JPH08307205A - Multiple input filter circuit - Google Patents

Abstract

(57) [Summary] [Object] To provide a positive feedback type quadratic function multi-input low-pass filter in which the required number of operational amplifiers OP is 1, and the mounting efficiency of a printed circuit board is improved. [Structure] _A resistor R _A having one end connected to a non-inverting input terminal of an operational amplifier OP, and a capacitor C ₁ having _one end connected to an output terminal of the operational amplifier OP and the other end connected to the other end of the resistor R _A. , A capacitor C ₂ connected between the non-inverting input terminal of the operational amplifier OP and the common line, and a resistor R connected between the inverting input terminal and output terminal of the operational amplifier OP.
_B , a resistor R _C connected between the inverting input terminal of the operational amplifier OP and a common line, and a plurality of input resistors R _{1 to} R _n each having one end connected to a common connection point of the resistor R _A and the capacitor C _1. And.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a low-pass filter used in a measuring circuit or an input circuit of a protective relay and represented by a quadratic function, and more particularly to a filter circuit having a plurality of analog inputs.

[0002]

2. Description of the Related Art Conventionally, as a low-pass filter represented by a quadratic function using an operational amplifier (operational amplifier), circuits of positive feedback type, multiple feedback type, state variable type, etc. are often used. The positive feedback type low-pass filter is as shown in FIG.
An operational amplifier OP, resistors R ₁ and R ₂ connected in series to an input terminal of the operational amplifier OP, a capacitor C ₁ connected between a common connection point of the resistors R ₁ and R ₂ and an output terminal of the operational amplifier OP, It is composed of an input terminal of the operational amplifier OP and a capacitor C ₂ connected between the ground.

The transfer function of the positive feedback circuit of FIG. 2 is expressed by the following equation.

[0004]

[Equation 1]

Here, S = jω (j is an imaginary unit, ω is an angular frequency of the input V ₁ , Q is a sharpness, H is a circuit constant, and ω ₀ is a cutoff frequency). Rewriting using ω,

[0006]

[Equation 2]

It is expressed as (V ₀ is an output voltage).

A multi-feedback low-pass filter is shown in FIG.
As described above, the low-pass filters of the state variable type are each configured as shown in FIG. The transfer function of these multiple feedback type and state variable type circuits is expressed by the following equation.

[0009]

(Equation 3)

This equation (3) is expressed by the same equation as the positive feedback type transfer function equation (2) except that the polarity is opposite.

Next, a low-pass filter when there are a plurality of inputs will be described. There is a method of preparing one low-pass filter for one input and adding the output of each low-pass filter, but this requires the same number of low-pass filters as the number of inputs. For this reason, this method is uneconomical, and the characteristics of each low-pass filter vary, so it is not a very good method.

However, when the state variable type low-pass filter shown in FIG. 4 is used, the inverting input terminal of the first-stage operational amplifier OP ₁ is in an imaginary short-circuit with the non-inverting input terminal at the common potential, so that addition is performed. Operates as a circuit. Resistors R ₁ more number of inputs as shown in the order 5
A multi-input low-pass filter can be realized simply by preparing ~ R _n .

A multi-input low-pass filter can also be realized by using a multiple feedback type filter circuit, for example, the circuit (bandpass filter) described in Japanese Utility Model Publication No. 6-9545 shown in FIG. A multi-feedback low-pass filter having n inputs and using the circuit of FIG. 6 is configured as shown in FIG. The circuit system of FIG. 7 requires only one operational amplifier as compared with the state variable circuit of FIG. 5, and is economical.

The output V ₀ of the circuit of FIG.
When expressed using the resistance R,

[0015]

[Equation 4]

[0016] Now where R _A / R ₁ = H ₁ , R _A /
_{R 2 = H 2 ... R A} / R n ... H n,

[0017]

(Equation 5)

[0018]

(Equation 6)

Putting it aside,

[0020]

(Equation 7)

Therefore, it can be seen that a multi-input low-pass filter is obtained.

3 to 7, C ₁ , C ₂ , C _A , and C _B
Is a capacitor, R _{1 to} R _n , R _{A to} R _E are resistors, OP, OP
_{1 to} OP ₃ are operational amplifiers.

[0023]

As described above, a multi-input low-pass filter could be realized by using a multiple-feedback type filter circuit, but a positive-feedback type in which the phase characteristics are inverted has a multi-input low-pass filter. The filter was not realized. Therefore, in the multiple-input low-pass filter having the positive feedback type phase output, as shown in FIG. 8, the inverter 10 including the resistors R _C and R _D and the operational amplifier OP ₂ is connected to the output side of the circuit of FIG. , Composed of two operational amplifiers in total.

However, the recent mounting density of printed boards has become extremely high, and there is a demand for omitting as much as possible even one resistor can be omitted. Nevertheless, adding one operational amplifier OP ₂ as an inverter as in the circuit of FIG. 8 is very difficult for the designer of the printed board and also leads to an increase in cost. Therefore, in a multi-input low-pass filter having a positive feedback type phase output, realization with one operational amplifier has been desired.

The present invention has been made in view of the above points, and an object thereof is to provide a positive feedback type quadratic function multiple-input low-pass filter in which the required number of operational amplifiers is one and the mounting efficiency of a printed board is improved. To provide.

