JP4438421B2 - Filter circuit and filter device having the same - Google Patents

Description

  The present invention relates to a filter circuit and a filter device having the circuit.

  Conventionally, filter devices in which a filter circuit is formed on a semiconductor substrate have been widely used in various electronic devices such as high-frequency communication devices and image processing devices.

  A wide variety of filter circuits have been considered for the filter device. In the case of an RC active filter circuit using a resistance element as a component of the filter circuit, the resistance element is formed on a semiconductor substrate. When the formed resistor is used, the accuracy of the resistance value is reduced due to variations in the formation of the resistor. Therefore, the resistance between the drain and source of the field effect transistor has come to be used as a resistance element. Yes.

  Thus, in the RC type active filter circuit in which the field effect transistor functions as a resistance element, a frequency adjustment circuit is provided in order to adjust the frequency characteristics (see Patent Document 1).

  This frequency adjustment circuit adjusts the voltage applied to the gate terminal of the field effect transistor using a reference clock signal, and adjusts the resistance value of the drain-source resistance thereby adjusting the RC type active filter circuit. The frequency characteristics were adjusted.

That is, as shown in FIG. 8, in the conventional frequency adjustment circuit 101, the reference clock signal f _ref is input to the filter circuit 102 serving as a slave having the same frequency characteristics as the RC type active filter circuit serving as a master. The phase difference between the output signal of the filter circuit 102 and the clock signal f _ref is detected by the phase detection circuit 103, and the center frequency of the filter circuit 102 is determined by the control voltage Vc generated by integrating the phase difference by the integration circuit 104. In addition to feedback control, the control voltage Vc is adjusted to the voltage applied to the gate terminal of the RC type active filter circuit as a master to adjust the frequency characteristics.
JP 2002-271173 A

  As described above, the conventional filter device uses a reference clock signal in order to adjust the frequency characteristic of the RC active filter circuit.

  However, the filter device is not necessarily provided with the reference clock signal, and if the reference clock signal is forced to be input from the outside to the filter device, the usability of the filter device may be reduced. there were.

  Further, even when a reference clock signal is provided inside the filter device, the clock signal is superimposed as a noise on the signal of the RC active filter circuit and its peripheral circuits, and the characteristics of the filter device Could deteriorate.

Therefore, in the present invention according to claim 1, the RC active filter circuit in which the field effect transistor functions as a resistance element, and the resistance value of the field effect transistor are adjusted to a set value to thereby adjust the RC active filter circuit. In the filter circuit having a frequency adjustment circuit for adjusting a frequency characteristic, the frequency adjustment circuit has a field effect transistor for adjustment having a similar characteristic to the field effect transistor, and the drain of the field effect transistor for adjustment The voltage applied to the gate terminal of the field effect transistor for adjustment is adjusted so that the resistance value between the source terminals becomes a set value, and the voltage is applied to the gate terminal of the field effect transistor of the RC active filter circuit. And the frequency adjustment circuit includes the field effect transistor and A first field effect transistor for adjustment having similar characteristics; a second field effect transistor for adjustment having a similar characteristic to the field effect transistor, and having a drain terminal connected to a source terminal of the first field effect transistor; A first input terminal connected to the drain terminal of the first field effect transistor, an inverting input terminal connected to the first power source, and an output terminal connected to the gate terminals of the first and second field effect transistors. An operational amplifier, a bipolar transistor having a collector terminal connected to the source terminal of the second field effect transistor, a non-inverting input terminal connected to the source terminal of the second field effect transistor, and a power supply voltage lower than that of the first power source A second power source is connected to an inverting input terminal, and the output terminal is connected to the base terminal of the bipolar transistor. A variable current source connected between a drain terminal of the first field effect transistor and a third power source having a power source voltage higher than that of the first power source, the variable current source comprising a fixed resistor And a plurality of resistors connected in parallel with externally controllable switches are connected in series .

