JP2002044777A - Microphone device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a microphone device capable of detecting small variations of sound wave input with high sensitivity and precision. SOLUTION: In the microphone device, an amplifying circuit is composed of a first operational amplifier 1 and a second operational amplifier 2. A feedback circuit network is composed of a second resistor 6 and a third resistor 11, an output terminal of the amplifier 2 is connected to a nonreversible input terminal of the amplifier 1 via a first capacitor 7, and a second capacitor 12 is connected between the nonreversible input terminal and an output terminal of the first amplifier 1. The output terminal of the amplifier 2 is connected to a signal output terminal 13. A capacitive sound wave sensor 8 is connected to the nonreversible input terminal of the amplifier 1. If AC electric voltage Vin is inputted into the device, an electric current Id running through basic capacitance Cd of the sensor 8 is supplyied from the amplifier 1 via the second capacitor 12, and an electric voltage Vout is obtained from the terminal 13.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a microphone device having a sound wave sensor for changing impedance such as capacitance.

[0002]

2. Description of the Related Art As a conventional example of a microphone device which inputs a sound wave signal, converts the sound signal into an electric signal, and outputs the electric signal, there is, for example, the one described in JP-A-11-88989. FIG. 5 is a circuit diagram showing a schematic configuration of the microphone device. This microphone device includes an electret-type acoustic wave sensor 41. The sound wave sensor 41 vibrates the vibrating membrane with a sound wave received from the outside, and changes the inter-terminal voltage Ve in accordance with a change in capacitance between the vibrating membrane and the electret layer due to the vibration. This change in the terminal voltage Ve is caused by the resistances 42, 43, 4
The output voltage Vd is output from the output terminal 46 after impedance conversion by the source follower circuit composed of the FET 4 and the FET 45.

[0003]

The capacitance Ce of a general condenser microphone including the sound wave sensor 41 is as follows.
It can be represented by the sum of a constant basic capacitance value Cd and a capacitance change ΔC that changes due to a sound wave received from the outside. Ce = Cd + ΔC However, the inter-terminal voltage Ve changes in accordance with a change in the overall capacitance value Ce of the condenser microphone. Therefore, when the capacitance change ΔC is smaller than the basic capacitance value Cd, there is a problem that the detection sensitivity of the capacitance change ΔC becomes extremely low. If the degree of amplification of the circuit is increased to solve this problem, the output voltage Vd will be saturated this time, causing a problem that the capacitance change ΔC cannot be detected accurately.

The present invention has been made to solve the above-mentioned conventional problems, and an object of the present invention is to provide a microphone device capable of accurately detecting a minute change in sound wave input with high sensitivity. It is in.

[0005]

Therefore, a microphone device according to the present invention comprises a sound wave sensor for changing impedance according to an input sound wave signal, a first operational amplifier, a second operational amplifier, and a first operational amplifier. A first terminal provided between one input terminal of the amplifier and the output terminal of the second operational amplifier.
And a signal line having one end connected to the input terminal of the first operational amplifier and the other end connected to the acoustic wave sensor, and an output terminal of the first operational amplifier and the signal line. And a signal output terminal connected to the output terminal of the second operational amplifier. The acoustic wave sensor receives a predetermined current from the output terminal of the first operational amplifier via the second impedance. Is supplied.

Further, the microphone device of the present invention comprises a sound wave sensor for changing impedance according to an input sound wave signal, a first operational amplifier provided with a feedback network for amplifying a signal between input and output terminals, Two operational amplifiers,
A first impedance provided between one input terminal of the first operational amplifier and an output terminal of the second operational amplifier, one end connected to the input terminal of the first operational amplifier and the other end connected to the other end; A signal line connected to the sound wave sensor, a current path having one end connected to the signal line, and a signal output terminal connected to an output terminal of a second operational amplifier; And a predetermined current is supplied from the power supply.

