JP3149560B2 - 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.

[0002]

2. Description of the Related Art FIGS. 4 and 5 show an active filter circuit having a secondary phase shift characteristic.

The active filter circuit shown in FIG. 4 uses an operational amplifier AMP0 having a non-inverting input and an inverting input, and the active filter circuit shown in FIG.
0 is used.

[0004]

The active filter circuit shown in FIG. 4 requires an operational amplifier having a high purchase price, which is disadvantageous in reducing the manufacturing cost. Also, this operational amplifier has a large shape in comparison with other constituent circuit components of the active filter circuit, but when the active filter circuit is mounted on a circuit board in the form of a component, the overall shape becomes large due to the operational amplifier. Become
This is a problem in the case where it is advantageous to promote downsizing of a device incorporating the circuit board. Further, in the case where the circuit configuration of FIG. 4 has a filter characteristic of the MHz band or higher, the component of the active filter circuit including the operational amplifier itself is inexpensive and small in shape, but is sufficiently low in the MHz band or higher. However, it is difficult to realize a filter having a good filter characteristic.

On the other hand, the active filter circuit of FIG. 5 has an advantage that the circuit configuration is simpler than that of FIG. 4 because it is composed of an inductor and a transistor. There is a problem that not only the noise is a problem but also the accuracy of the filter characteristic is significantly inferior.

Therefore, in the present invention, the operation amplifier is eliminated, so that the whole is inexpensive and advantageous in miniaturization. In addition, since no inductor is used, the generation of induction noise which is a disadvantage of the inductor is achieved. Accordingly, it is an object of the present invention to provide an active filter circuit having a high-precision filter characteristic.

[0007]

In order to achieve the above object, in the active filter circuit of the present invention, when the input voltage is Vi and the output voltage is Vo, the transfer function Vo / Vi [= T ( S)], T (S) = H ｛S ² − (ωo / Q) S + ωo ² ｝ / {S ² + (ωo / Q) S + ωo ² } (1) Further, H is the gain of the filter, S Is an active filter circuit that realizes a phase shift characteristic represented by jω, ωo is a central angular frequency, and Q is a sharpness of the filter, and has a first input terminal and a second input terminal.
A first series circuit including a first capacitor and a second capacitor is connected between the first input terminal and the input section of the first amplifier, and a first resistor, a second resistor, and a second resistor are connected in parallel with the first series circuit. And a third series circuit including a second amplifier, a fourth resistor, and a third resistor is connected between the second input terminal and the output of the first amplifier. A connection between the resistor and the third resistor, a connection between the first capacitor and the second capacitor, and a connection between the first resistor and the second resistor and an output of the first amplifier are connected to a third capacitor. Connected via
The resistances of the first to fourth resistors are r ₁ to r ₄ , the capacitances of the first to third capacitors are c _{1 to} c ₃ , the amplification of the first amplifier is K, the second is
The amplification degree of the amplifier is A, and an input signal Vi,
An input signal whose phase is inverted with respect to the input signal Vi is applied to a second input terminal.
When Vi is applied, the transfer function T (S) of the active filter circuit is expressed by the following equation (2): T (S) = K ｛S ³ + Cn ₂ · S ² + Cn ₁ · S + Cn ₀ / S _{^{3 + Cd 2 S 2 + Cd}} 1 S + Cd 0} ... (2) _{However, Cn 0 = (r 3 +} r 4) / (c 1 c 2 c 3 r 1 r 2 r 3 r 4) Cn 1 = (- Ac 2 r _{1 -Ac 2 r 3 + c 1} r 4 + c 2 r 4) / (c 1 c 2 c 3 r 1 r 2 r 4) Cn 2 = 1 / (c 3 r 1) + 1 / (c 3 r 2) - A / (c ₁ r ₄ ) Cd ₀ = (r ₃ + r ₄ ) / (c ₁ c ₂ c ₃ r ₁ r ₂ r ₃ r ₄ ) Cd ₁ = [(c ₁ / r ₁ r ₂ ) + (c _{2 / r 1 r 2) +} (c 2 / r 1 r 3) - (c 2 K / r 1 r 3) + (c 2 / r 2 r 3) + (c 3 / r 2 r 3) - ( _{c 2 K / r 2 r 3} ) - (c 3 K / r 2 r 3) + (c 2 / r 1 r 4) + (c 2 / _{2 r 4) + (c 3} / r 2 r 4) - (c 3 K / r 2 r 4) ] _{/ (c 1 c 2 c 3} ) Cd 2 = _{[(c 1 c 2 / r 1} ) + ( _{c 1 c 2 / r 2)} + (c 1 c 3 / r 2) + (c 2 c 3 / r 2) - (c 1 c 3 K / r 2) - (c 2 c 3 K / r 2) _{+ (c 2 c 3 / r} 3) - (c 2 c 3 K / r 3) + (c 2 c 3 / r 4) ] _{/ (c 1 c 2 c 3} ) the equation (2) (c ₁ + C ₂ ) / c ₃ = r ₁ · r ₂ (r ₃ + r
₄ ) / (r ₁ + r ₂ ) (r ₃ r ₄ ), c ₁ / c ₂ = r ₂ / r ₁ , second order degenerate, and c ₁ / c ₂ = r ₂ / r
The following equation (3) is obtained by rearranging r ₁ = x and c ₃ / c ₂ = y, and T (S) = K {S ² + Cn ₁ · S + Cn ₀ } / {S ² + Cd ₁ · S + Cd ₀ } ... (3) where, in the formula _{(3), Cn 0 = Cd} 0 = (r 3 + r 4) 2 · y / c 2 2 r 3 2 r 4 2 (1 + x) 3, Cn 1 = -A / c 2 r ₄ x Cd ₁ = _{{[r 3 + (1-k)} r 4 ] (1 + x) + (1 -k) (r 3 + r 4) y} / c 2 r 3 r 4 x (1 + x) and the It is characterized in that the capacitance and the resistance are selected so that the coefficients of S in the denominator and the numerator of both equations (1) and (3) are equal to each other.