According to the second aspect of the present invention, an RC active filter circuit in which a field effect transistor functions as a resistance element, and a resistance value of the field effect transistor adjusted to a set value to adjust the RC active filter circuit. In a filter device formed on a semiconductor substrate with a frequency adjustment circuit for adjusting frequency characteristics,
The frequency adjustment circuit includes an adjustment field effect transistor having similar characteristics to the field effect transistor, and the adjustment is performed so that a resistance value between a drain and a source terminal of the adjustment field effect transistor becomes a set value. Adjusting the voltage applied to the gate terminal of the field effect transistor for use, and applying the voltage to the gate terminal of the field effect transistor of the RC active filter circuit , the frequency adjusting circuit comprising: A first field effect transistor for adjustment having a similarity characteristic; a second field effect transistor for adjustment having a similarity characteristic to the field effect transistor and having a drain terminal connected to a source terminal of the first field effect transistor; A non-inverting input terminal is connected to a drain terminal of the first field effect transistor, and a first power source A first operational amplifier having an inverting input terminal connected and an output terminal connected to the gate terminals of the first and second field effect transistors; a bipolar transistor having a collector terminal connected to the source terminal of the second field effect transistor; A normal input terminal is connected to a source terminal of the second field effect transistor, an inverting input terminal is connected to a second power supply having a power supply voltage lower than that of the first power supply, and an output terminal is connected to a base terminal of the bipolar transistor. A variable current source connected between a second operational amplifier connected, a drain terminal of the first field effect transistor and a third power supply having a power supply voltage higher than the first power supply, and the variable current The source is configured by connecting in series a fixed resistor and a plurality of resistors in which externally controllable switches are connected in parallel .

  And in this invention, there exists an effect described below.

That is, in the present invention according to claim 1, the RC active filter circuit in which the field effect transistor functions as a resistance element, and the RC active filter circuit are adjusted by adjusting the resistance value of the field effect transistor to a set value. In the filter circuit having a frequency adjustment circuit for adjusting a frequency characteristic, the frequency adjustment circuit has a field effect transistor for adjustment having a similar characteristic to the field effect transistor, and the drain of the field effect transistor for adjustment The voltage applied to the gate terminal of the field effect transistor for adjustment is adjusted so that the resistance value between the source terminals becomes a set value, and the voltage is applied to the gate terminal of the field effect transistor of the RC active filter circuit. The frequency adjusting circuit is the field effect transistor A first field effect transistor for adjustment having a similarity characteristic; a second field effect transistor for adjustment having a similarity characteristic to the field effect transistor and having a drain terminal connected to a source terminal of the first field effect transistor; A first input terminal connected to the drain terminal of the first field effect transistor, an inverting input terminal connected to the first power source, and an output terminal connected to the gate terminals of the first and second field effect transistors. An operational amplifier, a bipolar transistor having a collector terminal connected to the source terminal of the second field effect transistor, a non-inverting input terminal connected to the source terminal of the second field effect transistor, and a power supply voltage lower than that of the first power source A second power source is connected to an inverting input terminal, and the output terminal is connected to the base terminal of the bipolar transistor. An operational amplifier, and a variable current source connected between a drain terminal of the first field effect transistor and a third power source having a power source voltage higher than that of the first power source. And a plurality of resistors connected in parallel with externally controllable switches connected in series, the frequency characteristics of the RC active filter circuit can be adjusted without using a reference clock signal. In addition, the shift of the center frequency caused by the parasitic capacitance of the field effect transistor that constitutes the RC active filter circuit can also be adjusted. Further, since the two field effect transistors for adjustment are cascade-connected, each field effect transistor cancels out even-order distortion from each other, and the error of the center frequency to be adjusted can be reduced. .

