JPH05122008A - Active high pass filter and active blow pass filter - Google Patents

Abstract

(57) [Summary] [Object] To provide an active high-pass filter capable of switching a frequency characteristic between a second-order filter characteristic and a flat characteristic with a simple configuration. [Configuration] In a Butterworth type second-order active high-pass filter, a first loop is provided in a feedback loop 5 of an amplifier 3.
The switching element Q1 is inserted, and the resistance value of the resistor forming the time constant circuit 6 is switched to the normal resistance value R2 and the resistance value R2 + R3 sufficiently larger than the normal resistance value R2 by the second switching element Q2. The feedback loop of the amplifier 3 is closed, and the resistance value of the resistor forming the time constant circuit 6 is switched to the normal resistance value R2 to set the frequency characteristic to 2
Use the next active high-pass filter characteristic. The feedback loop 5 of the amplifier 3 is opened, and the resistance value of the resistor forming the time constant circuit 6 is switched to a resistance value R2 + R3 sufficiently larger than the normal resistance value to make the frequency characteristic flat.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a Butterworth second order active filter.

[0002]

2. Description of the Related Art Conventionally, in a circuit using a Butterworth type second-order active filter (for example, a recording or reproducing system of a tape recorder), when the frequency characteristic is switched between the filter characteristic and the flat characteristic, the following is performed. The circuit configuration was adopted. That is, as shown in FIG.
Input terminals of the Butterworth secondary active filter, here the Butterworth secondary active high-pass filter 32 and the buffer amplifier 33 are connected, and their output terminals are connected to the input terminal of the buffer amplifier 35 through the changeover switch 34, and the changeover switch 34 is connected. By switching the frequency characteristic, the high-pass filter characteristic (in the case of removing wind noise in a voice signal, the cutoff frequency is, for example, 100 H
z) and the flat characteristic. still,
36 is an output terminal.

[0003]

In the structure of the conventional example, a circuit of two systems and a changeover switch are required, and the circuit structure becomes complicated.

In view of the above point, the present invention proposes an active high-pass filter and an active low-pass filter which can switch the frequency characteristic to the Butterworth type second-order filter characteristic and the flat characteristic with a simple structure.

[0005]

According to the present invention, in a Butterworth type second-order active high-pass filter, a first switching element Q1 inserted in a feedback loop 5 of an amplifier 3 and a resistor forming a time constant circuit 6 are provided. Second switching element Q for switching the resistance value of the device to a normal resistance value R2 and a resistance value R2 + R3 sufficiently larger than the normal resistance value R2.
2 and are provided. Then, the first switching element Q1 is operated to close the feedback loop of the amplifier 3, and the second switching element Q2 is operated to change the resistance value of the resistor forming the time constant circuit 6 to the normal resistance value R2. To the second-order active high-pass filter characteristic. By operating the first switching element Q1,
The feedback loop of the amplifier 3 is opened, and the second switching element Q2 is operated to switch the resistance value of the resistor forming the time constant circuit to the resistance value R2 + R3 which is sufficiently larger than the normal resistance value to change the frequency characteristic. Set to a substantially flat characteristic.

Further, according to the present invention, in the Butterworth type second-order active low-pass filter, the first switching element Q inserted in the feedback loop 15 of the amplifier 13 is provided.
11 and the second switching element Q1 for switching the capacitance value of the capacitor forming the time constant circuit 16 to the normal capacitance value C12 + C13 and the capacitance value C12 sufficiently smaller than this.
2 and are provided. The first switching element Q11 is operated to close the feedback loop 15 of the amplifier 13, and the second switching element Q12 is operated to change the capacitance value of the capacitor forming the time constant circuit 16 to the normal capacitance value C12 + C13. The frequency characteristic is changed to a second-order active low-pass filter by switching. The first switching element Q11 is operated to open the feedback loop 15 of the amplifier 13, and the second switching element Q12 is opened.
Is operated to switch the capacitance value of the capacitor forming the time constant circuit 16 to a capacitance value C12 sufficiently smaller than the normal capacitance value C12 + C13 to make the frequency characteristic substantially flat.

[0007]

Embodiments of the present invention will be described below with reference to the drawings. It should be noted that the reference numerals R and C in FIGS. 1 to 3 respectively followed by numerals indicate a resistor and its resistance value and a capacitor and its capacitance value, respectively.

[Active High-Pass Filter of Embodiment] (FIG. 1) First, an active high-pass filter of an embodiment of the present invention will be described with reference to FIG. 1 is an input terminal, 2 is a switching signal input terminal, 3 is a non-inverting amplifier, 4 is an output terminal, 5 is a feedback loop of the amplifier 3, and 6 is a time constant circuit. The amplification factor of the amplifier 3 is the resistance value R of the resistor R5 connected between its output terminal and inverting input terminal.
5 and a resistor R connected between the inverting input terminal and ground
It is determined by the ratio of the resistance value R4 of 4.

