JP2006005439A - Filter circuit and amplifier circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a filter circuit and an amplifier circuit capable of reducing the power consumption while suppressing an increase in noise index. <P>SOLUTION: A voltage gain (vo/vi) in the case of a DC is set on the basis of a trans-conductance gm and resistances of resistors R1, R2. Further, the frequency characteristic of the voltage gain (vo/vi) is set by the resistance of the resistors R1, R2 and the capacitance of capacitors C1, C2. That is, the resistors R1, R2 are shared with amplification of a voltage signal and setting of the filter characteristic. Thus, the number of the resistors on a signal path is reduced more, and deterioration in the noise index due to thermal noise of the resistors is suppressed more in comparison with the case with separate provision of an amplifier section and a filter section, and provision of respective resistors for the sections. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a low-pass filter circuit and an amplifier circuit using the same.

  FIG. 11 is a diagram illustrating a general configuration example of a secondary low-pass filter circuit.

  The filter circuit shown in FIG. 11 has an input terminal Ti, an output terminal To, npn transistors QA, QB, QC, resistors RA, RB, RC, RD, and constant current circuits IB, IC.

  The npn transistor QA has a base connected to the input terminal Ti, an emitter connected to the ground line G via the resistor RA, and a collector connected to the power supply line Vcc via the resistor RB.

  The npn transistor QB has a base connected to a connection point between the npn transistor QA and the resistor RA, a collector connected to the power supply line Vcc, and an emitter connected to the ground line G via the constant current circuit IB.

Resistor RC has one terminal connected to the emitter of npn transistor QB and the other terminal connected to ground line G via capacitor CC.
The resistor RD has one terminal connected to the connection point between the resistor RC and the capacitor CC, and the other terminal connected to the ground line G via the capacitor CD.
Resistors RC and RD and capacitors CC and CD are connected in a ladder shape as shown in FIG. 11, and constitute a secondary low-pass filter circuit.

  The npn transistor QC has a base connected to a connection point between the resistor RD and the capacitor CD, a collector connected to the power supply line Vcc, an emitter connected to the ground line G via the constant current circuit IC, and an output terminal To Connected to.

  A voltage signal vi is input to the input terminal Ti from a signal source Vi to which a DC bias voltage is applied by the bias circuit VB. The voltage signal vi is amplified in an amplifier circuit composed of an npn transistor QA and resistors RA and RB, and then passed through an emitter follower circuit composed of an npn transistor QB and a constant current circuit IB. (Resistors RC and RD, capacitors CC and CD). The signal from which the high-frequency component has been removed in the low-pass filter circuit is output as a voltage vo from the output terminal To via an emitter follower circuit composed of an npn transistor QC and a constant current circuit IC.

  The transfer function (vo / vi) of the filter circuit shown in FIG. 11 is expressed by the following equation.

In the above equation, “rA” and “rB” indicate resistance values of the resistors RA and RB.
'r1' and 'r2' indicate resistance values of the resistors RC and RD.
'c1' and 'c2' indicate capacitances of the capacitors CC and CD.
'Ω0' indicates a cut-off frequency, and 'Q' indicates a quality factor.

  The filter circuit shown in FIG. 11 is a passive filter circuit whose transfer function is basically determined by a resistor and capacitor circuit. In addition to this, for example, an active filter circuit as shown in FIG. It is used.

The filter circuit shown in FIG. 12 is obtained by separating the capacitor CC in the filter circuit shown in FIG. 11 from the ground line G and connecting it to the output terminal To.
A portion constituted by resistors RC and RD, capacitors CC and CD, and an emitter follower circuit (npn transistor QC, constant current circuit IC) is a secondary active low-pass filter circuit (see FIG. 9 of Patent Document 1). .

  The transfer function (vo / vi), cut-off frequency ω 0, and quality factor Q of the filter circuit shown in FIG. 12 are each expressed by the following equations.