In the microphone device, a predetermined current supplied to the sound wave sensor reduces a current that does not change in accordance with a change in impedance, out of the current flowing through the sound wave sensor, from the current flowing through the first impedance. It becomes possible. Further, by supplying the current for the reduction from the first operational amplifier via the second impedance, it is possible to reduce the size of the circuit.

Further, if the acoustic wave sensor, the first impedance, and the second impedance are respectively a capacitive acoustic wave sensor, a first capacitor, and a second capacitor, the influence due to the phase rotation can be suppressed, and It is possible to reliably reduce the amount of current that does not change according to the change in the capacitance of the acoustic wave sensor from the current flowing through the capacitor.

In the above microphone device,
Further, a shield means for electrically shielding at least a part of the signal line and a shield voltage applying means for applying a shield voltage based on the other input terminal voltage of the first operational amplifier to the shield means are provided. , Controlling the stray capacitance between the shield means and the signal line,
It becomes possible to detect the impedance change of the acoustic wave sensor with higher accuracy.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, specific embodiments of the microphone device of the present invention will be described in detail with reference to the drawings.

(First Embodiment) FIG. 1 is a circuit diagram showing a configuration of a microphone device according to a first embodiment. This microphone device includes an amplifier circuit including a first operational amplifier 1 and a second operational amplifier 2. A non-inverting input terminal of the first operational amplifier 1 is connected to a shielded signal line 9.
Are connected at one end.

At the other end of the signal line 9, a sound wave sensor 8 is provided. As shown in the longitudinal sectional view of FIG. 4, the acoustic wave sensor 8 includes a vibrating membrane 31 on one end side of a cylindrical metal outer wall 34, and a back electrode 32 supported by an insulator 33 on the cylindrical metal outer wall 34. , Are provided so as to be opposed to each other with a predetermined gap therebetween. A detection electrode 35 is formed at an end of the back pole 32 on the anti-vibration film side, and a negative electrode 36 is formed at an end of the cylindrical metal outer wall 34 on the anti-vibration film side. Then, the vibration film 31 vibrates according to the received sound wave, so that the relative position between the vibration film 31 and the back electrode 32 is changed, whereby the capacitance between the negative electrode 36 and the detection electrode 35 is changed. Cs is changed. Therefore, the signal line 9 is connected to the detection electrode 35 of the sound wave sensor 8, and the negative electrode 36 is grounded.

The second operational amplifier 2 has a non-inverting input terminal grounded, and an inverting input terminal having a first resistor (resistance value R).
1) One end of 5 and a second resistor (resistance value R2) 6 are connected to each other. The other end of the first resistor 5 is connected to an AC voltage generator (generated AC voltage Vin, angular frequency ω) 4, and the other end of the second resistor 6 is connected to the inverting input terminal of the first operational amplifier 1. . The output terminal of the second operational amplifier 2 is connected to a first capacitor (first impedance, capacitance value Cf) 7
To the non-inverting input terminal of the first operational amplifier 1. A third resistor (resistance value R3) 11 is connected between the inverting input terminal and the output terminal of the first operational amplifier 1,
A second capacitor (second impedance, capacitance value Cc) is connected to the electric circuit 17 provided between the non-inverting input terminal and the output terminal.
12 are interposed. The output terminal of the second operational amplifier 2 is connected to the signal output terminal 13.

Next, the operation of the microphone device configured as described above will be described. The total capacitance Cs of the acoustic wave sensor 8 can be expressed by Cs = Cd + ΔC (1) where Cd is the basic capacitance value and ΔC is the capacitance change. Then, an AC input voltage Vin having an angular frequency ω is supplied from the AC voltage generator 4. Since the input terminals of the operational amplifiers 1 and 2 are in an imaginary short state, the inverted input terminal voltage V1 of the second operational amplifier 2 is
0. Thus, the inverted input terminal voltage V2 of the first operational amplifier 1 becomes V2 =-(R2 / R1) .Vin --- (2) = Vp, and the output terminal voltage V3 of the first operational amplifier 1 becomes V3 = − ((R2 + R3) / R1) · Vin ――― (3) That is, the non-inverting input terminal voltage Vp and the output terminal voltage V3 of the first operational amplifier 1 are equal to the input voltage Vi, respectively.
That is, the voltage becomes proportional to n. And V3
Since / V2 = (R2 + R3) / R2, signal amplification is performed between the input and output terminals of the first operational amplifier 1, and a feedback network therefor is configured using the second resistor 6 and the third resistor 11. It can be said that there is.