According to the second aspect of the present invention, an RC active filter circuit in which a field effect transistor functions as a resistance element, and a resistance value of the field effect transistor adjusted to a set value to adjust the RC active filter circuit. In a filter device formed on a semiconductor substrate with a frequency adjustment circuit for adjusting frequency characteristics,
The frequency adjustment circuit includes an adjustment field effect transistor having similar characteristics to the field effect transistor, and the adjustment is performed so that a resistance value between a drain and a source terminal of the adjustment field effect transistor becomes a set value. Adjusting the voltage applied to the gate terminal of the field effect transistor for use, and applying the voltage to the gate terminal of the field effect transistor of the RC active filter circuit , the frequency adjusting circuit comprising: A first field effect transistor for adjustment having a similarity characteristic; a second field effect transistor for adjustment having a similarity characteristic to the field effect transistor and having a drain terminal connected to a source terminal of the first field effect transistor; A non-inverting input terminal is connected to a drain terminal of the first field effect transistor, and a first power source A first operational amplifier having an inverting input terminal connected and an output terminal connected to the gate terminals of the first and second field effect transistors; a bipolar transistor having a collector terminal connected to the source terminal of the second field effect transistor; A normal input terminal is connected to a source terminal of the second field effect transistor, an inverting input terminal is connected to a second power supply having a power supply voltage lower than that of the first power supply, and an output terminal is connected to a base terminal of the bipolar transistor. A variable current source connected between a second operational amplifier connected, a drain terminal of the first field effect transistor and a third power supply having a power supply voltage higher than the first power supply, and the variable current source consists a fixed resistor, because it is composed by connecting a plurality of resistors connected in parallel controllable switch externally in series, reference click Tsu can adjust the frequency characteristic of the RC type active filter circuit without using the click signal, moreover, possible to adjust the deviation of the center frequency caused by the parasitic capacitance of the field effect transistors constituting an RC-type active filter circuit Can do. Further, since the two field effect transistors for adjustment are cascade-connected, each field effect transistor cancels out even-order distortion from each other, and the error of the center frequency to be adjusted can be reduced. .

  In the filter device according to the present invention, a filter circuit including an RC active filter circuit and a frequency adjustment circuit for adjusting the frequency characteristic thereof is formed on a semiconductor substrate.

  The RC active filter circuit is configured to make the field effect transistor function as a resistance element by utilizing the drain-source resistance of the field effect transistor.

  The frequency adjustment circuit adjusts the resistance value of the drain-source resistance of the field effect transistor by adjusting the voltage applied to the gate terminal of the field effect transistor constituting the RC type active filter, and thereby the RC type The frequency characteristics such as the center frequency and cut-off frequency of the active filter circuit are adjusted.

  Moreover, in the filter circuit according to the present invention, the frequency adjustment circuit is provided with an adjustment field effect transistor having similar characteristics (including the same characteristics) as the field effect transistor constituting the RC active filter circuit. The voltage applied to the gate terminal of the field effect transistor for adjustment is adjusted so that the resistance value between the drain and source terminals of the field effect transistor becomes a preset value, and this voltage constitutes the RC active filter circuit. The voltage is applied to the gate terminal of the field effect transistor.

  As described above, the frequency characteristic of the RC active filter circuit is adjusted by adjusting the resistance value between the drain and source terminals of the adjustment field effect transistor having similar characteristics to the field effect transistor to be a set value. Therefore, the frequency characteristics of the RC active filter circuit can be improved by adjusting the variation of the resistance value caused by the variation of the threshold voltage and the voltage / current characteristic β of the field effect transistor constituting the RC active filter circuit. In addition, it is not necessary to separately prepare a reference clock signal for adjusting the frequency characteristics of the RC type active filter circuit, and the usability of the filter device can be improved.

  Further, since the reference clock signal is not supplied to the inside of the filter device, the clock signal is not superimposed on the signal of the RC type active filter circuit or its peripheral circuit as noise, and the filter due to the influence of noise is eliminated. It is possible to prevent deterioration of the characteristics of the apparatus.

  In particular, when the setting value of the resistance between the drain and source terminals of the adjustment field effect transistor can be changed, the setting value of the field effect transistor constituting the RC active filter circuit can be changed by appropriately changing the setting value. It is also possible to adjust the frequency characteristic shift caused by the manufacturing variation of the capacitance.

