JP3324338B2 - Active filter circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an active filter circuit, and more particularly to an active filter circuit used for arbitrarily adjusting a gain in a filter pass frequency band.

[0002]

2. Description of the Related Art FIG. 8 is a circuit diagram of an inversion type active filter circuit 10 which has been conventionally known. 8, the active filter circuit 10 connects the first impedance unit 1 between the inverting input terminal (−) of the operational amplifier A1 and the external input terminal 3, and connects the output terminal of the operational amplifier A1 to the external output terminal. 4 and the second impedance unit 2 is connected between the output terminal of the operational amplifier and the inverting input terminal (-) to form a feedback circuit, and the non-inverting input terminal (+) of the operational amplifier A1 is connected to It is configured to be connected to the external input terminal 5. Here, the first and second impedance units 1 and 2 are elements or a network composed of resistors, capacitors, and the like, and have impedance values of Za and Zb, respectively.

[0003]

When such an active filter circuit 10 is constructed as a circuit such as a low-pass filter, a delay equalizer, etc., the gain characteristic in the pass frequency band is different between the actual characteristic and the design characteristic. There was a problem that a large deviation occurred between them. This characteristic deviation occurs when the operational amplifier to be used is caused by a decrease in input impedance, an occurrence of input capacitance, a gain attenuation in a high frequency region, and the like as compared with an ideal operational amplifier. As can be guessed, it is very difficult to reduce the deviation of the frequency characteristics by taking measures against each of these causes.

Therefore, a main object of the present invention is to make it possible to easily reduce the deviation between the actual characteristic and the design characteristic in the gain characteristic in the pass frequency band.
An active filter circuit is provided.

[0005]

SUMMARY OF THE INVENTION An active filter circuit according to the present invention has the following features to attain the above object.
Is connected between the inverting input terminal and the external input terminal of the operational amplifier, the output terminal of the operational amplifier is connected to the external output terminal, and the second impedance portion is connected to the output terminal of the operational amplifier. connected between the inverting input terminal to form a feedback circuit, in the active filter circuit connected to the non-inverting input terminal of said operational amplifier to an external input terminal, said first impedance unit and before
The second impedance section is ideal for the operational amplifier.
It has a value so that the design characteristic can be obtained if it has the characteristic.
And connecting a third impedance part having a reactance property between a connection point between the inverting input terminal of the operational amplifier and the second impedance part in the feedback circuit and the ground, thereby improving design characteristics. Misalignment
It is characterized by being reduced .

A first impedance section is connected between an inverting input terminal and an external input terminal of the operational amplifier, an output terminal of the operational amplifier is connected to the external output terminal, and a second impedance section is connected. in the active filter circuit connected to form a feedback circuit between the output terminal and the inverting input terminal, and connecting the non-inverting input terminal of the operational amplifier to the external input terminal of the operational amplifier and the previous
A first impedance section and the second impedance section;
The sense unit is used when the operational amplifier has ideal characteristics.
It has a value to obtain the design characteristics,
Connecting a fourth impedance unit having an impedance characteristic in parallel with the second impedance unit in the feedback circuit.
Thus, the deviation from the design characteristics is reduced .

Further, the first impedance section is added to the calculation.
Connected between the inverting input terminal of the band and the external input terminal,
When connecting the output terminal of the operational amplifier to the external output terminal
The second impedance section is connected to the output of the operational amplifier.
Connected between the input terminal and the inverting input terminal to form a feedback circuit,
Connect the non-inverting input terminal of the operational amplifier to the external input terminal
In the active filter circuit described above,
The impedance section and the second impedance section are
Design characteristics are obtained when the operational amplifier has ideal characteristics.
And have reactance
A third impedance section that is connected to the feedback circuit;
An inverting input terminal of the operational amplifier and the second impedance
Between the ground connection point and ground, and
A fourth impedance unit having an actance property is connected in parallel with the second impedance unit in the feedback circuit.
To reduce the deviation from the design characteristics.
Features.

[0008]

[0009]

[0010]

[0011]

[0012]

According to the above construction, in the active filter circuit, the impedance section connected to the feedback circuit is provided with a capacitive reactance or an inductive reactance so that the gain characteristic in the pass frequency band can be changed according to the frequency. Can be adjusted. Therefore, it is possible to easily reduce the difference between the actual characteristic and the design characteristic in the gain characteristic in the pass frequency band.

[0013]

FIG. 1 is a circuit diagram showing an inversion type active filter circuit 20 according to a first embodiment of the present invention. Note that the same or equivalent parts as those of the conventional active filter circuit 10 are denoted by the same reference numerals and description thereof will be omitted.