[0008]

According to the above arrangement, since there is no operational amplifier and no inductor, it is inexpensive as a whole and is advantageous for miniaturization. In addition, since no inductor is used, the generation of induction noise which is a drawback of the inductor is reduced. Thus, an active filter circuit having high-accuracy filter characteristics is realized.

[0009]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a basic circuit diagram of an active filter circuit according to an embodiment of the present invention.

First, when the input voltage to the input terminal IN + of the active filter circuit is Vi and the output voltage from the output terminal OUT is Vo, the combined transfer function Vo / Vi = T
(S) is represented by the following equation (1).

T (S) = H ｛S ² − (ωo / Q) S + ωo ² ｝ / {S ² + (ωo / Q) S + ωo ² } (1) where H is the gain of the filter and S is jω , Ωo is the central angular frequency, and Q is the sharpness of the filter.

Next, the active filter circuit of the present embodiment has a first input terminal IN + and a second input terminal IN-, and a first input terminal IN + and an input portion of the first amplifier AMP1 are connected between the first input terminal IN + and the input section of the first amplifier AMP1. A first series circuit consisting of one capacitor C1 and a second capacitor C2 is connected, a second series circuit consisting of a first resistor R1 and a second resistor R2 is connected in parallel to the first series circuit, and a second input A third series circuit including a second amplifier AMP2, a fourth resistor R4, and a third resistor R3 is connected between the terminal IN- and the output of the first amplifier AMP1, and a fourth resistor R4
Between the first resistor C3 and the first capacitor C1
A connection portion with the capacitor C2 is connected, and a connection portion between the first resistor R1 and the second resistor R2 and an output portion of the first amplifier AMP1 are connected via a third capacitor C3. Then, the active filter circuit of this embodiment, first to the resistance of the fourth resistor R1~R4 and r ₁ ~r ₄ respectively, said first capacitor to the capacitance of the third capacitor C1 to C3 c ₁ respectively ~c ₃ , the amplification degree of the first amplifier AMP1 is K, the amplification degree of the second amplifier AMP2 is A, the input signal Vi is input to the first input terminal IN +, and the input signal Vi is inverted in phase to the input signal Vi at the second input terminal IN−. When the signal -Vi is applied, the transfer function T (S) of the active filter circuit is expressed by the following equation (2).