The time constant circuit 6 will be described. The input terminal 1 is connected to the non-inverting input terminal of the amplifier 3 through the capacitors C1 and C2 sequentially. The non-inverting input terminal of the amplifier 3 is grounded through the resistors R2 and R3 sequentially. Resistor R
The midpoint of connection of 2 and R3 is grounded through the capacitor C3 and the collector-emitter of the NPN transistor Q2 for switching. A series circuit between the collector and emitter of the resistor R1 and the switching NPN transistor Q1 is connected between the connection midpoint of the capacitors C1 and C2 and the output terminal of the amplifier 3.

The feedback loop 5 includes a capacitor C
A series circuit between the collector and emitter of a resistor R1 and a switching NPN transistor Q1 connected between the midpoint of the connection of C1 and C2 and the output terminal of the amplifier 3. Incidentally, C3 is a capacitor for suppressing the DC fluctuation when the transistor Q2 is turned on and off, and has a capacitance value C3.
Is a value sufficiently larger than the capacitance values C1 and C2.

The input terminal 2 is connected to the base of the transistor Q1 through the resistor R6 and the base of the transistor Q2 through the resistor R7, and the base is grounded through the resistor R8.

Next, the operation of this active high-pass filter will be described. When a positive voltage switching signal is supplied to the input terminal 2 and both the transistors Q1 and Q2 are turned on, the feedback loop 5 is closed and
The connection midpoint of the resistors R2 and R3 is grounded. In this case, the active high-pass filter functions as a Butterworth type second-order high-pass filter, and the cutoff frequency Fc _{2 at} this time is represented by the following equation. Fc ₂ = 1 / 2π (C1, C2, R1, R2) ^1/2

When the switching signal of the ground voltage is supplied to the input terminal 2 and both the transistors Q1 and Q2 are turned off, the feedback loop 5 is opened and, equivalently, the transistor Q2 and the capacitor C3 are not provided. Becomes In this case, the active high-pass filter is a cascade circuit of the primary CR high-pass filter and the amplifier 3. The cutoff frequency Fc _{1 at} this time is represented by the following equation. Fc ₁ = 1 / 2π {(R2 + R3) · C1 · C2 / (C1 + C2)}

Here, for simplicity, C1 = C2, R1 =
Assuming R2, the cutoff frequency Fc ₁ is expressed by the following equation. Fc ₁ = 1 / πC1 · (R2 + R3)

If the resistance value R3 is sufficiently larger than the resistance value R2, for example, R3 = 60R2, the cutoff frequency Fc ₁ is expressed by the following equation. Fc ₁ = 1 / π61C1 · R2

Incidentally, when C1 = C2 and R1 = R2, Fc ₂ is expressed by the following equation. Fc ₂ = 1 / 2πC1 · R2

Therefore, C1 = C2, R1 = R2, R3 =
When the ratio of both cutoff frequencies Fc ₂ and Fc ₁ at 60R2 is taken, the value is as in the following equation. Fc ₁ / Fc ₂ = 1 / 30.5 Therefore, if Fc ₂ = 200 Hz, then Fc ₁ = 6.5.
Hz. Therefore, it can be seen that the first-order high-pass filter characteristic is regarded as a substantially flat characteristic and there is no problem.

[Active Low-Pass Filter (1) of Embodiment] (FIG. 2) Next, the active low-pass filter (1) of the embodiment will be described with reference to FIG. 11 is an input terminal, 12 is a switching signal input terminal, 13 is a non-inverting amplifier, 14 is an output terminal, 15 is a feedback loop of the amplifier 3, 1
6 is a time constant circuit. The amplification factor of the amplifier 13 is determined by the ratio of the resistance value R15 of the resistor R15 connected between the output terminal and the inverting input terminal and the resistance value R14 of the resistor R14 connected between the inverting input terminal and the ground. ..

The time constant circuit 16 will be described. The input terminal 11 is connected to the non-inverting input terminal of the amplifier 13 through the resistors R11 and R12 sequentially. The non-inverting input terminal of the amplifier 13 is grounded by sequentially passing through the capacitor C12, and is also grounded through the series circuit of the capacitor C3 and the switching NPN transistor Q12. Between the connection midpoint of the resistors R11 and R12 and the output terminal of the amplifier 13, a capacitor C11 and an NPN for switching are provided.
A series circuit between the collector and emitter of the transistor Q11 is connected.

