JP3166681B2 - Active filter and integrated circuit for active filter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an active filter and an integrated circuit for an active filter.

[0002]

2. Description of the Related Art FIGS. 4 to 6 are circuit diagrams showing a configuration example of a conventional active low-pass filter.

First, FIG. 4 shows an active low-pass filter called a Sallen-key circuit, which comprises an operational amplifier 101, resistors 102 and 103, and capacitors 104 and 105. Although this active low-pass filter has a simple configuration, the input levels of the non-inverting input terminal (+) and the inverting input terminal (-) of the operational amplifier 101 change according to changes in the input signal. There is a drawback that the allowable range of the input signal for obtaining proper operation is restricted by the allowable range of the input level of the operational amplifier 101.

FIG. 5 shows a so-called multiple feedback type active low-pass filter.
01, resistors 202 to 204, and capacitors 205 and 206. In this active low-pass filter, the non-inverting input terminal (+) of the operational amplifier 201 is grounded, and the inverting input terminal (−) of the operational amplifier 201 is virtually grounded by the negative feedback operation performed in the operational amplifier 201. Accordingly, there is an advantage that the allowable range of the input signal for obtaining the proper operation is not restricted by the allowable range of the input level of the operational amplifier 201, and is wider than that shown in FIG.

FIG. 6 shows a so-called biquad active low-pass filter, which is composed of operational amplifiers 301 to 303, resistors 311 to 316, and capacitors 321 and 322. This biquad active low-pass filter also has an advantage in that the inverting input terminals (−) of the operational amplifiers 301 to 303 are virtually grounded, so that the allowable range of an input signal for obtaining proper operation is wide. Further, this biquad active low-pass filter includes operational amplifiers 301 and 30.
There is an advantage that a sufficiently high Q (selectivity) can be obtained by adjusting each of the resistors and capacitors connected to the switch 2.

[0006]

Incidentally, each capacitor used in this type of active filter generally needs to have a relatively large capacitance value. Therefore, when the active filter is formed by an integrated circuit, it is difficult to form these capacitors on a semiconductor chip due to the limitation of the chip area. Therefore, these capacitors are generally provided externally to the integrated circuit. However, as shown in FIGS. 4 to 6, the conventional active filter employs a configuration in which a capacitor is inserted between the input and output of an operational amplifier to form an integral element or a proportional element. In order to be used as an external capacitor of an integrated circuit, it is necessary to provide two pins for connecting both ends of the capacitor to the integrated circuit. Therefore, the conventional active filter has a problem that the number of necessary pins increases when it is configured as an integrated circuit.

SUMMARY OF THE INVENTION The present invention has been made in view of the circumstances described above, and provides an active filter that requires a small number of pins for connecting external elements, and an active filter integrated circuit for forming the active filter. It is intended to provide.

[0008]

Means for Solving the Problems As a result of studying the above problems, the present inventor has arrived at one idea. That is, the reason why the number of pins for externally attaching the impedance of the capacitor or the like to the integrated circuit in the active filter increases in the first place is that such impedance is interposed between the input and output of the operational amplifier, and one end of the impedance. Can be reduced from the operational amplifier, the number of pins to be provided on the integrated circuit can be reduced. In view of this, the inventor of the present application has intensively studied the improvement of the conventional active filter. As a result, if the following improvements are made, the present active filter has the same transfer function as the conventional active filter, and furthermore, integrates the external impedance. We concluded that an active filter with a small number of pins for connecting to the circuit could be obtained.

A. In the conventional active filter, the current supplied to the input terminal that is virtually grounded flows through an external impedance inserted between the input and output of the operational amplifier. The applied output voltage was obtained from the output terminal of the operational amplifier. On the other hand, in the present invention, such a voltage output type operational amplifier whose input terminal is virtually grounded is connected to a current operational amplifier (current output type operational amplifier) whose input terminal is virtually grounded, that is, an input terminal which is virtually grounded. The input current is replaced with a means for amplifying and outputting the current, and the external impedance is inserted between the output terminal of the current operational amplifier and a ground line or a reference power supply. Even if such a configuration change is made, the current supplied to the input terminal that is virtually grounded flows through the external impedance, so that a voltage corresponding to the output voltage of the operational amplifier before the change is applied from both ends of the external impedance. can get. In addition, after the change, one end of the external impedance may be connected to the ground line or the reference power supply, so that the number of pins required for connecting the external impedance to the integrated circuit can be reduced.