  Further, when two adjustment field effect transistors are cascade-connected, each field effect transistor cancels out even-order distortion with each other, and an error in frequency to be adjusted can be reduced.

  The specific configuration of the filter circuit according to the present invention will be described below with reference to the drawings.

  The filter circuit according to the present invention includes an RC active filter circuit 1 and a frequency adjustment circuit 2 for adjusting the frequency characteristic (center frequency) thereof.

  As shown in FIG. 1, the RC-type active filter circuit 1 is composed of a biquad band-pass filter, which filters the signals input to the input terminals 3 and 4 and outputs them from the output terminals 5 and 6. Like to do.

  In this RC type active filter circuit 1, the field effect transistor M1 is made to function as a resistance element by utilizing the resistance between the drain and source terminals of the field effect transistor M1, and the control terminal is connected to the gate terminal of the field effect transistor M1. The center frequency of the RC active filter circuit 1 can be adjusted by adjusting the value of the resistance between the source and drain terminals according to the voltage Vc applied via 7. FIG. 2 is an equivalent circuit diagram of the RC active filter circuit 1.

  As shown in FIG. 3, the frequency adjustment circuit 2 has a field effect transistor M2 for adjustment having similar characteristics to the field effect transistor M1 constituting the resistance element of the RC active filter circuit 1. The voltage Vc applied to the gate terminal of the field effect transistor M2 for adjustment is adjusted so that the resistance value between the drain and source terminals of the field effect transistor M2 for use becomes a preset set value, and the voltage Vc is the RC type. The active filter circuit 1 is configured to be applied to the gate terminal of the field effect transistor M1.

  Specifically, the frequency adjustment circuit 2 connects a power supply E1 having a voltage Vref equal to the reference voltage of the RC type active filter circuit 1 to the source terminal of the field effect transistor M2 for adjustment, and a ground terminal GND to the power supply E1. On the other hand, a current source I1 that supplies a predetermined current Iref is connected to the drain terminal of the adjustment field effect transistor M2, and this current source I1 is connected to the ground terminal GND.

  The frequency adjusting circuit 2 connects the drain terminal of the field effect transistor M2 for adjustment to the normal input terminal of the operational amplifier A1, and connects the power source E2 having the voltage Vref + ΔV to the inverting input terminal of the operational amplifier A1. The ground terminal GND is connected to the power supply E2, and the output terminal of the operational amplifier A1 is connected to the gate terminal of the field effect transistor M2 for adjustment.

Thereby, the frequency adjustment circuit 2 feeds back the gate terminal of the adjustment field-effect transistor M2 with the operational amplifier A1 so that the potential of the drain terminal of the adjustment field-effect transistor M2 becomes the voltage Vref + ΔV, thereby adjusting the electric field. The resistance value R between the drain and source of the adjusting field effect transistor M2 is set such that the voltage ΔV and the current Iref are passed between the drain and source of the effect transistor M2.
R = ΔV / Iref
The set value is determined by.

  In the filter circuit having the above configuration, the adjustment field effect transistor M2 of the frequency adjustment circuit 2 has similar characteristics to the field effect transistor M1 constituting the RC active filter circuit 1, and therefore the voltage at the output terminal of the operational amplifier A1. By applying Vc to the gate terminal of the field effect transistor M1, the resistance value between the drain and source terminals of the field effect transistor M1 can be adjusted to a set value.

  As a result, in the filter circuit, the resistance value of the field effect transistor M1 of the RC active filter circuit 1 is adjusted to the set value even when manufacturing variations of the RC active filter circuit 1, temperature changes, or power supply voltage changes occur. It is possible to prevent fluctuations in the frequency characteristics of the RC active filter circuit.

  The frequency adjustment circuit 2 shown in FIG. 3 is configured using one adjustment field effect transistor M2, which is a frequency having a configuration in which two adjustment field effect transistors M3 and M4 are cascade-connected. The adjustment circuit 8 may be used.