The feedback loop 15 is a series circuit between the collector and emitter of the capacitor C11 and the switching NPN transistor Q11 connected between the midpoint of connection between the capacitors C11 and C12 and the output terminal of the amplifier 13. Composed.

The input terminal 12 is connected to the base of the transistor Q11 through a resistor R16, and the resistor R16
Connected to the base of transistor Q12 through 17,
Its base is grounded through a resistor R18.

Next, the operation of this active high-pass filter will be described. When a positive voltage switching signal is supplied to the input terminal 2 and both the transistors Q1 and Q2 are turned on, the feedback loop 15 is closed and the capacitors C12 and C13 connect the non-inverting input terminal of the amplifier 13 and the ground. Are connected in parallel. in this case,
The active high-pass filter functions as a Butterworth type second-order low-pass filter, and the cutoff frequency Fc _{2 at} this time is expressed by the following equation. Fc ₂ = 1 / 2π {C11 ・ (C12 + C13) ・ R11 ・ R12} ^1/2

When the switching signal of the ground voltage is supplied to the input terminal 12 and both the transistors Q11 and Q12 are turned off, the feedback loop 15 is opened and, equivalently, the capacitors C11 and C13 are not provided. Becomes In this case, the active low-pass filter is a cascade circuit of the primary RC low-pass filter and the amplifier 13. The cutoff frequency Fc _{1 at} this time is represented by the following equation. Fc ₁ = 1 / 2π (R11 + R12) ・ C12

Here, for simplicity, R11 = R12, C
If 11 = C12, the cutoff frequency Fc ₁ is expressed by the following equation. Fc ₁ = 1 / 4πR11 · C12

If the capacitance value C12 is sufficiently smaller than C13, for example, C12 = C13 / 60, the cutoff frequency Fc ₁ is expressed by the following equation. Fc ₁ = 15 / πR11 · C13

Incidentally, C11 = C13, R11 = R1
In the case of 2, C12 = C13 / 60, Fc ₂ is expressed by the following equation. Fc ₂ = 1 / 2π (61/60) ^1/2 · C13 · R11

Therefore, C11 = C13, R11 = R1
2. When the ratio of both cutoff frequencies Fc ₂ and Fc ₁ when C12 = C13 / 60 is taken, the following value is obtained. Fc ₁ / Fc ₂ ≈30 Therefore, if Fc ₂ = 100 Hz, then Fc ₁ = 3 kH
z. Therefore, it can be seen that the first-order low-pass filter characteristic is regarded as a substantially flat characteristic and there is no problem.

[Active Low-Pass Filter (2) of the Embodiment] (FIG. 3) Next, the active low-pass filter (2) of the embodiment will be described with reference to FIG. This active low-pass filter (2) corresponds to the active low-pass filter (1) of FIG. 2 in which the capacitor C12 is omitted. In this case, the switching transistor Q11,
When both Q12 are turned off, the circuit has a completely flat characteristic in which the series circuit of the resistors R11 and R12 is connected to the non-inverting input terminal of the amplifier 13. Other configurations and operations are the same as those of the active low-pass filter (1) in FIG. 2, and thus the description thereof will be omitted.

[Speech enhancement circuit to which active high-pass filter is applied] (FIG. 4) Next, the speech enhancement circuit to which the high-pass filter of FIG. 1 is applied will be described with reference to FIG. Input terminals TL1, TR1
The left and right audio signals L and R from the amplifier 2 for extracting the middle frequency band
1 and the left and right adders 22 and 23, respectively. In the adders 22 and 23, the left and right audio signals L and R are output from the amplifier 21, that is, the sum audio signal L + R. Area components are added. The respective outputs of the adders 22 and 23 are supplied to the left and right low-frequency cut circuits 24 and 55 to cut the low-frequency components, respectively, and then to low-frequency amplifiers (may be buffer amplifiers) A1 and A2. Supplied and amplified, left and right output terminals TLO, TR respectively
Supplied to O.

The low-frequency cut circuits 24 and 25 described above are provided in FIG.
Apply the high-pass filter described in. As for the low frequency cut frequencies of the left and right low frequency cut circuits 24 and 25, the pass band frequency of the mid frequency extraction amplifier 21 is 500 Hz to 5 kHz.
In the case of z, it is 400 Hz, and in the case of 1 kHz to 3 kHz, it is 900 Hz. Reference numeral 28 denotes a switching signal input terminal, which outputs a switching signal from the above-mentioned transistors Q1 and Q of the low-frequency cut circuits 24 and 25, respectively.
2 is supplied with a switching signal.