B. Change in connection between operational amplifiers If the output signal of the voltage output type operational amplifier that has been replaced with the current operational amplifier is supplied to the operational amplifier whose input terminal is virtually grounded, if no measures are taken, A part of the output current of the current output type operational amplifier flows into the subsequent operational amplifier. In this case, the transfer function between the input and output of the current operational amplifier is different from the transfer function between the input and output of the conventional voltage output type operational amplifier. Therefore, in order to avoid such a situation, the operational amplifier in the subsequent stage is replaced with a current output type operational amplifier having two output terminals, and the output current obtained from one of the output terminals is output from the current output type operational amplifier in the preceding stage. Return to terminal or input terminal. That is, all the current supplied to the input terminal of the current output type operational amplifier of the subsequent stage is covered by the output current of the current output type operational amplifier of the subsequent stage, and all the output current of the current output type operational amplifier of the preceding stage is supplied to the external impedance. It is.

The present invention has been obtained by adding the above improvement to a conventional active filter.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments will be described to make the present invention easier to understand. Such an embodiment shows one aspect of the present invention, and does not limit the present invention, and can be arbitrarily changed within the scope of the present invention.

FIG. 1 is a circuit diagram showing a configuration of a biquad active filter according to an embodiment of the present invention.
A circuit including the resistors 11, 12, and 16 and the current operational amplifiers 1 and 2 among the components of the biquad active filter is formed on a semiconductor substrate as a part of a certain analog integrated circuit 100. The capacitors 21 and 22 and the resistor 17, which are the remaining components of the biquad active filter, are provided outside the analog integrated circuit 100, and are connected via the pins 31 and 32 of the analog integrated circuit 100 to the biquad active filter. Is connected to other components.
Reference numeral 10 denotes an input terminal of the biquad active filter, and reference numeral 20 denotes an output terminal.

The biquad active filter according to this embodiment is obtained by applying the above-described improvements a and b to the biquad active filter disclosed in FIG. Therefore, the biquad active filter according to the present embodiment shown in FIG. 1 will be described below with reference to the configuration disclosed in FIG. 6 described above.

First, the resistors 11 and 16 inserted between the input and output of the biquad active filter shown in FIG.
Corresponds to the resistors 311 and 316 in FIG. 6, and the resistance of the resistor 11 is equal to the resistance of the resistor 311;
The resistance value of the resistor 16 is equal to the resistance value of the resistor 316.

In FIG. 6, the non-inverting input terminal (+) of the operational amplifier 301 is grounded, and a capacitor 321 is connected between the output terminal and the inverting input terminal (-) of the operational amplifier 301. One end of the resistor 311 is connected to the inverting input terminal (-) of the operational amplifier 301.

According to this configuration, the inverting input terminal (-) of the operational amplifier 301 is virtually grounded, and the input current supplied to the inverting input terminal (-) via the resistor 311 flows through the capacitor 321. Therefore, an output voltage corresponding to the result of integrating the input current and inverting the polarity of the integrated value is obtained from the output terminal of the operational amplifier 301.

On the other hand, in the configuration shown in FIG. 1, the operational amplifier 301 in FIG. 6 is replaced with the current operational amplifier 1, and the non-inverting input terminal (+) of the current operational amplifier 1 is grounded, and the inverting input terminal (- ) Is connected to one end of the resistor 11. Here, inside the current operational amplifier 1, a negative feedback operation is performed to match the input level of the inverting input terminal (-) with the input level of the non-inverting input terminal (+) (details will be described later). Therefore, the inverting input terminal (-) of the current operational amplifier 1 is also virtually grounded like the operational amplifier 301 in FIG. Further, in the configuration shown in FIG. 1, a capacitor 21 is interposed between the output terminal of the current operational amplifier 1 and the ground line. This capacitor 21 corresponds to capacitor 321 in FIG. 6, and its capacitance value is equal to the capacitance value of capacitor 321.

According to such a configuration, an input current flows into the virtual grounded inverting input terminal (-) via the resistor 11,
An output current having the same magnitude as the input current is output from the current operational amplifier 1 and flows into the capacitor 21. Therefore,
An output voltage corresponding to the integrated value of the input current is obtained from the output terminal of the current operational amplifier 1.