The active filter circuit 20 has a third impedance section 6 connected between the inverting input terminal (-) of the operational amplifier A1 and the ground. Here, the third
Is an element or a network composed of a resistor, a capacitor, a coil, and the like, and has an impedance value of Zc.

Hereinafter, in the gain characteristic in the pass frequency band, the gain characteristic of the active filter circuit 20 is changed by the change of the impedance value Zc of the third impedance section 6, whereby the gain of the conventional active filter circuit 10 is changed. A description will be given of how the characteristic changes in comparison with the characteristic.

In the conventional active filter circuit 10, the voltage applied to the external input terminal is Vin, the voltage output from the external output terminal 4 is Vout, and the voltage at the external input terminal 5 is Vo. Further, if Kirchhoff's law is applied assuming that the current flowing into the operational amplifier A1 is zero, that is, the input impedance of the operational amplifier A1 is infinite, the following equation S1 is established.

[0017]

(Equation 1)

By rearranging the expression S1, the following expression S2 is established as the voltage value Vout output from the external output terminal 4.

[0019]

(Equation 2)

On the other hand, the active filter circuit 2 of the present invention
When Kirchhoff's law is applied to 0 under the same conditions as in the active filter circuit 10, the following equation S3 is established.

[0021]

(Equation 3)

When the equation S3 is arranged, the following equation S4 holds as the voltage value Vout output from the external output terminal 4.

[0023]

(Equation 4)

Comparing equation S2 with equation S4, equation S4
Is a form in which the term (Zb / Zc) · Vo is added to equation S2.

Here, as the third impedance section 6, an element or a network having a capacitive reactance such as a capacitor is used, the imaginary number is j, the angular velocity is w, and the capacitance is C.
Then, the impedance value Zc becomes 1 / jwC, and
Zc decreases as the frequency increases. As a result, (Z
The value of b / Zc) · Vo increases as the frequency increases, and the frequency characteristic curve of the gain of the active filter circuit 20 is, as shown in the frequency characteristic curve 21 of the gain of FIG.
Frequency characteristic curve 1 of gain of active filter circuit 10
As the frequency increases as compared with 1, the gain moves in the direction of increasing the gain.

As the third impedance section 6,
Assuming that an imaginary number is j, an angular velocity is w, and a self-inductance is L using an element or a network having an inductive reactance such as a coil, the value of Zc is jwL, and the impedance value Zc increases as the frequency increases. as a result,
The value of (Zb / Zc) · Vo decreases as the frequency increases, and the frequency characteristic curve of the gain of the active filter circuit 20 is, as shown in the frequency characteristic curve 22 of the gain of FIG. As the frequency increases as compared with the frequency characteristic curve 11 of FIG. As described above, when the active filter circuit 20 uses an element or a network having a capacitive reactance such as a capacitor as the third impedance section 6, the gain increases as the frequency increases, and the inductive reactance such as a coil increases. When an element or a network having the following is used, the gain decreases as the frequency increases. Therefore, by adopting a circuit configuration such as the active filter circuit 20, the gain in the frequency band of the active filter circuit can be adjusted.

FIG. 3 is a circuit diagram showing an inversion type active filter circuit 30 according to a second embodiment of the present invention.
The same reference numerals are given to the same or equivalent parts as those of the conventional active filter circuit 10 and the active filter circuit 20 according to the first embodiment of the present invention, and description thereof will be omitted.

The active filter circuit 30 is configured by connecting the fourth impedance section 7 in parallel with the second impedance section 2 forming a feedback circuit. Here, the fourth impedance section 7 is an element or a network composed of a resistor, a capacitor, and the like, and has an impedance value of Zd.

The following equation S5 holds as the combined impedance value Ze of the parallel circuit section composed of the second impedance section 2 and the fourth impedance section 7.

[0030]

(Equation 5)

Here, when an element or a circuit network having a capacitive reactance such as a capacitor is used as the fourth impedance section 7, the impedance value Zd decreases as the frequency increases, as described above. As a result, the impedance value Ze decreases as the frequency increases, as can be seen from Equation S3, and the frequency characteristic curve of the gain changes as shown by the frequency characteristic curve 31 of the gain of the active filter circuit 10 in FIG. As the frequency increases as compared to the characteristic curve 11, the gain moves in the direction of smaller gain.

As the fourth impedance section 7,
When an element or a network having an inductive reactance such as a coil is used, the impedance value Zd becomes
It increases as the frequency increases. As a result, Ze has the formula S
As can be seen from 3, the higher the frequency, the larger
The gain frequency characteristic curve moves in a direction of increasing the gain as the frequency becomes higher than the gain frequency characteristic curve 11 of the active filter circuit 10, as shown by the gain frequency characteristic curve 32 in FIG.