JP-A-11-261372

Incidentally, in the filter circuits shown in FIGS. 11 and 12, an amplifying unit (npn transistor QA, resistors RA and RB) for amplifying the voltage signal and a filter unit for determining frequency characteristics are separately provided. . For this reason, in the signal path from the input terminal Ti to the output terminal To, there are separately provided resistors RA and RB used for signal amplification and resistors RC and RD that determine the frequency characteristics of the filter.
Thus, if there are many resistors on the signal propagation path, the thermal noise generated in each resistor is superimposed on the signal, so there is a disadvantage that the noise index increases and the signal quality decreases.

  In the configuration in which the amplification unit and the filter unit are provided separately, a buffer circuit (npn transistor QB, constant current circuit IB) is required to connect the two. Therefore, the power consumption increases due to the bias current of the buffer circuit, the signal dynamic range becomes narrower due to the voltage shift (voltage shift between the base and emitter of the npn transistor QB) generated in the buffer circuit, the number of parts increases, and so on. There are disadvantages.

  The present invention has been made in view of such circumstances, and an object thereof is to provide a filter circuit capable of reducing power consumption while suppressing an increase in noise figure, and an amplifier circuit configured using such a filter circuit. It is to provide.

  In order to achieve the above object, a filter circuit of the present invention includes an input terminal and an output terminal, a transconductance amplifier circuit that converts a voltage signal input from the input terminal into a current signal with a predetermined transconductance, and outputs the current signal. A buffer circuit that outputs a voltage signal proportional to a voltage signal generated at a current signal output node of the transconductance amplifier circuit to the output terminal, one terminal is connected to the current signal output node, and the other terminal is constant. A series circuit of a first resistor and a second resistor held at a potential; a first capacitor having one terminal connected to the current signal output node and the other terminal held at a constant potential; One terminal is connected to the connection point between the first resistor and the second resistor, and the other terminal is connected to the output terminal. Has a second capacitor which is held at a constant potential.

  The amplifier circuit of the present invention includes a plurality of the filter circuits of the present invention each having a predetermined gain, and the plurality of filter circuits are connected in cascade.

According to the present invention, the gain of the voltage signal at the output terminal with respect to the voltage signal at the input terminal is, in direct current, the transconductance of the transconductance amplifier circuit, and the resistance values of the first resistor and the second resistor. It has a value according to. The frequency characteristic of the gain has characteristics corresponding to resistance values of the first resistor and the second resistor and capacitances of the first capacitor and the second capacitor.
That is, since the first resistor and the second resistor are commonly used for the amplification of the voltage signal and the setting of the filter characteristics, when the amplification unit and the filter unit are separately provided and the resistors are used respectively. In comparison, the number of resistors on the signal path is reduced.
In addition, the buffer circuit that is necessary when the amplification unit and the filter unit are provided separately becomes unnecessary, and the power consumption corresponding to the buffer circuit is reduced.

In the present invention, the transconductance amplifier circuit includes a first transistor having a base connected to the input terminal, a collector connected to the current signal output node, and one terminal serving as an emitter of the first transistor. And a third resistor connected to the other terminal and held at a constant potential. The buffer circuit has a base connected to the current signal output node and an emitter connected to the output terminal. A second transistor may be included.
In this case, the first transistor and the second transistor may be bipolar transistors.

In the present invention, the transconductance amplifier circuit includes a third transistor having a gate connected to the input terminal, a drain connected to the current signal output node, and one terminal connected to the third transistor. A third resistor connected to the source and having the other terminal held at a constant potential. The buffer circuit has a gate connected to the current signal output node and a source connected to the output terminal. A fourth transistor may be included.
In this case, the third transistor and the fourth transistor may be field effect transistors.

In the present invention, the transconductance amplifier circuit may change the transconductance according to an input control signal.
As a result, the DC gain of the voltage signal at the output terminal with respect to the voltage signal at the input terminal changes according to the control signal.