In the configuration shown in FIG. 6, the operational amplifier 3
01 is integrated and the voltage obtained by inverting the polarity of the integrated value is output from the operational amplifier 301. In the present embodiment, as described above, the voltage corresponding to the integrated value of the input current to the current operational amplifier 1 remains unchanged. Is output with the polarity of. Therefore, in the present embodiment, it is not necessary to provide an equivalent to the phase inversion circuit including the operational amplifier 303 and the resistors 314 and 315 in FIG.

Next, in FIG. 6, the non-inverting input terminal (+) of the operational amplifier 302 is grounded, and the operational amplifier 3
02 between the output terminal and the inverting input terminal (−)
The output terminal of the operational amplifier 301 is connected to the inverting input terminal (−) of the operational amplifier 302 via the resistor 312.

According to this configuration, since the inverting input terminal (−) of the operational amplifier 302 is virtually grounded, an input current corresponding to a value obtained by dividing the output voltage of the operational amplifier 301 by the resistance value of the resistor 312 is applied to the operational amplifier 302. The input current is supplied to the inverting input terminal (−),
22 and the resistance 313. As a result, a voltage drop occurs in the impedance due to the passage of the input current, and an output voltage having the polarity of the voltage drop inverted is obtained from the output terminal of the operational amplifier 302.

On the other hand, in the configuration shown in FIG. 1, the operational amplifier 302 in FIG. The non-inverting input terminal (+) of the current operational amplifier 2 is grounded, and the inverting input terminal (−) is connected to the non-inverting output terminal (+) of the current operational amplifier 1 via the resistor 12.

Here, inside the current operational amplifier 2, a negative feedback operation for matching the input level of the inverting input terminal (-) with the input level of the non-inverting input terminal (+) is performed. 6 as well as the operational amplifier 302 in FIG. Therefore, an input current obtained by dividing the output voltage of the current operational amplifier 1 by the resistance value of the resistor 12 flows into the inverting input terminal (−) of the current operational amplifier 2.

The current operational amplifier 2 has a non-inverting output terminal (+)
A current having the same magnitude as the input current is output from the non-inverting output terminal (+), and the input current has the same magnitude, and
The current whose polarity has been inverted is output from the inverted output terminal (-).

Of these output currents, the current output from the non-inverting output terminal (+) of the current operational amplifier 2 is fed back to point A, which is the output terminal of the current operational amplifier 1. Therefore,
Inverting input terminal (-) of the current operational amplifier 2 via the resistor 12
All of the current flowing into the current operational amplifier 2 is covered by the current output from the non-inverting output terminal (+) of the current operational amplifier 2, and the output current from the current operational amplifier 1 to the inverting input terminal (-) of the current operational amplifier 2 is reduced. No supply is made. The reason why such a configuration is adopted is that all of the output current of the current operational amplifier 1 flows through the capacitor 21. In the configuration shown in FIG. 1, the output current of the non-inverting output terminal (+) of the current operational amplifier 2 is fed back to the point A, but this output current is supplied to the point B in FIG. You may make it return to an end (-).

Capacitors 22 and 17 are inserted in parallel between the inverted output terminal (-) of the current operational amplifier 2 and the ground line. The inverting output terminal (-) of the current operational amplifier 2 is connected to one end of a resistor 16, and the other end of the resistor 16 is virtually grounded at the inverting input terminal (-) of the current operational amplifier 1. Therefore, the capacitor 22 and the resistors 17 and 16 are inserted in parallel between the inverted output terminal (-) of the current operational amplifier 2 and the ground line.

Here, the capacitor 22 corresponds to the capacitor 322 in FIG. 6, and its capacitance value is equal to the capacitance value of the capacitor 322. Further, the resistance value of the resistor 17 is determined based on the respective resistance values of the resistor 16 and the resistor 313 in FIG. That is, when the resistance of the resistor 313 is R1, the resistance of the resistor 316 is R2, and the resistance of the resistor 17 is R3, the resistance R3 is determined to satisfy the following relationship. R1 = R2R3 / (R2 + R3) (1)

Therefore, in the present embodiment, an input current corresponding to the output voltage of the current operational amplifier 1 divided by the resistance value of the resistor 12 is supplied to the inverting input terminal (-) of the current operational amplifier 2, and this input current And the current whose polarity is inverted is equal to that of the current operational amplifier 2.
, And this output current flows through a parallel connection of the capacitor 22 and the resistors 17 and 16. Here, the resistance value of the parallel connection of the resistors 17 and 16 is, as shown in the equation (1), the resistance value of FIG.
Is equal to the resistance value R1 of the resistor 313 in FIG. Accordingly, a voltage corresponding to the output voltage of the operational amplifier 302 in FIG. 6 is generated at the inverted output terminal (-) of the current operational amplifier 2.
This voltage is supplied to another subsequent circuit (not shown) as a final output signal of the biquad active filter shown in FIG.

