a controllable Gm value;

a capacitor;

a detecting circuit configured to detect a Gm value based on a voltage of the capacitor which is charged by an output of the OTA circuit;

a control circuit comprising a code generating circuit coupled to an adjustable current source for generating a control signal to set a cutoff frequency of the GmC filter circuit to a certain value by controlling the Gm value of the OTA circuit constant based on the Gm value detected by the detecting circuit; and

a first switch, provided between an output terminal of the OTA circuit and an electrode of the capacitor, and configured to charge the capacitor by supplying the output of the OTA circuit to the capacitor for a certain time.

7. The GmC filter circuit as claimed in claim 6 , wherein the detecting circuit comprises:

a comparator circuit configured to compare the voltage of the capacitor which is charged and a voltage of the capacitor when the Gm value having a certain value is obtained;

wherein the control circuit increases the Gm value of the OTA circuit when the comparator circuit outputs a signal indicating that the voltage of the capacitor which is charged is lower than the voltage of the capacitor when the Gm value has the certain value.

8. The GmC filter circuit as claimed in claim 7 , further comprising:

a second switch configured to carry out an initializing process by short-circuiting the capacitor prior to detecting the Gm value by the detecting circuit.

9. The GmC filter circuit as claimed in claim 8 , further comprising:

a third switch configured to supply the voltage of the capacitor which is charged to the comparator circuit.

10. The GmC filter circuit as claimed in claim 8 , wherein the control circuit controls a timing when the capacitor is short-circuited by the second switch, a timing when the capacitor is charged by supplying the output of the OTA circuit to the capacitor for the certain time by the first switch, and a timing when the third switch supplies the voltage of the capacitor which is charged to the comparator circuit and the comparator circuit compares the voltage of the capacitor which is charged and the voltage of the capacitor when the Gm value having the certain value is obtained.

11. A semiconductor device comprising:

a GmC filter circuit;

said GmC filter circuit comprising:

an Operational Transconductance Amplifier (OTA) circuit with a controllable Gm value;

a capacitor;

a detecting circuit configured to detect a Gm value based on a voltage of the capacitor which is charged by an output of the OTA circuit;

a control circuit comprising a code generating circuit coupled to an adjustable current source for generating a control signal to set a cutoff frequency of the GmC filter circuit to a certain value by controlling the Gm value of the OTA circuit constant based on the Gm value detected by the detecting circuit; and

a first switch, provided between an output terminal of the OTA circuit and an electrode of the capacitor, and configured to charge the capacitor by supplying the output of the OTA circuit to the capacitor for a certain time.

12. The semiconductor device as claimed in claim 11 , wherein the detecting circuit comprises:

a comparator circuit configured to compare the voltage of the capacitor which is charged and a voltage of the capacitor when the Gm value having a certain value is obtained;

wherein the control circuit increases the Gm value of the OTA circuit when the comparator circuit outputs a signal indicating that the voltage of the capacitor which is charged is lower than the voltage of the capacitor when the Gm value has the certain value.

13. The semiconductor device as claimed in claim 12 , further comprising:

a second switch configured to carry out an initializing process by short-circuiting the capacitor prior to detecting the Gm value by the detecting circuit.

14. The semiconductor device as claimed in claim 13 , further comprising:

a third switch configured to supply the voltage of the capacitor which is charged to the comparator circuit.

15. The semiconductor device as claimed in claim 13 , wherein the control circuit controls a timing when the capacitor is short-circuited by the second switch, a timing when the capacitor is charged by supplying the output of the OTA circuit to the capacitor for the certain time by the first switch, and a timing when the third switch supplies the voltage of the capacitor which is charged to the comparator circuit and the comparator circuit compares the voltage of the capacitor which is charged and the voltage of the capacitor when the Gm value having the certain value is obtained.

16. The semiconductor device as claimed in claim 11 , further comprising:

one or a plurality of GmC filter circuits each having an OTA circuit identical to said OTA circuit and a capacitor identical to said capacitor,

wherein a control signal for controlling the Gm value of the OTA circuit constant is supplied to said OTA circuit and to each OTA circuit of said one or a plurality of GmC filter circuits in order to adjust the cutoff frequency of two or more GmC filter circuits.