T (S) = K {S ³ + Cn ₂ · S ² + Cn ₁ · S + Cn ₀ } / {S ³ + Cd ₂ S ² + Cd ₁ S + Cd ₀ } (2) where Cn ₀ = (r ₃ + r _{4) ) / (c 1 c 2 c} 3 r 1 r 2 r 3 r 4) Cn 1 = (- Ac 2 r 1 -Ac 2 r 3 + c 1 r 4 + c 2 r 4) / (c 1 c 2 c 3 r _{1 r 2 r 4) Cn 2} = 1 / (c 3 r 1) + 1 / (c 3 r 2) -A / (c 1 r 4) Cd 0 = (r 3 + r 4) / (c 1 c 2 c _{3 r 1 r 2 r 3 r} 4) Cd 1 = _{[(c 1 / r 1 r 2} ) + (c 2 / r 1 r 2) + (c 2 / r 1 r 3) - (c 2 K / r _{1 r 3) + (c 2} / r 2 r 3) + (c 3 / r 2 r 3) - (c 2 K / r 2 r 3) - (c 3 K / r 2 r 3) + (c 2 _{/ r 1 r 4) + (} c 2 / r 2 r 4) + (c 3 / r 2 r 4) - (c 3 K / r 2 r 4) ] _{/ (c 1 c 2 c 3} ) Cd 2 = _{[(C 1 c 2 / r 1} ) + (c 1 c 2 / r 2) + (c 1 c 3 / r 2) + (c 2 c 3 / r 2) - (c 1 c 3 K / r _{2) - (c 2 c 3} K / r 2) + (c 2 c 3 / r 3) - (c 2 c 3 K / r 3) + (c 2 c 3 / r 4) ] / (c ₁ c ₂ c ₃ ) This equation (2) is calculated as (c ₁ + c ₂ ) / c ₃ = r ₁ · r ₂ (r ₃ + r
₄ ) / (r ₁ + r ₂ ) (r ₃ r ₄ ), c ₁ / c ₂ = r ₂ / r ₁ , second order degenerate, and c ₁ / c ₂ = r ₂ / r
The following equation (3) is obtained by rearranging r ₁ = x and c ₃ / c ₂ = y, and T (S) = K {S ² + Cn ₁ · S + Cn ₀ } / {S ² + Cd ₁ · S + Cd ₀ } ... (3) where, in the formula _{(3), Cn 0 = Cd} 0 = (r 3 + r 4) 2 · y / c 2 2 r 3 2 r 4 2 (1 + x) 3, Cn 1 = -A / c 2 r ₄ x Cd ₁ = _{{[r 3 + (1-k)} r 4 ] (1 + x) + (1 -k) (r 3 + r 4) y} / c 2 r 3 r 4 x (1 + x) Then, the The active filter circuit according to the embodiment is characterized in that the capacitance and the resistance are selected so that the denominator and the numerator of the two equations (1) and (3) have the same S coefficient.

Therefore, by selecting the capacitance and the resistance of each capacitor so that the coefficients of S in the denominator and the numerator of the above equations (1) and (3) become equal to each other, the active filter circuit of the embodiment can obtain a desired value. Filter characteristics can be obtained.

FIG. 2 is a circuit diagram in the case where each amplifier in the basic circuit of the active filter circuit of the present invention shown in FIG. 1 is constituted by a transistor. It is attached.

In FIG. 2, the first transistor TR1
Corresponds to the first amplifier AMP1, and the second transistor TR2 corresponds to the second amplifier AMP2. The emitter of the second transistor TR2 is connected to the first input terminal IN + of FIG. Amplifier AMP2
And a connection portion between the second resistor R4 and the fourth resistor R4. The emitter of the first transistor TR1 corresponds to the output of the first amplifier AMP1 in FIG. 1, and the base thereof corresponds to the input of the first amplifier AMP1.

FIG. 3 shows a first amplifier AMP1 in the basic circuit of the active filter circuit of the present invention shown in FIG.
FIG. 2 is a circuit diagram in the case where P2 is configured by a differential amplifier AMP3, and portions corresponding to FIG. 1 are denoted by the same reference numerals.

In FIG. 3, one output of the differential amplifier AMP3 is connected to the first amplifier AMP1 and the fourth resistor R4 of FIG.
Is connected to the first input terminal IN + of FIG.
Respectively. Also, the third transistor TR
3, the emitter of which is the second amplifier AMP3 of FIG.
And its base corresponds to the input of the second amplifier AMP2.

[0019]

As is apparent from the above description, according to the present invention, by eliminating the operational amplifier, it is possible to reduce the size as a whole and to obtain an advantage in miniaturization. It is possible to provide an active filter circuit having high-accuracy filter characteristics without generating induction noise which is a drawback of the inductor.

[Brief description of the drawings]

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

FIG. 2 is a circuit diagram showing a specific example of the active filter circuit of the present invention.

FIG. 3 is a circuit diagram showing another specific example of the active filter circuit of the present invention.

FIG. 4 is a circuit diagram of a conventional active filter circuit.

FIG. 5 is a circuit diagram of another conventional active filter circuit.