  According to the present invention, by sharing the resistor used for amplification of the voltage signal and the resistor used for setting the filter characteristics, it is possible to reduce power consumption while suppressing an increase in noise figure.

  Hereinafter, four embodiments of the present invention will be described with reference to the drawings.

<First Embodiment>
FIG. 1 is a diagram illustrating an example of a configuration of a filter circuit according to the first embodiment of the present invention.
The filter circuit shown in FIG. 1 has an input terminal Ti and an output terminal To, a transconductance amplifier circuit 10, resistors R1 and R2, capacitors C1 and C2, and a buffer circuit 20.

The input terminal Ti and the output terminal To are an embodiment of the input terminal and output terminal of the present invention.
The transconductance amplifier circuit 10 is an embodiment of the transconductance amplifier circuit of the present invention.
The resistors R1 and R2 are an embodiment of the first resistor and the second resistor of the present invention.
Capacitor C1 is an embodiment of the first capacitor of the present invention.
Capacitor C2 is an embodiment of the second capacitor of the present invention.
The buffer circuit 20 is an embodiment of the buffer circuit of the present invention.

The transconductance amplifier circuit 10 converts the voltage signal vi input from the input terminal Ti into a current signal is with a transconductance gm, and outputs the current signal is to the node N1.
The current signal is is expressed by the following equation using the voltage signal vi and the transconductance gm.

The buffer circuit 20 outputs a voltage signal vo proportional to the voltage signal generated at the node N1 to the output terminal To.
Since the buffer circuit 20 functions as an impedance converter having a high input impedance and a low output impedance, most of the current signal is flowing into the node N1 is shunted to the resistor R2 and the capacitor C2.

The resistors R1 and R2 are connected in series, and one terminal of the series circuit is connected to the node N1, and the other terminal is connected to the ground line G. In the example of FIG. 11, the resistor R2 is connected to the node N1, and the resistor R1 is connected to the ground line G.
Note that the terminal on the side not connected to the node N1 in this resistor series circuit is only required to be held at a constant potential, so that it is connected to the power supply line Vcc, for example, as in the filter circuit of FIG. May be.

  Capacitor C1 has one terminal connected to the connection point of resistors R1 and R2, and the other terminal connected to output terminal To.

  Capacitor C2 has one terminal connected to node N1 and the other terminal connected to ground line G. The other terminal may be connected to the power supply line Vcc, for example, as long as it is held at least at a constant potential.

  The transfer function (vo / vi), cut-off frequency ω0, and quality factor Q of the filter circuit shown in FIG.

In the above equation, “r1” and “r2” indicate resistance values of the resistors R1 and R2.
'c1' and 'c2' indicate capacitances of the capacitors C1 and C2.

FIG. 2 is a diagram illustrating an example of frequency characteristics of the transfer function of the filter circuit shown in FIG.
As shown in FIG. 2, DC gain A0 of the filter circuit shown in FIG. 1 has a value corresponding to transconductance gm and resistance values r1 and r2.
Further, it operates as a secondary low-pass filter at a frequency higher than the cutoff frequency ω0. That is, every time the frequency is doubled, the gain decreases by about 12 dB.

  However, at a frequency higher than the frequency ω1 shown in the following equation, the filter operates as a first-order low-pass filter due to the influence of the first-order term of the variable s included in the numerator of the equation (8).

  Therefore, by setting the values of the resistor and the capacitor so that the frequency ω1 is sufficiently larger than the frequency ω0, the filter circuit shown in FIG.

  Here, a more specific configuration of the filter circuit shown in FIG. 1 will be described.

FIG. 3 is a diagram showing a specific example of the filter circuit shown in FIG.
The filter circuit shown in FIG. 3 has an input terminal Ti and an output terminal To, resistors R1, R2, and R3, capacitors C1 and C2, npn transistors Q1 and Q2, and a constant current circuit I2.