  That is, as shown in FIG. 4, the frequency adjustment circuit 8 includes two adjustment field effect transistors M3 and M4 instead of one adjustment field effect transistor M2 of the frequency adjustment circuit 2 shown in FIG. The source terminal of this adjusting field effect transistor M3 and the drain terminal of the adjusting field effect transistor M4 are directly connected, and the gate terminals of these adjusting field effect transistors M3 and M4 are the output terminals of the operational amplifier A2. Connected to.

  The frequency adjusting circuit 8 connects a power source E3 having a voltage Vref−ΔV / 2 to the source terminal of the adjusting field effect transistor M4, and connects the ground terminal GND to the power source E3. A current source I2 that supplies a predetermined current Iref is connected to the drain terminal of the effect transistor M3, a ground terminal GND is connected to the current source I2, and a field effect transistor for adjustment is connected to the normal input terminal of the operational amplifier A2. The drain terminal of M3 is connected, while the power supply E4 having the voltage Vref + ΔV / 2 is connected to the inverting input terminal of the operational amplifier A2, and the ground terminal GND is connected to the power supply E4, thereby adjusting each field effect transistor M3 for adjustment. , M4 is applied with a voltage ΔV / 2 between the drain and source thereof.

  In this way, in the frequency adjustment circuit 8 in which the two adjustment field effect transistors M3 and M4 are cascade-connected, the respective adjustment field effect transistors M3 and M4 cancel each other out even-order distortions, and the even-order distortions. The influence of distortion can be reduced, and thereby the error in the frequency to be adjusted can be reduced.

  In addition, the voltage between the drain and source of each of the field effect transistors M3 and M4 for adjustment can be increased, and in this case, the influence of the offset voltage can also be reduced, and the adjustment is performed accordingly. Frequency error can be reduced.

  As shown in FIG. 5, two adjustment field effect transistors M3 and M4 connected in cascade are replaced with one adjustment field effect transistor M5 having the same gate width W and twice the gate length L. However, the same effect is obtained.

  In the frequency adjustment circuits 2 and 8 shown in FIGS. 3 and 4, the gate terminal voltage is adjusted by directly feedback controlling the potentials of the gate terminals of the field effect transistors M2, M3, and M4 for adjustment. However, the frequency adjustment circuit 9 can also be configured to adjust the voltage at the gate terminal by feedback controlling the current passed between the drain and source of the adjustment field effect transistors M6 and M7.

  That is, the frequency adjustment circuit 9 directly connects the source terminal of the adjustment field effect transistor M6 and the drain terminal of the adjustment field effect transistor M7 as shown in FIG. By connecting the gate terminals of the transistors M6 and M7 in common, two adjustment field effect transistors M6 and M7 are cascade-connected, and the source terminal of the adjustment field effect transistor M7 has a voltage Vref−ΔV / 2. Connect the power supply E5, connect the ground terminal GND to this power supply E5, and connect the emitter terminal of the npn-type bipolar transistor Q1 to the drain terminal of the field effect transistor M6 for adjustment, and the collector terminal of this bipolar transistor Q1 Is connected to a current source I3 that supplies a predetermined current Iref, a ground terminal GND is connected to the current source I3, and further, a bipolar transistor is connected. The collector terminal of Njisuta Q1 is connected to the gate terminal of the field effect transistors M6, M7 for adjustment.

  The frequency adjusting circuit 9 connects the drain terminal of the field effect transistor M6 for adjustment to the inverting input terminal of the operational amplifier A3, and connects the power supply E6 having the voltage Vref + ΔV / 2 to the normal input terminal of the operational amplifier A3. A ground terminal GND is connected to the power source E6, and a voltage ΔV / 2 is applied between the drain and source of each of the field effect transistors M6 and M7 for adjustment.

  Further, the frequency adjustment circuit 9 connects the output terminal of the operational amplifier A3 to the base terminal of the bipolar transistor Q1.

  In the frequency adjusting circuit 9, feedback is applied by the operational amplifier A3 so that the current Iref is supplied between the drain and source terminals of the adjusting field effect transistors M6 and M7.