As described above, the operation of each unit in this embodiment is completely equivalent to the operation of each unit shown in FIG. 6 described above. Therefore, the biquad active filter according to this embodiment is the same as that shown in FIG. It has exactly the same transfer function as a quad active filter. The biquad active filter according to the present embodiment has a function equivalent to that of FIG.
It is superior to the Biquad active filter in the above. That is, in FIG. 6 described above, since the capacitors 321 and 322 are interposed between the input and output of the operational amplifier, two connecting pins are required for each capacitor in order to make each capacitor external. However, in this embodiment, since one end of each external capacitor is grounded as shown in FIG. 1, only one connecting pin needs to be used for each capacitor. Therefore, in the case of the present embodiment, the number of pins to be provided on the analog integrated circuit for connecting the external impedance can be reduced as compared with that of FIG.

Next, the current operational amplifiers 1 and 2 used in this embodiment will be described in detail. Various types of current operational amplifiers can be considered as this type of current operational amplifier. FIG. 2 illustrates an example of the current operational amplifier 1 and FIG. 3 illustrates an example of the current operational amplifier 2. First, FIG.
, A configuration example of the current operational amplifier 1 will be described.

In FIG. 2, the sources of P-channel MOS (metal oxide semiconductor structure) transistors P1 and P2 are commonly connected. This common connection point and the positive power supply VDD
A constant current source CC1 capable of supplying a constant current 2I10 is interposed between the two. Here, the gate of the P-channel MOS transistor P1 is connected to one end of the resistor 11,
The gate of P channel MOS transistor P2 is grounded. The drain of P channel MOS transistor P1 is connected to the drain and gate of N channel MOS transistor N1 and the gate of N channel MOS transistor N2, and the source of N channel MOS transistor N1 is connected to negative power supply VSS. .
On the other hand, the drain of P-channel MOS transistor P2 is connected to the drain of N-channel MOS transistor N2.
Are connected to the negative power supply VSS.

The circuit composed of the components described above constitutes a differential amplifier. Here, the operation of the differential amplifier will be described assuming an ideal case. First, P
When the gate level of channel MOS transistor P1 matches the ground level, gate bias of the same magnitude is applied to P channel MOS transistors P1 and P2. At this time, N-channel MOS transistors N1 and N2 operate as a current mirror. Therefore, the current 2I10 of the constant current source CC1 is divided into two, and currents I10 of the same magnitude flow through the P-channel MOS transistor P1 and the P-channel MOS transistor P2, respectively.

However, the P-channel MOS transistor P
When the gate level of 1 becomes higher than the ground level, the gate bias of the P-channel MOS transistor P1 decreases, so that the current flowing to the P-channel MOS transistor P1 becomes, for example, I10-.DELTA.I, and the current flowing to the P-channel MOS transistor P2 becomes I10 + ΔI. As a result, the level of the drain of the N-channel MOS transistor N2 changes in the positive direction, and conversely, the level of the drain of the N-channel MOS transistor N1 changes in the negative direction.

The P-channel MOS transistor P1
When the gate level of the N-channel MOS transistor N2 becomes lower than the ground level, the operation is completely opposite to that described above.
The level of the drain of the S transistor N1 changes in the positive direction.

The drain of the N-channel MOS transistor N2 forms the output terminal of the differential amplifier described above, and is connected to the gate of the N-channel MOS transistor N3. The source of the N-channel MOS transistor N3 is connected to the negative power supply VSS, and the drain is connected to the positive power supply VDD via a constant current source CC2 capable of supplying a constant current I20.
It is connected to the. The drain of the N-channel MOS transistor N3 is connected to the gate of the P-channel MOS transistor P1 in the differential amplifier. That is, the output signal of the differential amplifier is N-channel MO
This is a configuration in which negative feedback is provided to the differential amplifier via the S transistor N3.