The npn transistor Q1 has a base connected to the input terminal Ti, a collector connected to the node N1, and an emitter connected to the ground line G via the resistor R3.
Npn transistor Q1 and resistor R3 correspond to transconductance amplifier circuit 10 in the filter circuit shown in FIG. When the resistance value of the resistor R3 is “r3”, the transconductance gm is approximately equal to the reciprocal (1 / r3) of the resistance value r3.

Npn transistor Q2 has a base connected to node N1, a collector connected to power supply line Vcc, and an emitter connected to output terminal To and one terminal of constant current circuit I2. The other terminal of the constant current circuit I2 is connected to the ground line G.
The npn transistor Q2 and the constant current circuit I2 correspond to the buffer circuit 20 in the filter circuit shown in FIG.

The resistors R1 and R2 are connected in series, and one terminal of the series circuit is connected to the node N1, and the other terminal is connected to the power supply line Vcc.
Capacitor C1 has one terminal connected to the connection point of resistors R1 and R2, and the other terminal connected to output terminal To.
Capacitor C2 has one terminal connected to node N1 and the other terminal connected to ground line G.

In the filter circuit shown in FIG. 3, the voltage signal vi is generated from the signal source Vi to which the DC bias voltage is applied by the bias circuit VB, and is input to the input terminal Tin. Voltage signal vi is converted into a current signal in a transconductance amplifier circuit composed of npn transistor Q1 and resistor R3, and is output to node N1. The voltage signal at the node N1 is shifted to the low voltage side by the base-emitter voltage of the npn transistor Q2 in the buffer circuit composed of the npn transistor Q2 and the constant current circuit I2, and is output as the voltage signal vo from the output terminal To. The
The transfer function of the filter circuit shown in FIG. 3 is expressed by the equations (8) to (10) as in the filter circuit shown in FIG.

  The filter circuit shown in FIG. 1 can be configured using a bipolar transistor as shown in FIG. 3, but is not limited thereto, and can also be configured using a field effect transistor such as a MOS transistor.

FIG. 4 is a diagram showing an example in which the filter circuit shown in FIG. 1 is configured using MOS transistors.
The filter circuit shown in FIG. 4 is obtained by replacing the npn transistors Q1 and Q2 in the filter circuit shown in FIG. 3 with n-type MOS transistors Q3 and Q4, respectively, and the other configurations are the same.

In the n-type MOS transistor Q3, the gate is connected to the input terminal Ti, the drain is connected to the node N1, and the source is connected to the ground line G via the resistor R3.
The n-type MOS transistor Q4 has a gate connected to the node N1, a drain connected to the power supply line Vcc, and a source connected to the output terminal To.

  In the filter circuit shown in FIG. 4, when the transconductance of the n-type MOS transistor Q3 is 'gm (Q3)', the transconductance gm of the transconductance amplifier circuit composed of the n-type MOS transistor Q3 and the resistor R3 is approximately It is expressed as an expression.

Therefore, when r3 >> 1 / gm (Q3) is satisfied, the transconductance gm is substantially equal to the reciprocal (1 / r3) of the resistance value r3.
Conversely, when r3 << 1 / gm (Q3) is established, the transconductance gm is substantially equal to the mutual conductance gm (Q3) of the n-type MOS transistor Q3.

  The transfer function of the filter circuit shown in FIG. 4 is also expressed by the equations (8) to (10) as in the filter circuit shown in FIG.

As described above, the gain (vo / vi) of the filter circuit according to the present embodiment has a value (gm × (r1 + r2)) corresponding to the transconductance gm and the resistance values r1 and r2 in direct current. The frequency characteristic of the gain (vo / vi) has the characteristics of a second-order low-pass filter corresponding to the resistance values r1 and r2 and the capacitances c1 and c2.
That is, the resistors R1 and R2 can be shared for amplifying the voltage signal and setting the filter characteristics. Therefore, the number of resistors on the signal path can be reduced as compared to the case where the amplifying unit and the filter unit are separately provided and the resistors are respectively used as in the conventional filter circuit shown in FIGS. Can do. For example, in the filter circuit shown in FIG. 12, there are four resistors (RA, RB, RC, RD) on the signal path, whereas in the filter circuit shown in FIG. R2, R3). As a result, it is possible to improve the quality of the output signal by suppressing an increase in noise figure due to thermal noise generated by the resistor.

