JPH0619216Y2 - Active filter circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention relates to an active filter circuit.

(Prior Art) FIG. 2 is a circuit diagram of a conventional active filter circuit. In FIG. 2, 1 is an active filter circuit, 2
Is an amplifier circuit used for gain adjustment of the active filter circuit 1.

The active filter circuit 1 includes an operational amplifier 6 having a first input section 3, a second input section 4 and an output section 5. The input terminal 7 of this active filter circuit 1 and the operational amplifier 6
The first resistor 8 and the series circuit of the second resistor 9 and the first capacitor 10 are connected in parallel with the first input unit 3. A fourth resistor 11 is connected between the input terminal 7 and the second input section 4 of the operational amplifier 6, and a fifth resistor 12 is connected between the second input section 4 and the ground. The third resistor 13 is provided between the output section 5 of the operational amplifier 6 and the first input section 3.
Is connected to the connection between the second resistor 9 and the first capacitor 10 and the third
The second capacitors 14 are connected between the resistor 13 and the connection between the output part 5 of the operational amplifier 6 and the second capacitor 14, respectively.

(Problems to be solved by the invention) By the way, the input voltage is Vi, the output voltage of the output unit 5 is Vo, the gain thereof is H, the angular frequency represented by jω is S, the cutoff frequency is ωc, and the transmission zero point is When the frequency is ωp and the sharpness is Q, the general form of the reaching function Vo / Vi is
When the active filter circuit represented by the above is applied to the active filter circuit 1 having the above-mentioned configuration, its transfer function is represented by the following expression (2).

_{However, H = H '= 5 /} (R 4 + R 5) ωP 2 = X = (1 / C 1 C 2 R 2) · _{[1 / R 2 -R 4 / (} R 5 R 1) ] .omega.c / Q ＝ Y ₁ ＝ (1 / R ₃ ) ・ (1 / C ₁ + 1 / C ₂ ) ＝ (R ₄ / R ₅ ) ・ {(1 / R ₁ ) ・ (1 / C ₁ + 1 / C ₂ ) + 1 / (C ₂ R ₂ )} ωc ² = Y ₂ = 1 / (C ₁ C ₂ R ₂ R ₃ ), therefore, as is clear from the above equations (1) and (2), the active filter circuit 1 having the above configuration The gain is H '=
It will be expressed as R ₅ / (R ₄ + R ₅ ). Since such a gain is always a value smaller than 1, conventionally, as shown in FIG. 2, from the practical point of view, the gain adjusting amplifier circuit 2 is used.
Was connected to the output section 5 of the active filter circuit 1.

Therefore, in the conventional active filter circuit 1, since it is necessary to provide such an amplifier circuit 2 at the output portion, the cost is high.

Therefore, an object of the present invention is to provide an active filter circuit which is configured so as to obtain a sufficient gain without using an amplifier circuit for gain adjustment and which can be manufactured at low cost.

Embodiment An embodiment of the present invention will be described in detail below with reference to the drawings. FIG. 1 is a circuit diagram of an active filter circuit according to an embodiment of the present invention, and parts corresponding to those in FIG. 2 are designated by the same reference numerals.

This active filter circuit 20 uses an operational amplifier 6 having a first input section 3, a second input section 4 and an output section 5, and a first resistor between the input terminal 7 and the first input section 3 of the operational amplifier 6. 8 and a series circuit of the second resistor 9 and the first capacitor 10 are connected in parallel, and a fourth resistor 11 is connected between the input end 7 and the second input portion 4 of the operational amplifier 6,
A fifth resistor 12 is connected between the second input unit 4 and the ground, and a series circuit including a third resistor 13 and a sixth resistor 15 between the output unit 5 of the operational amplifier 6 and the first input unit 3. Connect the second
The connection between the resistor 9 and the first capacitor 10 and the third resistor 13
Between the second resistor 14 and the connecting portion of the sixth resistor 15
And a seventh resistor 16 is connected between the connection between the third resistor 13 and the sixth resistor 15 and the ground.

The resistance values of the first to seventh resistors 8 to 16 are respectively set to R _{1 to} R ₇
When the capacitances of the first and second capacitors 10 and 14 are C ₁ and C ₂ , respectively, the transfer function Vo / Vi of the active filter circuit is represented by the following equation (3).

However, H ″ = {[R ₅ / (R ₄ + R ₅ )] ・ [(R ₆ + R ₇ ) / R ₇ ] − [R ₄ / (R ₄ + R ₅ )] ・₆・ (1 / R _{1 + 1 / R 2)},} A 1 = 1 / R 3 -R 4 / (R 5 R 1) - (R 4 R 6 R 7) / _{[R 5 · (R 6 + R} 7) · R 1 R 3 ] , A ₂ = 1 / (C ₁ C ₂ R ₂ ), A ₃ = 1- (R ₄ / R ₅ ) ・ [R ₆ R ₇ / (R ₆ + R ₇ )] ・ (1 / R ₁ + 1 / R ₂ ), B ₁ = (1 / R ₃ ) ・ (1 / C ₁ + 1 / C ₂ ), B ₂ = 1 / (C ₁ C ₂ R ₂ R ₃ ), and [(R ₅ / R ₄ ) ・ (1 / R ₃ ) − (1 / R ₁ )] ・ (C ₁ + C ₂ ) ＝ {[(R ₆ R ₇ ) / (R ₆ + R ₇ )] ・ [1 / (R ₁ R ₂ R ₃ )]} · {(R ₁ + R ₂ ) · (C ₁ + C ₂ ) + C ₂ R ₃ } + C ₁ / R ₂ Therefore, in the active filter circuit 20 represented by this transfer function, its gain H ″ is R ₆ and R ₇
When the values are approximated to values sufficiently smaller than the respective values of R ₁ , R ₂ and R ₃ , they are expressed by the following equation (4).