[0026]

According to the present invention, in a positive feedback type low pass filter having one operational amplifier and represented by a quadratic function, one end is connected to a non-inverting input terminal of the operational amplifier. One resistor, a first capacitor having one end connected to the output terminal of the operational amplifier and the other end connected to the other end of the first resistor, and a non-inverting input terminal of the operational amplifier and a common potential line. A connected second capacitor, a second resistor connected between the inverting input terminal and the output terminal of the operational amplifier, and a third resistor connected between the inverting input terminal of the operational amplifier and the common potential line. , And a plurality of input resistors each having one end connected to a common connection point of the first resistor and the first capacitor.

[0027]

The circuit of the present invention operates as a multi-input low-pass filter represented by a positive feedback type quadratic function by one operational amplifier, a capacitor and a resistor without using a plurality of operational amplifiers.

[0028]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. In FIG. 1, one end of a resistor R _A and one end of a capacitor C ₂ are connected to the non-inverting input terminal of the operational amplifier OP. The other end of the capacitor C ₂ is connected to the common line. A resistor R _B is connected between the inverting input terminal and the output terminal of the operational amplifier OP. Operational amplifier OP
A resistor R _C is connected between the inverting input terminal and the common line. One end of the capacitor C ₁ is connected to the output terminal of the operational amplifier OP, and one end of each of the _n resistors R _{1 to} R _n is connected to the common connection point of the other end of the capacitor C _{1 and} the other end of the resistor _RA. It is connected.

The figure _{i 1, i 2 ~i n,} i A, i B, i C denotes the current flowing through the illustrated arrows moiety, V _A is the common connection point voltage of the resistor R ₁ ~R _n, V _B Is the voltage at the non-inverting input terminal of the operational amplifier OP, V _C is the voltage at the inverting input terminal of the operational amplifier OP, V ₁ , V _{2 to} V _n are the input voltages, and V _O is the output voltage.

In the circuit configured as described above, the relational expression (8) shown in the following "" is established. V _C = V _B i _C = V _O / (R _B + R _C ) = V _C / R _C i _B = jωC ₂ V _B = (V _A −V _B ) / R _A V _A = V _B + i _{B RA i A = jωC 1 (V O} -V A) i 1 = (V 1 -V A) / R 1 i 2 = (V 2 -V A) / R 2 · · · i n = (V n -V A _{) / R n i 1 + i} 2 + ... + i n = i a + i B ...... (8) i 1 from these equations _{(8), i 2 ... i} n, i a, i B, i C,
When V _A , V _B and V _C are erased and V _O is arranged,

[0031]

(Equation 8)

It becomes here,

[0033]

[Equation 9]

[0034]

[Equation 10]

[0035]

[0036]

[Equation 11]

Then, now here,

[0038]

(Equation 12)

[0039]

[0040]

(Equation 13)

Therefore, it can be seen that a multi-input low-pass filter is realized in the same manner as the positive feedback type transfer function described in the above equations (1) and (2).

In the above equations (8) to (14), j is an imaginary unit, ω is an angular frequency of the input voltage, Q is a sharpness, H is a circuit constant, and ω ₀ is a cutoff frequency.

[0043]

As described above, according to the present invention, a first resistor having one end connected to the non-inverting input terminal of the operational amplifier, one end connected to the output terminal of the operational amplifier, and the other end connected to the first terminal A first capacitor connected to the other end of the resistor 1; a second capacitor connected between the non-inverting input terminal of the operational amplifier and a common potential line; and an inverting input terminal of the operational amplifier and an output terminal. The connected second resistor, the third resistor connected between the inverting input terminal of the operational amplifier and the common potential line, and one end connected to the common connection point of the first resistor and the first capacitor, respectively. Since it has a plurality of input resistors, it is possible to realize a positive feedback type quadratic function multiple-input low-pass filter with one operational amplifier, a capacitor, and a resistor.

Therefore, there are advantages such as (1) improvement in reliability by reducing the number of parts, (2) reduction in printed circuit board design time due to reduction in the number of parts, and (3) realization of a high-density circuit for the printed board by reducing the number of parts. It is possible to obtain a great effect in improving reliability and reducing cost.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention.

FIG. 2 is a circuit diagram of a positive feedback low-pass filter represented by a conventional quadratic function.

FIG. 3 is a circuit diagram of a conventional multiple-feedback low-pass filter.

FIG. 4 is a circuit diagram of a conventional state variable low-pass filter.

FIG. 5 is a circuit diagram of a conventional state variable low-pass filter having n inputs.

FIG. 6 is a circuit diagram of a filter circuit described in Japanese Utility Model Publication No. 6-9545.

FIG. 7 is a circuit diagram of a conventional multi-feedback low-pass filter having n inputs.

FIG. 8 is a circuit diagram of a conventional low-pass filter having two inputs of n operational amplifiers.

[Explanation of symbols]

_{C 1, C 2, C A} , C B ... capacitor _{R 1 ~R n, R A ~R} E ... resistance OP, OP ₁ ~OP ₃ ... op

Claims (1)

[Claims] 1. A positive feedback low-pass filter having one operational amplifier, which is expressed by a quadratic function, wherein one end is connected to a non-inverting input terminal of the operational amplifier, and one end is connected to the first resistor. A first capacitor connected to the output terminal of the operational amplifier, the other end of which is connected to the other end of the first resistor, and a second capacitor connected between the non-inverting input terminal of the operational amplifier and the common potential line. A second resistor connected between the inverting input terminal and the output terminal of the operational amplifier, a third resistor connected between the inverting input terminal of the operational amplifier and the common potential line, and one end of the first resistor And a plurality of input resistors respectively connected to a common connection point of the first capacitors.