In addition, the buffer circuit that is necessary because the amplifier unit and the filter unit are separately provided in the conventional filter circuit is not necessary in the filter circuit according to the present embodiment. As a result, the power consumption in the buffer circuit can be reduced, and the power consumption can be reduced.
Moreover, since the voltage shift that occurs in the buffer circuit is eliminated, the dynamic range of the signal can be expanded. For example, in the filter circuit shown in FIG. 12, the signal is shifted to the low voltage side by the voltage between the gate and source of the npn transistor QA. However, in the filter circuit shown in FIG. Can be wide. As a result of the wide dynamic range, the power supply voltage can be lowered, so that the power consumption can be further reduced.

  Further, according to the filter circuit of the present embodiment, the number of necessary resistors is reduced by sharing the resistors R1 and R2 for the amplification of the voltage signal and the setting of the filter characteristics. Since the buffer circuit becomes unnecessary, the number of circuit elements is reduced, and the configuration can be simplified.

Further, the cutoff frequency ω0 and the quality factor Q can be arbitrarily set by adjusting the values of the resistors R1 and R2 and the capacitors C1 and C2.
Also, by setting the quality factor Q high, it is possible to attenuate the signal more steeply than the filter circuit shown in FIG.

<Second Embodiment>
Next, a second embodiment of the present invention will be described.
FIG. 5 is a diagram illustrating an example of a configuration of a filter circuit according to the second embodiment of the present invention.

  The filter circuit shown in FIG. 5 is obtained by disconnecting the capacitor C1 in the filter circuit shown in FIG. 1 from the output terminal To and connecting it to the ground line G. Except for this point, the configurations of the two circuits are the same. .

  The transfer function (vo / vi), cut-off frequency ω0, and quality factor Q of the filter circuit shown in FIG.

  As can be seen by comparing the equations (8) to (10) and the equations (13) to (15), the filter circuit shown in FIG. 5 has the characteristics of a second-order low-pass filter similar to the filter circuit shown in FIG. have.

FIG. 6 is a diagram showing a specific example of the filter circuit shown in FIG.
The filter circuit shown in FIG. 6 is obtained by disconnecting the capacitor C1 in the filter circuit shown in FIG. 3 from the output terminal To and connecting it to the ground line G. Except for this point, the configurations of the two circuits are the same. .
The transfer function of the filter circuit shown in FIG. 6 is expressed by equations (13) to (15), similarly to the filter circuit shown in FIG.

FIG. 7 is a diagram showing an example in which the filter circuit shown in FIG. 6 is configured using MOS transistors.
The filter circuit shown in FIG. 7 is obtained by disconnecting the capacitor C1 in the filter circuit shown in FIG. 4 from the output terminal To and connecting it to the ground line G. Except for this point, the configurations of the two circuits are the same. .
The transfer function of the filter circuit shown in FIG. 7 is expressed by equations (13) to (15), similarly to the filter circuit shown in FIG.

  As described above, the filter circuit according to the present embodiment in which the capacitor C1 is connected from the output terminal To to the ground line G operates as a secondary low-pass filter as in the filter circuit according to the first embodiment. It is possible to achieve the same effect as this.

<Third Embodiment>
Next, a third embodiment of the present invention will be described.
FIG. 8 is a diagram illustrating an example of a configuration of a filter circuit according to the third embodiment of the present invention.

  The filter circuit shown in FIG. 8 is obtained by adding npn transistors Q11 and Q12 and a resistor R4 to the filter circuit shown in FIG. 3. In other configurations, the two circuits are the same.