With such a configuration, the P channel M
The gate of the OS transistor P1 is always in a virtual ground state. That is, if the P-channel MOS transistor P
When the gate level of 1 is going to be higher than the ground level, the level of the drain of N-channel MOS transistor N2 changes in the positive direction, thereby increasing the gate bias of N-channel MOS transistor N3. The conductance increases, preventing an increase in the level of the gate of P-channel MOS transistor P1. Conversely, when the gate level of P-channel MOS transistor P1 is going to be lower than the ground level, the level of the drain of N-channel MOS transistor N2 changes in the negative direction.
Since the gate bias of the S transistor N3 decreases,
The conductance of N-channel MOS transistor N3 is reduced, and a decrease in the level of the gate of P-channel MOS transistor P1 is prevented. Thus, the P-channel MO
The operation of matching the level of the gate of the S transistor P1 to the ground level is always performed.

Then, a P-channel MOS transistor P
Since gate 1 is thus virtually grounded,
An input current I corresponding to a value obtained by dividing an input voltage to the input terminal 10 by a resistance value of the resistor 11 flows through the resistor 11, and a current I20 + I obtained by adding the input current I and the constant current I20 from the constant current source CC2 is N. It flows to the channel MOS transistor N3. At this time, the operating point of N-channel MOS transistor N3 is in the saturation region, and a gate voltage required for flowing drain current I20 + I to N-channel MOS transistor N3 is output from the drain of N-channel MOS transistor N2. That is, when the gate voltage required to flow drain current I20 + I is not applied to N-channel MOS transistor N3, N-channel
Increase in drain level of S transistor N3 → decrease in gate bias of P channel MOS transistor P1 →
The operation of increasing the gate voltage of the N-channel MOS transistor N3 → preventing the rise in the level of the drain of the N-channel MOS transistor N3 is performed. As a result, the gate voltage required to allow the drain current I20 + I to flow is N-channel M
This is given to the OS transistor N3.

The source of the N-channel MOS transistor N4 is connected to the negative power supply VSS, and the gate of the N-channel MOS transistor N4 is the same as that of the N-channel MOS transistor N3.
An output voltage is supplied from the drain of the OS transistor N2. Here, N-channel MOS transistor N4
Has the same size as N-channel MOS transistor N3. Therefore, when drain current I20 + I flows through N channel MOS transistor N3, drain current I20 + I also flows through N channel MOS transistor N4.
Flows.

The sources of the P-channel MOS transistors P5 and P6 are connected to the positive power supply VDD.
The drain of the N-channel MOS transistor N4 is connected to the drain and gate of the P-channel MOS transistor P5 and the P-channel MOS transistor P6.
Connected to the gate. Here, P-channel MOS
Transistors P5 and P6 form a current mirror. Therefore, when the drain current I20 + I flows through the N-channel MOS transistor N4, the P-channel MO
The drain current I20 + I also flows through the S transistor P6.

Next, the constant current source CC3 is connected to the constant current source CC2.
Similarly, the constant current source can supply the constant current I20. The constant current source CC3 has one end connected to the positive power supply VDD, and the other end connected to the drain and gate of the N-channel MOS transistor N5 and the gate of the N-channel MOS transistor N6. These N-channel MOS transistors N5 and N6 have their sources connected to the negative power supply VSS. Here, N channel MOS transistors N5 and N6 form a current mirror, and N
The constant current I20 from the constant current source CC4 flows through the channel MOS transistor N5. Therefore, the same drain current I20 flows through the N-channel MOS transistor N6.

The drain of the N-channel MOS transistor N6 is connected to the P-channel MOS transistor P
6 is connected to the drain. Where P channel M
As described above, the drain current I is supplied to the OS transistor P6.
20 + I flows. On the other hand, the constant current I20 flows through the N-channel MOS transistor N6. Therefore, the N-channel MO
The current I flows from the connection point between the drains of the S transistor N6 and the P-channel MOS transistor P6 to the outside. This is the output current of the non-inverting output terminal (+) of the current operational amplifier 1. The above is the details of the current operational amplifier 1 illustrated in FIG.

Next, an example of the configuration of the current operational amplifier 2 will be described with reference to FIG. Note that in FIG. 3, the same reference numerals are given to portions common to the respective portions in FIG. 2, and description thereof will be omitted.

This current operational amplifier 2 is obtained by adding a circuit comprising a constant current source CC4 and an N-channel MOS transistor N7 surrounded by a broken line in FIG. 3 to the current operational amplifier 1 shown in FIG. Here, the N-channel MOS transistor N7 is changed to an N-channel MOS transistor N3.
The source is connected to the negative power supply VSS, and the gate is supplied with the output voltage from the drain of the N-channel MOS transistor N2. Therefore, when drain current I20 + I flows through N channel MOS transistor N3, drain current I20 + I also flows through N channel MOS transistor N7.

