JPH0614507Y2 - Active filter circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to an active filter circuit capable of obtaining a combined characteristic of a primary low-pass filter characteristic and a secondary notch filter characteristic.

[Conventional technology]

Conventionally, as shown in FIG. 2, for example, as shown in FIG. 2, an active filter circuit of the prior art includes an operational amplifier 2 for buffer and an operational amplifier 2 in front of a secondary notch filter 3 having an operational amplifier 30.
The first-order low-pass filter 1 is connected and configured.

[Problems to be solved by the invention]

However, in the active filter circuit described above, in order to obtain the desired combined characteristic as described above, the operational amplifier 3
In addition to 0, the operational amplifier 2 for a buffer is always required to ensure the independence of both filters 1 and 3, which hinders circuit size reduction, cost reduction, and power reduction.

Therefore, an object of the present invention is to provide an active filter circuit which uses one operational amplifier and can realize the above-mentioned combined characteristic of the primary low-pass filter characteristic and the secondary notch filter characteristic.

〔Example〕

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

The active filter circuit 4 uses an operational amplifier 50 having a first input section (that is, a positive side input section) 51, a second input section (that is, a negative side input section) 52, and an output section 53. A first resistor 71, a second capacitor 62, and a third capacitor 63, which are connected in series in this order, are connected between the first input units 51 of 50 to connect the first resistor 71 and the second resistor 71. The first capacitor 61, the fourth resistor 74, and the sixth resistor 76 are connected between the connection portion with the capacitor 62 and the ground, between the first input portion 51 and the second input portion 52 of the operational amplifier 50, respectively. Then, the third resistor 7 is connected between the connection between the second capacitor 62 and the third capacitor 63 and the ground, and between the output 53 of the operational amplifier 50.
3 and the second resistor 72 are connected to each other, the fifth resistor 75 is connected between the second input portion 52 and the output portion 53 of the operational amplifier 50, and the output portion 53 of the operational amplifier 50 is connected to the output terminal 42. Connected to.

The input voltage is V _i and the output voltage is V ₀ .
To the capacitances of the third capacitors 61 to 63, respectively.
_{1 to} C ₃ and the conductances (reciprocals of resistances) of the _{first to sixth} resistors 71 to 76 are g ₁ to g ₆ , respectively, the transfer function V _{0 of the} active filter circuit 4 concerned.
/ V _i is expressed by the following equation. Here, S is the angular frequency represented by jω.

However, On the other hand, the target combined characteristic of the primary low-pass filter characteristic and the secondary notch filter characteristic is expressed by the following equation.

Here, ω _a is the cutoff angular frequency of the primary low-pass filter, ω ₀ is the resonance point of the secondary notch filter, Q is the sharpness of the secondary notch filter, and S is the angular frequency represented by jω.

Therefore, the coefficient and the constant of the frequency S in the denominator numerator of the above equations (1) and (2) are equal to each other,
That is, K ₁ = ω _a , K ₂ = 0, K ₃ = ω _a ω ₀ ² , K ₄ =
(Ω ₀ / Q) + ω _a , K ₅ = ω ₀ ² + (ω _a ω ₀ / Q)
And the capacitance C so as to satisfy K ₆ = ω _a ω ₀ ²⁰
By selecting _{1 to} C ₃ and conductances g _{1 to} g ₆ , the active filter circuit 4 can obtain the desired composite characteristics as described above, and the active filter circuit 4 makes such selection. .

To supplement the above explanation, since the active filter circuit 4 of FIG. 1 has a shape in which the primary low-pass filter circuit and the secondary notch filter circuit are directly connected, the independence of both filter circuits It collapses, and its characteristic does not become a simple combination of the original characteristics of both filter circuits. However, the active filter circuit 4 of FIG.
Since the transfer function of is as shown in the equation (1), the active filter circuit 4 is regarded as one circuit as a whole,
As described on page 7, line 7 and below,
Capacitance C ₁ ~ so that each coefficient and constant of the frequency S in the transfer function of the equation (1) become equal to the coefficient and constant of the frequency S in the transfer function of the equation (2) representing the target composite characteristic. C ₃ and conductance g ₁ ~
If g ₆ is selected, as a result, the active filter circuit 4 of FIG.
Thus, it is possible to realize the target combined characteristic, that is, the combined characteristic of the primary low-pass filter characteristic and the secondary notch filter characteristic.