As described above, the active filter circuit 30
When an element or a network having a capacitive reactance such as a capacitor is used as the fourth impedance section 7, the gain decreases as the frequency increases, and an element or a network having an inductive reactance such as a coil is used. Then, the gain increases as the frequency increases. Therefore, by adopting a circuit configuration such as the active filter circuit 30, the gain in the frequency band of the active filter circuit can be adjusted.

FIG. 5 is a circuit diagram of an inversion type active filter circuit 40 according to a third embodiment of the present invention.
Note that the same or equivalent parts as those of the conventional active filter circuit 10 and the active filter circuits 20 and 30 are denoted by the same reference numerals and description thereof is omitted.

The active filter circuit 40 connects the third impedance section 6 between the inverting input terminal of the operational amplifier A1 and the ground, and connects the fourth impedance section 7 to the second impedance section 2 forming a feedback circuit. Both ends are connected in parallel with the second impedance section 2.

When an element or a network having a capacitive reactance such as a capacitor is used as the third and fourth impedance units 6 and 7, the frequency characteristic curve of the gain of the active filter circuit 40 becomes As compared with the frequency characteristic curve, as in the case of the active filter circuits 20 and 30, the third impedance unit 6 moves in the direction of increasing the gain as the frequency increases, and the fourth impedance unit 7 increases the frequency. In the direction of smaller gain.

Here, for example, the active filter circuit 1
When the frequency characteristic curve of the gain of 0 has a drop around 5 to 15 MHz as in the frequency characteristic curve 12 of the gain shown in FIG. 6, the drop of the gain in the frequency region around 5 to 15 MHz rises. Then, the impedance value Zc of the third impedance section 6 is selected (the frequency characteristic curve of the gain in this state is set to 41), so that the gain in the frequency region higher than around 10 MHz, which is extraly raised, is reduced. By selecting the impedance value Zd of the fourth impedance unit 7 (the frequency characteristic curve of the gain in this state is assumed to be 42), the frequency characteristic curve of the gain can be made flat like 42.

As described above, the active filter circuit 10
The third and fourth impedance units 6 and 7 are connected to the active filter circuit 40 to have a circuit configuration similar to that of the active filter circuit 40. By selecting the impedance values Zc and Zd, the frequency characteristic curve of the gain is flattened. be able to.

The third and fourth impedance units 6,
An element or a network having an inductive reactance such as a coil is used as the element 7, or an element or a network having an inductive reactance such as a capacitor is used in combination with an element or a network having an inductive reactance such as a coil. The active filter circuit 10
It is also possible to adjust the frequency characteristics of the gain.

FIG. 7 is a circuit diagram of a Stefan type group delay equivalent circuit 50 which is a kind of an active filter circuit according to a fourth embodiment of the present invention. The Stefan type group delay equivalent circuit 50 is configured by connecting capacitors C3 and C4 to a circuit network 50a which is a well-known Stefan type group delay equivalent circuit. The network 50a includes an operational amplifier A2, capacitors C1 and C2, resistors R1, R2, R3, R4, and R5, an external input terminal In, and an external output terminal Out. . Network 5
Of the capacitors C3 and C4 connected to Oa, the capacitor C3 is connected between the operational amplifier A2 and the ground. Further, the capacitor C4 is connected in parallel with the resistor R4.

Here, the Stefan type group delay equivalent circuit 50
Is compared with the active filter circuit 40 described above.
The network 50b corresponds to the first impedance section 1, the resistor R4 corresponds to the second impedance section 2, the capacitor C3 corresponds to the third impedance section 6, and the capacitor C4 corresponds to the fourth impedance section 7. It is equivalent.

The frequency characteristic curve of the gain of the Stefan type group delay equivalent circuit consisting of only the network 50a is shown in FIG.
When there is a dip in the vicinity of 5 to 15 MHz as in the gain characteristic curve 12 shown in FIG. 7, the gain frequency characteristic curve of the Stefan type group delay equivalent circuit is flattened similarly to the third embodiment. be able to.

[0043]

As described above, according to the active filter circuit of the present invention, the third impedance section is connected to the connection point between the inverting input terminal of the operational amplifier and the second impedance section in the feedback circuit. When a device or a circuit network having a capacitive reactance such as a capacitor is used as the third impedance unit connected to the ground, the gain increases as the frequency increases, and the coil is used as the third impedance unit. When an element or a network having an inductive reactance such as is used, the gain decreases as the frequency increases. Therefore, the gain in the frequency band of the active filter circuit can be adjusted.