The drain of the N-channel MOS transistor N7 is connected to a constant current source CC4. The constant current source CC4 is a constant current source capable of supplying a constant current I20. On the other hand, the drain current I20 + I flows through the N-channel MOS transistor N7 as described above. Therefore, the current -I is externally drawn into the N-channel MOS transistor N7. This is the output current of the inverting output terminal (-) of the current operational amplifier 2. The above is the details of the current operational amplifier 2 illustrated in FIG.

In the above embodiment, the case where the present invention is applied to a biquad active filter has been described as an example. However, the scope of the present invention is not limited to this, and the present invention is applicable to any active filter. Applicable.

[0047]

As described above, according to the present invention, an active filter having exactly the same transfer function as a conventional active filter and having a smaller number of pins for connecting an external impedance than the conventional active filter. There is an effect that a filter can be realized.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a configuration of a biquad active filter according to an embodiment of the present invention.

FIG. 2 is a circuit diagram illustrating a configuration example of a current operational amplifier 1 according to the first embodiment.

FIG. 3 is a circuit diagram illustrating a configuration example of a current operational amplifier 2 according to the first embodiment.

FIG. 4 is a circuit diagram showing a configuration example of a conventional active filter.

FIG. 5 is a circuit diagram showing a configuration example of a conventional active filter.

FIG. 6 is a circuit diagram showing a configuration example of a conventional active filter.

[Explanation of symbols]

1, 2,..., Current operational amplifier (current output type operational amplifier),
21, 22... External capacitors, 17... External resistors, 11, 12, 16,.

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int. Cl. ⁷ , DB name) H03H 11/12

Claims (4)

(57) [Claims] An input terminal is virtual grounded, and a current output type operational amplifier for outputting a current corresponding to an input current inputted to the input terminal from one or more output terminals, and one end of the current output type operational amplifier. A plurality of operation elements connected to one output terminal of the amplifier and having the other end constituted by an impedance fixed to a ground level or a reference power supply level are included. The current output from the output end of the current output type operational amplifier constituting the element to which the impedance is not connected is connected to one end of the impedance or the input end of the current output type operational amplifier constituting the preceding operation element. An active filter, characterized in that it is configured to supply. 2. An input terminal that is grounded virtually and outputs a current corresponding to a current input to the input terminal from an output terminal.
A first impedance having one end connected to the output terminal of the first current output type operational amplifier, the other end fixed to a ground level or a reference power supply level, and one end connected to the first A resistor connected to the output terminal of the current output type operational amplifier, and a current supplied to the input terminal via the resistor, the input terminal being virtually grounded and the other end of the resistor being connected to the input terminal. Is output from two output terminals, and one output terminal of the two output terminals is connected to one end of the resistor or the input terminal of the first current output type amplifier. A current output type operational amplifier, a second impedance having one end connected to the other output end of the second current output type operational amplifier, and the other end fixed to a ground level or a reference power supply level; Output current Active filter, characterized by comprising interpolated and a feedback resistor through between the other output terminal of the type operational amplifier and the input terminal of said first current output type amplifier. 3. An operational amplifier comprising: a plurality of current output type operational amplifiers whose input terminals are virtually grounded and output a current corresponding to an input current input to the input terminal from one or more output terminals;
At least two current output type operational amplifiers in the plurality of current output type operational amplifiers are connected in cascade, and an interconnection point of the two cascaded current output type operational amplifiers is an external impedance terminal. Connected to
The latter one of the two current output type operational amplifiers has at least two output terminals, and one of these output terminals is connected to the external impedance connection terminal or the former stage of the two current output type operational amplifiers. An integrated circuit for an active filter, wherein the integrated circuit is connected to an input terminal of an object. 4. An input terminal that is virtually grounded and outputs a current proportional to a current input to the input terminal from an output terminal.
And a first external impedance terminal connected to the output terminal of the first current output type operational amplifier, and one end connected to the output terminal of the first current output type operational amplifier. And the other end of the resistor is connected to the input terminal, and a current corresponding to a current input to the input terminal via the resistor is output from the two output terminals. And a second current output type operational amplifier in which one of the two output terminals is connected to one end of the resistor or an input terminal of the first current output type amplifier; and A second external impedance terminal connected to the other output terminal of the current output type operational amplifier; the other output terminal of the second current output type operational amplifier; and an input terminal of the first current output type amplifier Return interposed between Active filter integrated circuit characterized by comprising a resistor.