The current value Is flowing through the acoustic wave sensor 8 is represented by the following formula: Is = jωCs · Vp = jωCs · V2 (4) The current Ic flowing through the second capacitor 12
Is as follows: Ic = jωCc · (V3−V2) (5), and the current If flowing through the first capacitor 7 is If = Is−Ic (6). That is, the electric circuit 17 functions as a current path for supplying the predetermined current Ic to the sound wave sensor 8 connected to the signal line 9.

On the other hand, the signal output terminal voltage (detection voltage) Vo
ut is expressed by: Vout = If / jωCf + V2 (7) Therefore, this equation (7) is replaced by the above equations (2) to (6).
Is obtained, the following equation is obtained: Vout = − (R2 + Cs · R2 / Cf−Cc · R3 / Cf) · Vin / R1 (8) Further, by substituting the equation (1) into the equation (8), Vout = − (R2 + Cd · R2 / Cf + ΔC · R2 / Cf−Cc · R3 / Cf) · Vin / R1 (9) it can.

The basic capacitance value Cd of the sound wave sensor 8 is known. Therefore, in the microphone device, Cd · R2 =
Resistors 5 and 6 and capacitor 12 so as to be Cc · R3
Is selected. Then, the voltage Vout becomes: Vout = − (1 + ΔC / Cf) · Vin · R2 / R1 (10) Therefore, by detecting Vout from the signal output terminal 13, the capacitance change ΔC of the acoustic wave sensor 8 can be obtained.

In the microphone device, a voltage Vout having a value corresponding to the current If flowing through the first capacitor 7 is obtained. The current Is flowing through the acoustic wave sensor 8 is represented by the sum of the current If and the current Ic flowing through the second capacitor 12, as in Expression (6). Is = If + Ic (11) On the other hand, the current Is is a current Id flowing through a portion corresponding to the basic capacitance value Cd of the capacitance Cs of the acoustic wave sensor 8 and a current ΔI flowing through a portion corresponding to the capacitance change ΔC. And can be decomposed into Is = Id + ΔI (12) Then, all of the current Id is supplied from the output terminal of the first operational amplifier 1 via the second capacitor 12. Id = Ic (13) Therefore, the current If flowing through the first capacitor 7 is equal to the current ΔI
If = ΔI (14) It is possible to obtain a signal output terminal voltage Vout independent of the magnitude of the basic capacitance value Cd. Accordingly, even when the capacitance change ΔC is extremely small compared to the basic capacitance value Cd, the capacitance change ΔC can be detected with high sensitivity. Further, even if the detection sensitivity is increased by reducing the capacitance Cf of the first capacitor 7, the current If does not include the current Id, so that the voltage Vout can be easily set regardless of the magnitude of the basic capacitance value Cd. Saturation can be avoided. Therefore, even a small change in sound wave input can be detected with high sensitivity. And the current Ic
Are supplied from the first operational amplifier 1 in the amplifier circuit. Therefore, the detection circuit for detecting the change in capacitance of the sound wave sensor 8 can be made compact.