  As a result, the frequency adjusting circuit 9 adjusts the voltage at the gate terminals of the adjusting field effect transistors M6 and M7 by adjusting the current passed between the drain and source of the adjusting field effect transistors M6 and M7. The voltage at the gate terminal is applied to the gate terminal of the field effect transistor M1 of the RC type active filter circuit 1 via the control terminal 7.

  In the frequency adjustment circuits 2, 8, and 9 described above, the current values at the current sources I1, I2, and I3 and the voltage values at the voltage sources E1 to E6 are constant, but these current values and voltage values are changed. As the variable current source and variable voltage source that can be made, it is also possible to change the set values of the resistance values of the field effect transistors M2 to M7 for adjustment by changing these current values and voltage values. .

  For example, the frequency adjustment circuit 10 shown in FIG. 7 is configured so that the current value generated by the current source can be changed using 4-bit data input from the outside.

  That is, the frequency adjusting circuit 10 directly connects the source terminal of the adjusting field effect transistor M8 and the drain terminal of the adjusting field effect transistor M9 as shown in FIG. By connecting the gate terminals of the transistors M8 and M9 in common, two adjustment field effect transistors M8 and M9 are cascaded, and the variable current source 11 is connected to the drain terminal of the adjustment field effect transistor M8. A ground terminal GND is connected to the variable current source 11.

  The variable current source 11 includes a fixed resistor R0, switching resistors R1 to R4 in which switches S1 to S4 are connected in parallel, and a voltage source E9 having a voltage Vref + ΔV / 2 + VR. The fixed resistor R0 and the switching resistors R1 to R4 The voltage VR is applied between the switching resistors R1 to R4, and the switches S1 to S4 connected in parallel to these switching resistors R1 to R4 are input from the outside. The combined resistance value is changed by intermittent control using the 4-bit data, so that the current value generated by the variable current value 11 can be increased or decreased.

  The frequency adjusting circuit 10 connects the source terminal of the adjusting field effect transistor M8 to the normal input terminal of the operational amplifier A4, and connects the power source E7 having the voltage Vref + ΔV / 2 to the inverting input terminal of the operational amplifier A4. The ground terminal GND is connected to the power source E7, and the gate terminals of the field effect transistors M8 and M9 for adjustment are connected to the output terminal of the operational amplifier A4.

  Further, the frequency adjustment circuit 10 connects the collector terminal of the npn-type bipolar transistor Q2 to the source terminal of the field-effect transistor M9 for adjustment, connects the ground terminal GND to the emitter terminal of the bipolar transistor Q2, While the collector terminal of the bipolar transistor Q2 is connected to the normal input terminal of the operational amplifier A5, the power supply E8 having the voltage Vref−ΔV / 2 is connected to the inverting input terminal of the operational amplifier A5, and the ground terminal GND is connected to the power supply E8. Connected.

  The frequency adjustment circuit 10 provides feedback so that the drain terminal of the adjustment field effect transistor M8 becomes the voltage Vref + ΔV / 2 by the operational amplifier A4, and the source terminal of the adjustment field effect transistor M9 becomes the voltage by the operational amplifier A5. By applying feedback so as to be Vref−ΔV / 2, the voltage of the gate terminals of the field effect transistors M8 and M9 for adjustment is adjusted, and the voltage at the gate terminal is supplied to the RC active filter via the control terminal 7 The voltage is applied to the gate terminal of the field effect transistor M1 of the circuit 1.

  In this frequency adjustment circuit 10, the set value of the resistance value between the drain and source terminals of the field effect transistors M8 and M9 for adjustment can be changed by increasing or decreasing the current value generated at the variable current value 11. By appropriately changing this set value, it is possible to adjust the deviation of the frequency characteristics due to manufacturing variations in the capacitance of the field effect transistor M1 constituting the RC type active filter circuit 1.