  Npn transistor Q11 is inserted on a wiring connecting the collector of npn transistor Q1 and node N1. That is, the collector of npn transistor Q11 is connected to node N1, and the emitter of npn transistor Q11 is connected to the collector of npn transistor Q1.

  The npn transistor Q12 has an npn transistor Q11 and an emitter connected in common. The collector of npn transistor Q12 is connected to power supply line Vcc via resistor R4. The resistor R4 has the same resistance value as the series circuit of the resistors R1 and R2, for example.

According to the above configuration, the current ratio K (= Ics / Ic1) of the collector current Ics of the npn transistor Q11 to the collector current Ic1 of the npn transistor Q1 depends on the control signal vc input between the bases of the npn transistors Q11 and Q12. Change.
For example, when the npn transistors Q11 and Q12 are transistors having the same characteristics, if the potential difference between the bases is zero, the collector currents of the two are equal and the current ratio K = 0.5.
When the base potential of the npn transistor Q11 is slightly higher than the base potential of the npn transistor Q12, the current ratio K becomes larger than 0.5. Conversely, when the base potential of the npn transistor Q11 is slightly lower than the base potential of the npn transistor Q12. The current ratio K is smaller than 0.5.

  On the other hand, when npn transistors Q1, Q11, Q12 and resistors R4, R3 are regarded as one transconductance amplifier circuit, transconductance gm is proportional to current ratio K.

As shown in Expression (8), the direct current gain of the filter circuit is proportional to the transconductance gm. Therefore, when the transconductance gm changes, the direct current gain of the filter circuit also changes accordingly.
Therefore, according to the filter circuit shown in FIG. 8, by changing the transconductance gm according to the control signal vc, the DC gain can be arbitrarily changed while keeping the cutoff frequency ω0 and the quality factor Q constant. It becomes possible.

8 is obtained by adding npn transistors Q11 and Q12 and a resistor R4 to the filter circuit shown in FIG. 3. However, the filter circuit shown in FIG. 6 may have the same configuration. Also in this case, the gain can be adjusted according to the control signal, as in the filter circuit shown in FIG.
4 and 7 can be adjusted in gain according to the control signal by adding an n-type MOS transistor having a similar function and a resistor.

<Fourth Embodiment>
Next, a fourth embodiment of the present invention will be described.
FIG. 9 is a diagram illustrating an example of a configuration of an amplifier circuit according to the fourth embodiment of the present invention.

  The amplifier circuit shown in FIG. 9 is obtained by cascading filter circuits 101 to 103 according to the first and second embodiments in multiple stages.

  In general, an amplifier circuit having a very high gain of 60 dB or more easily oscillates even when a small signal is fed back from the output unit to the input unit via a power supply line or a parasitic capacitance component. End up.

  Therefore, as shown in FIG. 9, when low-pass filter circuits each having a predetermined gain are cascaded in multiple stages, high-frequency signals are attenuated at the output of each stage. Feedback can be suppressed. As a result, it is possible to achieve a very high gain of 60 dB while effectively preventing oscillation due to high-frequency signal feedback.

  Furthermore, by using the filter circuit of the above-described embodiment as a low-pass filter circuit, it is possible to maintain the signal quality while suppressing an increase in noise figure even though the circuit is connected in multiple stages. In addition, effects such as reduction of power consumption, securing of a dynamic range, and simplification of a circuit can be achieved.

  Note that the amplifier circuit of the present embodiment may be configured by cascading filter circuits 104 to 106 according to the fourth embodiment in multiple stages as shown in FIG. 10, for example. In this case, the total gain of the amplifier circuit can be arbitrarily changed by supplying the control signal vc to the filter circuit at each stage.

  As mentioned above, although some embodiment of this invention was described, this invention is not limited only to these forms, A various variation is included.

  For example, in the above-described embodiment, an example using an npn transistor or an n-type MOS transistor has been described. However, the present invention is not limited to this, and a similar filter circuit can be configured using a pnp transistor or a p-type MOS transistor. Further, not only bipolar transistors and field effect transistors, but a similar filter circuit can be configured using other various transistors having similar functions.