H ″ = [R ₅ / (R ₄ + R ₅ )] · [(R ₆ + R ₇ ) / R ₇ ] (4) Then, in the active filter circuit 20, the gain is [R ₅ / ( [R ₄ + R ₅ )] is greater than 1 [(R ₆ +
R ₇ ) / R ₇ ], which is a value larger than the conventional value, and as a result, the amplifier circuit used for adjusting the gain of the output of the active filter circuit in the related art can be omitted.

Note that the gain of the output of the active filter circuit 20 can be set to a value larger than 1 by appropriately setting the resistance value and the capacitance value of each resistor and capacitor without performing the above approximation.

Further, in the circuit configuration of the embodiment, the gain can be adjusted within the allowable range of the frequency characteristics of the active filter circuit 20 by variably adjusting the ratio of the resistance values of the sixth resistor 15 and the seventh resistor 16. Become. Further, the value of the sharpness Q in the active filter circuit 20 of the embodiment is the same as that of the conventional example, and the sharpness of the active filter circuit 20 of the embodiment is the sixth resistance 15 and the seventh resistance 1.
Since it is not involved in 6, even if its frequency characteristic is shifted,
Readjustment is possible by the resistor 8, the sixth resistor 15, and the seventh resistor 17.

(Effect) As is apparent from the above description, according to the present invention,
Since a simple circuit configuration in which two resistors are simply added to the output section of the operational amplifier can be used to obtain a sufficient gain without using an amplifier circuit for gain adjustment, the cost can be reduced. An active filter circuit can be provided.

[Brief description of drawings]

FIG. 1 is a circuit diagram of an active filter circuit according to an embodiment of the present invention, and FIG. 2 is a circuit diagram of a conventional active filter circuit. 3 ... 1st input part, 4 ... 2nd input part, 5 ... output part, 6 ... operational amplifier, 7 ... input terminal, 8, 9, 11, 12, 13, 1
5, 16 ... Resistor, 10, 14 ... Capacitor.

Claims (1)

[Scope of utility model registration request] 1. An input voltage is Vi, an output voltage is Vo, a gain is H,
When the angular frequency represented by jω is S, the cutoff frequency is ωc, the frequency of the transfer zero point is ωp, and the sharpness is Q, the transfer function Vo / Vi is It is an active filter circuit which realizes the characteristic represented by the following, wherein an operational amplifier having a first input section, a second input section and an output section is used, and a first resistor is provided between the input end and the first input section of the operational amplifier. And a series circuit of the second resistor and the first capacitor are connected in parallel, and the input terminal and the second circuit of the operational amplifier are connected.
A fourth resistor is connected between the input unit and a fifth resistor between the second input unit and the ground, and a third resistor and a third resistor are connected between the output unit and the first input unit of the operational amplifier. A series circuit with a sixth resistor is connected, and a second capacitor is connected between a connecting portion between the second resistor and the first capacitor and a connecting portion between the third resistor and the sixth resistor, and the third resistor and the third resistor are connected together. A seventh resistor is connected between the connection portion with the 6 resistor and the ground, and the resistance values of the _first to seventh resistors are R _{1 to} R ₇ , respectively, and the capacitances of the first and second capacitors are Each
When C ₁ and C ₂ are used, the transfer function Vo / Vi of the active filter circuit is expressed by the following equation, However, H ″ = {[R ₅ / (R ₄ + R ₅ )] ・ [(R ₆ + R ₇ ) / R ₇ ] − [R ₄ / (R ₄ + R ₅ )] ・ R ₆・ (1 / R _{1 + 1 / R 2)}} , A 1 = 1 / R 3 -R 4 / (R 5 R 1) - (R 4 R 6 R 7) / _{[R 5 · (R 6 + R} 7) · R 1 R 3 ], A ₂ = 1 / (C ₁ C ₂ R ₂ ), A ₃ = 1- (R ₄ / R ₅ ) ・ [R ₆ R ₇ / (R ₆ + R ₇ )] ・ (1 / R ₁ + 1 / R ₂ ), B ₁ = (1 / R ₃ ) ・ (1 / C ₁ + 1 / C ₂ ), B ₂ = 1 / (C ₁ C ₂ R ₂ R ₃ ), and [(R ₅ / R ₄ ) ・ (1 / R ₃ ) − (1 / R ₁ )] ・ (C ₁ + C ₂ ) ＝ {[(R ₆ R ₇ ) / (R ₆ + R ₇ )] ・ [1 / (R ₁ R ₂ R ₃ )]} ・ {(R ₁ + R ₂ ) ・ (C ₁ + C ₂ ) + C ₂ R ₃ } + C ₁ / R ₂ , and in the above equations (1) (2), H = H ″ Relationship and S
The active filter circuit is characterized in that the capacitance and the resistance value are selected so as to match the coefficient of each.