(Embodiment 2) FIG. 2 is a circuit diagram showing a configuration of a microphone device of Embodiment 2. This device differs from that of the first embodiment in that the inverting input terminal voltage of the first operational amplifier 1 is applied to a shield line (shield means) 10 via a voltage follower or a buffer (shield voltage application means) 3. Is a point. Shield wire 10
Is for electrically shielding the signal line 9. Since both input terminals of the first operational amplifier 1 are imaginarily short-circuited, a shield voltage equal to the non-inverting input terminal voltage can be applied to the shield line 10 in this manner.
As a result, the voltage of the signal line 9 and the voltage of the shield line 10 become equal, and it is possible to avoid the generation of stray capacitance between the two. Further, since the shield voltage is applied using the voltage follower 3 having a high input impedance, it is possible to prevent a current from flowing from the feedback network of the amplifier circuit to the voltage follower 3 side, and to reduce the output voltage V3 of the first operational amplifier 1. It is possible to prevent an error from occurring in the signal output terminal voltage Vout due to fluctuation.

Here, the voltage follower 3 is used as the shield voltage applying means to simplify the circuit configuration. However, if the circuit scale is not limited, a circuit capable of compensating the phase and amplitude of the inverting input terminal voltage of the first operational amplifier 1 may be used as the shield voltage applying means. With such a circuit,
Even when the input voltage frequency ω is extremely high, the potential between the signal line 9 and the shield line 10 can be reliably set to the same potential to prevent generation of stray capacitance. And this phase
When an amplitude compensation circuit having an input impedance as high as the input impedance of the operational amplifier or the gate input impedance of the MOSFET is used, it is possible to reliably prevent an error from occurring in the voltage Vout. Further, in the case where a phase amplitude compensation circuit is used as the shield voltage applying means, this is connected between the shield wire 10 and the AC voltage generator 4, and the voltage Vin of the AC voltage generator 4 is subjected to phase amplitude compensation. You may make it apply to the shield wire 10. Since the inverting input terminal voltage V2 of the first operational amplifier 1 is formed based on the voltage Vin, the shield voltage based on the inverting input terminal voltage V2 of the first operational amplifier 1 is also applied to the shield line 10 in this case. This is because they can be applied.

Although the specific embodiments of the present invention have been described above, the present invention is not limited to the above embodiments, and can be implemented with various modifications within the scope of the present invention. In the above, a capacitive sensor is used as the sound wave sensor 8, but this may be an electret sensor in which an electret layer is formed on the back electrode 32 or the vibration film 31. Further, an inductive sensor or a resistance sensor may be used as the acoustic wave sensor 8. The impedances 7 and 12 are not limited to the above-described capacitance elements, but may be resistance elements or inductive elements. In addition, the reason why the capacitive element (second capacitor 12) is used as the impedance for supplying Ic in the above-described embodiment is that a capacitive element is used as the acoustic wave sensor 8, so that Ic equal to Id is reduced to the first operational amplifier. This is because the sound wave can be reliably supplied to the sound wave sensor 8 from 1. Therefore, when an inductive sensor is used as the acoustic wave sensor 8, it is desirable that the impedance for supplying Ic is also an inductive element.
When a resistance type sensor is used as the acoustic wave sensor 8,
It is desirable that the impedance is also a resistance element.

Further, the reason why the capacitive element (first capacitor 7) is used as the impedance for flowing the current If is that the capacitive element is used as the acoustic wave sensor 8, and thus the phase between If and Id (or Is) is increased. This is because the influence of the surroundings can be suppressed, and Id can be reliably reduced from If. Therefore, when an inductive sensor is used as the acoustic wave sensor 8, it is desirable that the impedance for flowing If is also an inductive element, and when a resistive sensor is used as the acoustic wave sensor 8, the impedance is also a resistive element. It is desirable.

Further, the impedance 12 may be constituted by a plurality of kinds of impedances. For example,
The capacitance element and the resistance element can be connected in parallel. If the current Id flowing to the basic capacitance value Cd is supplied by the current flowing through the capacitive element, and the current flowing into the first operational amplifier 1 from the non-inverting input terminal is supplied by the current flowing through the resistive element, the first The current If flowing through the capacitor 7 can be made very equal to the current ΔI.