The mid-range extraction amplifier 21 has a frequency band (for example, 500 Hz to 5) substantially equal to the frequency band of the human voice of the sum audio signal L + R from the left and right audio signals L and R.
(1 kHz to 3 kHz, 1 kHz to 3 kHz) is extracted and amplified with an amplification factor of +8 dB, for example. A collector-emitter of a switching NPN transistor 26 is connected between the output side of the amplifier 21 and the ground.

Then, the transistor 26 is turned off by the switching signal from the input terminal 27, and the amplifier 2
The midrange component of the sum audio signal L + R from 1 is added to the adder 22,
23, and at the same time, the transistors Q1 and Q2 of the low-frequency cut circuits 24 and 25 are turned on by the switching signal from the input terminal 28 to operate as Butterworth second-order active high-pass filters, respectively. To do so.

Further, the transistor 26 is turned on by the switching signal from the input terminal 27, and the middle frequency component of the sum audio signal L + R from the amplifier 21 is added to the adders 22 and 23.
Not to be supplied to the input terminal 2 at the same time.
By the switching signal from 8, the low frequency cut circuit 2
The transistors Q1 and Q2 of Nos. 4 and 25 are turned off to form a circuit having flat characteristics.

According to such a voice emphasizing circuit, the mid-range extraction amplifier 21 for amplifying the mid-range component of the sum sound signal L + R of the left and right sound signals L, R and the middle-range extraction for the left and right sound signals L, R are provided. Since the left and right voice adders 22 and 23 for adding the outputs of the amplifier 21 are provided, the voice of the person in the center can be easily heard. Left and right voice adders 22, 23
Since the low-frequency cut circuits 24 and 25 are respectively provided on the output side of, the low-frequency noise such as the wind noise and the sound at the time of operating the device is reduced, and the voice of the person in the center can be more easily heard.

In the voice emphasizing circuit of FIG. 4, the mid-range extraction amplifier 21 is omitted and the low-range cut circuits 24 and 2 are provided.
It is also possible to set the cut frequency of 5 to 100 Hz and use it as a circuit for removing wind noise in the audio signal from the microphone.

[0036]

According to the present invention described above, it is possible to obtain an active high-pass filter and an active low-pass filter that can switch the frequency characteristic to the Butterworth type second-order filter characteristic and the flat characteristic with a simple structure.

[Brief description of drawings]

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

FIG. 2 is a circuit diagram showing an active low-pass filter (1) of an embodiment.

FIG. 3 is a circuit diagram showing an active low-pass filter (2) of an embodiment.

FIG. 4 is a block diagram showing a voice enhancement circuit to which an active high-pass filter according to an embodiment is applied.

FIG. 5 is a block diagram showing a conventional active high-pass filter.

[Explanation of symbols]

 1 Input Terminal 2 Input Terminal 3 Amplifier 4 Output Terminal 5 Feedback Loop 6 Time Constant Circuit C1, C2 Capacitor R1, R2 Resistor Q1, Q2 Switching Transistor 11 Input Terminal 12 Input Terminal 13 Amplifier 14 Output Terminal 15 Feedback Loop 16 Time Constant Circuit C11, C12 Capacitor R11, R12 Resistor Q1, Q2 Switching transistor 21 Mid range extraction amplifier 22, 23 Adder 24, 25 Low range cut circuit

Claims (2)

[Claims] 1. In a Butterworth type second-order active high-pass filter, the first switching element inserted in the feedback loop of the amplifier and the resistance value of the resistor forming the time constant circuit are set to the normal resistance value A second switching element for switching to a large resistance value is provided, and the first switching element is operated to close the feedback loop of the amplifier, and the second switching element is operated to operate the time constant circuit. By switching the resistance value of the resistor to the normal resistance value, the frequency characteristic becomes the second-order active high-pass filter characteristic, and the first switching element is operated to open the feedback loop of the amplifier. Operate the second switching element to change the resistance value of the resistor forming the time constant circuit to the above value. An active high-pass filter characterized in that the frequency characteristic is made substantially flat by switching to a resistance value sufficiently larger than a normal resistance value. 2. In a Butterworth type second-order active low-pass filter, the capacitance value of the first switching element inserted in the feedback loop of the amplifier and the capacitance value of the capacitor forming the time constant circuit are set to the normal capacitance value and sufficiently smaller than this. A second switching element for switching to a capacitance value is provided, and the first switching element is operated to close the feedback loop of the amplifier, and the second switching element is operated to configure the time constant circuit. The capacitance value of the capacitor to be used is switched to the normal capacitance value, the frequency characteristic becomes a second-order active low-pass filter, the first switching element is operated to open the feedback loop of the amplifier, and the second The capacitance of the capacitor that constitutes the time constant circuit by operating the switching element An active low-pass filter characterized in that a frequency characteristic is made substantially flat by switching the value to a capacitance value sufficiently smaller than the normal capacitance value.