[Explanation of symbols]

 AMP1 First amplifier AMP2 Second amplifier IN + First input terminal IN- Second input oscillation output C1 to C3 Capacitors R1 to R4 Resistance

Claims (1)

(57) [Claims] 1. A input voltage Vi, when the output voltage is Vo, the transfer function Vo / Vi [= T (S)] is, T (S) = H { S 2 - (ωo / Q) S + ωo 2} / {S ² + (ωo / Q) S + ωo ² } (1) where H is the gain of the filter, S is the angular frequency represented by jω, ωo is the central angular frequency, and Q is the sharpness of the filter. An active filter circuit for realizing a phase shift characteristic having a first input terminal and a second input terminal,
A first series circuit including a first capacitor and a second capacitor is connected between the first input terminal and the input section of the first amplifier, and a first resistor, a second resistor, and a second resistor are connected in parallel with the first series circuit. And a third series circuit including a second amplifier, a fourth resistor, and a third resistor is connected between the second input terminal and the output of the first amplifier. A connection between the resistor and the third resistor, a connection between the first capacitor and the second capacitor, and a connection between the first resistor and the second resistor and an output of the first amplifier are connected to a third capacitor. The first to fourth resistors are respectively represented by r ₁ to r ₄ , the capacitances of the _first to _third capacitors are respectively represented by c _{1 to} c _3, and the amplification degree of the first amplifier is Is K, the second
The amplification degree of the amplifier is A, and an input signal Vi,
An input signal whose phase is inverted with respect to the input signal Vi is applied to a second input terminal.
When Vi is applied, the transfer function T (S) of the active filter circuit is expressed by the following equation (2): T (S) = K ｛S ³ + Cn ₂ · S ² + Cn ₁ · S + Cn ₀ / S _{^{3 + Cd 2 S 2 + Cd}} 1 S + Cd 0} ... (2) _{However, Cn 0 = (r 3 +} r 4) / (c 1 c 2 c 3 r 1 r 2 r 3 r 4) Cn 1 = (- Ac 2 r _{1 -Ac 2 r 3 + c 1} r 4 + c 2 r 4) / (c 1 c 2 c 3 r 1 r 2 r 4) Cn 2 = 1 / (c 3 r 1) + 1 / (c 3 r 2) - A / (c ₁ r ₄ ) Cd ₀ = (r ₃ + r ₄ ) / (c ₁ c ₂ c ₃ r ₁ r ₂ r ₃ r ₄ ) Cd ₁ = [(c ₁ / r ₁ r ₂ ) + (c _{2 / r 1 r 2) +} (c 2 / r 1 r 3) - (c 2 K / r 1 r 3) + (c 2 / r 2 r 3) + (c 3 / r 2 r 3) - ( c ₂ K / r ₂ r ₃ ) − (c ₃ K / r ₂ r ₃ ) + (c ₂ / r ₁ r ₄ ) + (c ₂ / _{r 2 r 4) + (c} 3 / r 2 r 4) - (c 3 K / r 2 r 4) ] _{/ (c 1 c 2 c 3} ) Cd 2 = _{[(c 1 c 2 / r 1} ) + (C ₁ c ₂ / r ₂ ) + (c ₁ c ₃ / r ₂ ) + (c ₂ c ₃ / r ₂ ) − (c ₁ c ₃ K / r ₂ ) − (c ₂ c ₃ K / r ₂₎ ) + (C ₂ c ₃ / r ₃ ) − (c ₂ c ₃ K / r ₃ ) + (c ₂ c ₃ / r ₄ )] / (c ₁ c ₂ c ₃ ) ₁ + c ₂ ) / c ₃ = r ₁ · r ₂ (r ₃ + r
₄ ) / (r ₁ + r ₂ ) (r ₃ r ₄ ), c ₁ / c ₂ = r ₂ / r ₁ , second order degenerate, and c ₁ / c ₂ = r ₂ / r
The following equation (3) is obtained by rearranging r ₁ = x and c ₃ / c ₂ = y, and T (S) = K {S ² + Cn ₁ · S + Cn ₀ } / {S ² + Cd ₁ · S + Cd ₀ } ... (3) where, in the formula _{(3), Cn 0 = Cd} 0 = (r 3 + r 4) 2 · y / c 2 2 r 3 2 r 4 2 (1 + x) 3, Cn 1 = -A / c 2 r ₄ x Cd ₁ = _{{[r 3 + (1-k)} r 4 ] (1 + x) + (1 -k) (r 3 + r 4) y} / c 2 r 3 r 4 x (1 + x) and the An active filter circuit, wherein the capacitance and the resistance are selected such that the coefficients of S in the denominator and the numerator of the equations (1) and (3) are equal to each other.