To briefly explain how to calculate the coefficient equal values, target characteristics are given by ω _a , ω _0, and Q, and these are known. Therefore, from the values of ω _a , ω ₀ and Q, all the values of K _{1 to} K ₆ can be obtained as described in the fifth and subsequent lines from the bottom of page 7. That K _{1 to} K ₆
So as to satisfy the _values, nine variables in total of C 1 -C ₃ and _g 1 to g _6, is calculated using the six equations in (1) of provisos. Here, the number of unknowns is 9, whereas the number of equations is 6, so a solution cannot be obtained as it is. However, for example, by giving three of g _{1 to} g ₃ with a certain value, the number of unknowns becomes 6, and since there are 6 equations,
All of the remaining C _{1 to} C ₃ and g _{4 to} g ₆ can be calculated and obtained. What is given is not limited to g _{1 to} g ₃ , but C
It may be _{1 to} C _3, etc., and the point is that it can be solved by giving three constants.

The calculation of the coefficient equivalence value can be performed by a well-known numerical calculation method.

[Effect of device]

As described above, according to the present invention, the combined characteristic of the primary low-pass filter characteristic and the secondary notch filter characteristic can be realized by using one operational amplifier, and as a result, the circuit can be downsized and the cost can be reduced. And power saving can be achieved.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an active filter circuit according to an embodiment of the present invention. FIG. 2 is a circuit diagram showing a conventional active filter circuit having a combined characteristic of a primary low-pass filter characteristic and a secondary notch filter characteristic. 4 ... Active filter circuit according to the embodiment, 41 ...
Input end, 42 ... Output end, 50 ... Operational amplifier, 51 ...
... first input section, 52 ... second input section, 53 ... output section,
61-63 ... Capacitors, 71-76 ... Resistors.

Claims (1)

[Scope of utility model registration request] 1. When the input voltage is V _i and the output voltage is V ₀ , the transfer function V ₀ / V _i is Where ω _a is the cutoff angular frequency of the primary low-pass filter, ω ₀ is the resonance point of the secondary notch filter, Q is the sharpness of the secondary notch filter, and S is the angular frequency represented by jω. An active filter circuit for realizing characteristics, which comprises an operational amplifier (50) having a first input section (51), a second input section (52) and an output section (53), and an input terminal (41) and an operational amplifier. A first resistor (71) and a second capacitor (62) are provided between the first input section (51) of (50).
And a third capacitor (63) connected in series in this order are connected to connect the connection between the first resistor (71) and the second capacitor (62) and the ground, and the operational amplifier (5
0) between the first input parts (51) and the second input part (52)
Between the first capacitor (61), the fourth resistor (74)
And a sixth resistor (76), respectively, between the connection between the second capacitor (62) and the third capacitor (63) and ground, and the output (5) of the operational amplifier (50).
3) Between the third resistor (73) and the second resistor (7
2) are respectively connected, and a fifth resistor (75) is provided between the second input section (52) and the output section (53) of the operational amplifier (50).
And the output (53) of the operational amplifier (50)
Is connected to the output terminal (42), the capacities of the _first to _third capacitors (61 to 63) are C _{1 to} C ₃ , respectively, and the capacitances of the first to sixth resistors (71 to 76) are Conductances g ₁ to g ₆ respectively
, The transfer function V of the active filter circuit is
₀ / _{V i} is expressed by the following equation, However, Then, K ₁ = ω _a , K ₂ = 0, K ₃ = ω _a ω ₀ ² , K
₄ = (ω ₀ / Q) + ω _a , K ₅ = ω ₀ ² + (ω _a ω ₀ /
Q) and K ₆ = ω _a ω ₀ ^{2 The} above-mentioned capacitances C _{1 to} C ₃ and conductances g _{1 to} g ₆ are selected so that the active filter circuit is characterized.