The circuit diagram which shows an RC type active filter circuit. Equivalent circuit diagram. The circuit diagram which shows a frequency adjustment circuit. The circuit diagram which shows a frequency adjustment circuit. Explanatory drawing which shows the field effect transistor for adjustment. The circuit diagram which shows a frequency adjustment circuit. The circuit diagram which shows a frequency adjustment circuit. The circuit diagram which shows the conventional frequency adjustment circuit.

Explanation of symbols

1 RC active filter circuit
2,8,9,10 Frequency adjustment circuit
3,4 input terminals
5,6 Output terminal 7 Control terminal
11 Variable current source
M1 field effect transistor
M2-M9 field effect transistors for adjustment
I1 to I3 Current source
E1 to E9 power supply
Q1, Q2 Bipolar transistor
A1-A5 operational amplifier

Claims (2)

An RC active filter circuit in which a field effect transistor functions as a resistance element; and a frequency adjustment circuit that adjusts a frequency characteristic of the RC active filter circuit by adjusting a resistance value of the field effect transistor to a set value. In the filter circuit,
The frequency adjustment circuit includes an adjustment field effect transistor having similar characteristics to the field effect transistor, and the adjustment is performed so that a resistance value between a drain and a source terminal of the adjustment field effect transistor becomes a set value. Adjusting the voltage applied to the gate terminal of the field effect transistor, and applying the voltage to the gate terminal of the field effect transistor of the RC active filter circuit ,
The frequency adjustment circuit includes:
A first field effect transistor for adjustment having similar characteristics to the field effect transistor;
A second field effect transistor for adjustment having a similar characteristic to the field effect transistor and having a drain terminal connected to a source terminal of the first field effect transistor;
A first input terminal connected to the drain terminal of the first field effect transistor, an inverting input terminal connected to the first power source, and an output terminal connected to the gate terminals of the first and second field effect transistors. An operational amplifier,
A bipolar transistor having a collector terminal connected to a source terminal of the second field effect transistor;
A normal input terminal is connected to a source terminal of the second field effect transistor, an inverting input terminal is connected to a second power supply having a power supply voltage lower than that of the first power supply, and an output terminal is connected to a base terminal of the bipolar transistor. A connected second operational amplifier;
A variable current source connected between a drain terminal of the first field effect transistor and a third power source having a power source voltage higher than that of the first power source;
The variable current source is a filter circuit configured by connecting in series a fixed resistor and a plurality of resistors in which externally controllable switches are connected in parallel . An RC active filter circuit in which a field effect transistor functions as a resistance element, and a frequency adjustment circuit that adjusts a frequency characteristic of the RC active filter circuit by adjusting a resistance value of the field effect transistor to a set value In the filter device formed on the substrate,
The frequency adjustment circuit includes an adjustment field effect transistor having similar characteristics to the field effect transistor, and the adjustment is performed so that a resistance value between a drain and a source terminal of the adjustment field effect transistor becomes a set value. Adjusting the voltage applied to the gate terminal of the field effect transistor, and applying the voltage to the gate terminal of the field effect transistor of the RC active filter circuit ,
The frequency adjustment circuit includes:
A first field effect transistor for adjustment having similar characteristics to the field effect transistor;
A second field effect transistor for adjustment having a similar characteristic to the field effect transistor and having a drain terminal connected to a source terminal of the first field effect transistor;
A first input terminal connected to the drain terminal of the first field effect transistor, an inverting input terminal connected to the first power source, and an output terminal connected to the gate terminals of the first and second field effect transistors. An operational amplifier,
A bipolar transistor having a collector terminal connected to a source terminal of the second field effect transistor;
A normal input terminal is connected to a source terminal of the second field effect transistor, an inverting input terminal is connected to a second power supply having a power supply voltage lower than that of the first power supply, and an output terminal is connected to a base terminal of the bipolar transistor. A connected second operational amplifier;
A variable current source connected between a drain terminal of the first field effect transistor and a third power source having a power source voltage higher than that of the first power source;
The variable current source is a filter device configured by connecting in series a fixed resistor and a plurality of resistors in which externally controllable switches are connected in parallel .

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