When the fourth impedance section is connected in parallel with the feedback circuit and an element or a network having a capacitive reactance such as a capacitor is used as the fourth impedance section, the gain increases as the frequency increases. When an element or a network having an inductive reactance such as a coil is used as the fourth impedance section,
The gain increases as the frequency increases. Therefore, the gain in the frequency band of the active filter circuit can be adjusted.

A third impedance section is connected between the inverting input terminal of the operational amplifier and the ground, and a fourth impedance section is connected in parallel with the feedback circuit.
By selecting the impedance value of each of the third and fourth impedance units, the frequency characteristic curve of the gain of the active filter circuit can be made flat.

[0046]

As described above, according to the active filter circuit of the present invention, it is possible to easily reduce the deviation between the actual characteristic and the design characteristic in the gain characteristic in the pass frequency band. Become.

[Brief description of the drawings]

FIG. 1 is a circuit configuration diagram of an inversion type active filter circuit according to a first embodiment of the present invention.

FIG. 2 is a frequency characteristic curve diagram showing a frequency characteristic of a gain of the inversion type active filter circuit according to the first embodiment of the present invention.

FIG. 3 is a circuit configuration diagram of another inversion type active filter circuit according to a second embodiment of the present invention.

FIG. 4 is a frequency characteristic curve diagram showing a frequency characteristic of a gain of another inversion type active filter circuit according to the second embodiment of the present invention.

FIG. 5 is a circuit configuration diagram of another inversion type active filter circuit according to a third embodiment of the present invention.

FIG. 6 is a frequency characteristic curve diagram showing the frequency characteristic of the gain of another inversion type active filter circuit according to the third embodiment of the present invention.

FIG. 7 is a circuit diagram of a Stefan type group delay equivalent circuit according to a fourth embodiment of the present invention.

FIG. 8 is a circuit diagram of a conventional inversion type active filter circuit.

[Explanation of symbols]

 A1, A2 Operational amplifiers 1, 2, 6, 7 Impedance section 3, 5 External input terminal 4 External output terminal 20, 30, 40 Active filter circuit C3, C4 Capacitor 50 Stefan type group delay equivalent circuit 50a, 50b Circuit network

Continuation of the front page (56) References JP-A-51-3833 (JP, A) JP-A-61-73411 (JP, A) JP-A-59-193613 (JP, A) JP-A-50-152641 (JP, A) , A) Hikaru Hei 4-55823 (JP, U) JP-B-57-54015 (JP, B1) (58) Fields investigated (Int. Cl. ⁷ , DB name) H03H 11/12 H03H 11/04

Claims (3)

(57) [Claims] 1. A first impedance unit is connected between an inverting input terminal and an external input terminal of an operational amplifier, an output terminal of the operational amplifier is connected to an external output terminal, and a second impedance unit is connected. connected between the inverting input terminal and the output terminal of said operational amplifier to form a feedback circuit, in the active filter circuit connected to the non-inverting input terminal of said operational amplifier to an external input terminal, said first impedance unit and said The second impey
The dance part is when the operational amplifier has ideal characteristics.
A third impedance section having a value such that design characteristics can be obtained , and having a reactance property, a connection point between the inverting input terminal of the operational amplifier in the feedback circuit and the second impedance section, An active filter circuit characterized in that deviation from design characteristics is reduced by being connected to a ground. A first impedance section connected between an inverting input terminal and an external input terminal of the operational amplifier; an output terminal of the operational amplifier connected to the external output terminal; and a second impedance section. Is connected between the output terminal and the inverting input terminal of the operational amplifier to form a feedback circuit,
An active filter circuit in which the non-inverting input terminal of the operational amplifier is connected to the external input terminal, wherein the first impedance section and the second impedance section are connected to each other.
The dance part is when the operational amplifier has ideal characteristics.
Will have a value such that design features can be obtained, a fourth impedance unit having a reactive reduces the displacement between the design characteristics by connecting in parallel with the second impedance portion in the feedback circuit An active filter circuit, characterized in that: 3. The operational amplifier according to claim 1, wherein the first impedance section is an operational amplifier.
Connected between the inverting input terminal and external input terminal of
Connect the output terminal of the amplifier to the external output terminal.
And a second impedance section connected to the output terminal of the operational amplifier.
Connected between the input terminal and the inverting input terminal to form a feedback circuit.
Connect the non-inverting input terminal of the operational amplifier to the external input terminal.
In the active filter circuit, the first impedance section and the second impedance section
The dance part is when the operational amplifier has ideal characteristics.
The impedance section having a reactance property has a value such that the design characteristic is obtained.
An inverting input terminal of the operational amplifier in the feedback circuit;
Between the connection point with the second impedance section and the ground
And a fourth impedance section having reactance is connected in parallel with the second impedance section in the feedback circuit to reduce deviation from design characteristics.
Wherein the allowed, active filter circuit.