It is a figure which shows an example of a structure of the filter circuit which concerns on 1st Embodiment. It is the figure which illustrated an example of the frequency characteristic of the transfer function of the filter circuit shown in FIG. FIG. 2 is a first diagram illustrating a specific example of the filter circuit illustrated in FIG. 1. FIG. 3 is a second diagram illustrating a specific example of the filter circuit illustrated in FIG. 1. It is a figure which shows an example of a structure of the filter circuit which concerns on 2nd Embodiment. FIG. 6 is a first diagram illustrating a specific example of the filter circuit illustrated in FIG. 5. FIG. 6 is a second diagram illustrating a specific example of the filter circuit illustrated in FIG. 5. It is a figure which shows an example of a structure of the filter circuit which concerns on 3rd Embodiment. It is a figure which shows an example of a structure of the amplifier circuit which concerns on 4th Embodiment. It is a figure which shows the other structural example of the amplifier circuit which concerns on 4th Embodiment. It is a 1st figure which shows the general structural example of a secondary low-pass filter circuit. It is a 2nd figure which shows the general structural example of a secondary low-pass filter circuit.

Explanation of symbols

  Ti ... input terminal, To ... output terminal, 10 ... transconductance amplifier circuit, 20 ... buffer circuit, R1 to R4 ... resistor, C1, C2 ... capacitor, Q1, Q2, Q11, Q12 ... npn transistor, Q3, Q4 ... n MOS transistor

Claims (6)

Input and output terminals;
A transconductance amplifier circuit that converts a voltage signal input from the input terminal into a current signal with a predetermined transconductance and outputs the current signal;
A buffer circuit that outputs to the output terminal a voltage signal proportional to a voltage signal generated at a current signal output node of the transconductance amplifier circuit;
A series circuit of a first resistor and a second resistor, with one terminal connected to the current signal output node and the other terminal held at a constant potential;
A first capacitor having one terminal connected to a connection point between the first resistor and the second resistor and the other terminal connected to the output terminal or held at a constant potential;
A second capacitor having one terminal connected to the current signal output node and the other terminal held at a constant potential;
A filter circuit. The transconductance amplifier circuit is
A first transistor having a base connected to the input terminal and a collector connected to the current signal output node;
A third resistor having one terminal connected to the emitter of the first transistor and the other terminal held at a constant potential;
The buffer circuit is
A second transistor having a base connected to the current signal output node and an emitter connected to the output terminal;
The filter circuit according to claim 1. The transconductance amplifier circuit is
A third transistor having a gate connected to the input terminal and a drain connected to the current signal output node;
A third resistor having one terminal connected to the source of the third transistor and the other terminal held at a constant potential;
The buffer circuit is
A fourth transistor having a gate connected to the current signal output node and a source connected to the output terminal;
The filter circuit according to claim 1. The transconductance amplifier circuit changes transconductance according to an input control signal.
The filter circuit according to claim 1. A plurality of cascaded filter circuits each having a predetermined gain;
The filter circuit is
Input and output terminals;
A transconductance amplifier circuit that converts a voltage signal input from the input terminal into a current signal with a predetermined transconductance and outputs the current signal;
A buffer circuit that outputs to the output terminal a voltage signal proportional to a voltage signal generated at a current signal output node of the transconductance amplifier circuit;
A series circuit of a first resistor and a second resistor, with one terminal connected to the current signal output node and the other terminal held at a constant potential;
A first capacitor having one terminal connected to a connection point between the first resistor and the second resistor and the other terminal connected to the output terminal or held at a constant potential;
A second capacitor having one terminal connected to the current signal output node and the other terminal held at a constant potential;
Having
Amplification circuit. The transconductance amplifier circuit changes transconductance according to an input control signal.
The amplifier circuit according to claim 5.

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