If at least part of the current Id is covered by the current Ic, the contribution of Id to the detection voltage Vout can be reduced, and the effect of the present invention can be obtained. However, in order to detect the capacitance change ΔC with high sensitivity, it is preferable to cover almost the entire amount of the current Id with the current Ic and make If = ΔI as close as possible.

The voltage Vout from the signal output terminal voltage Vout
A differential circuit for subtracting and outputting 2 may be constituted by an operational amplifier or the like, and this differential circuit may be included in the detection circuit. By doing so, a signal not including V2 can be obtained from the output of the difference circuit, so that the subsequent signal processing circuit can be simplified.

Further, in the above, the current I is supplied via the electric path 17 provided between the output terminal of the first operational amplifier 1 and the signal line 9.
Although c is supplied to the sound wave sensor 8, another predetermined current source may be provided, and the current Ic may be supplied via an electric path provided between the current source and the signal line 9. The current source may be, for example, an AC voltage generator and an impedance connected thereto, or, for example, an operational amplifier and an impedance connected to an output terminal thereof.

[0027]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the circuit shown in FIG. 1, R1 = R2 = 1k
Ω, R3 = 10 kΩ, and Cc = 2 pF. As the sound wave sensor 8, a basic capacitance value Cd = 20 pF as shown in FIG.
And a capacitance Cs of 17 pF to 23
A sound input was given that varied to pF. A case where the current Ic is not supplied to the sound wave sensor 8 is set as a comparison value, and Cf =
The signal output terminal voltage Vout was measured for each of the cases of 5 pF and Cf = 1 pF.

FIG. 3A shows the result when Cf = 5 pF, and FIG. 3B shows the result when Cf = 1 pF. When Cf = 5 pF, Vout according to the change of the capacitance Cs in both the example of the present invention and the comparative example.
Has been detected. However, since the change width of the voltage Vout is about 0.15 V, it can be said that it is not easy to accurately capture a minute change in the capacitance Cs. On the other hand, Cf = 1
In the case of pF, in the comparative example, the output voltage Vout approaches the power supply voltage and saturates, and it is extremely difficult to detect the capacitance Cs. In the embodiment of the present invention, the change in the capacitance Cs can be detected even in this case, and the change width of the voltage Vout at this time is about 0.65V. Therefore, a small change in capacitance Cs,
That is, it is clear that a minute change in the sound wave input can be accurately detected with high sensitivity.

[0029]

According to the microphone device of the present invention, of the current flowing through the acoustic wave sensor, the current that does not change in accordance with the impedance change can be reduced from the current flowing through the first impedance. Therefore, it is possible to accurately detect a minute change in the sound wave input with high sensitivity. Further, since the circuit can be made compact, it is suitable for making a chip.

[Brief description of the drawings]

FIG. 1 is a circuit diagram illustrating a configuration of a microphone device according to a first embodiment of the present invention.

FIG. 2 is a circuit diagram illustrating a configuration of a microphone device according to a second embodiment.

FIG. 3 is a graph showing a relationship between a signal output terminal voltage and a change in capacitance in an example, in comparison with a comparative example.

FIG. 4 is a longitudinal sectional view showing an example of a sound wave sensor used in the microphone device of the present invention.

FIG. 5 is a circuit diagram showing a configuration of a conventional microphone device.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 1st operational amplifier 2 2nd operational amplifier 7 1st capacitor 8 Sound wave sensor 9 Signal line 10 Shield line 12 2nd capacitor 13 Signal output terminal

Claims (5)

[Claims] A first operational amplifier; a second operational amplifier; one input terminal of the first operational amplifier; and a second operational amplifier. The first terminal provided between the
And a signal line having one end connected to the input terminal of the first operational amplifier and the other end connected to the acoustic wave sensor, and an output terminal of the first operational amplifier and the signal line. And a signal output terminal connected to the output terminal of the second operational amplifier. The acoustic wave sensor receives a predetermined current from the output terminal of the first operational amplifier via the second impedance. And a microphone device. A first operational amplifier, a second operational amplifier, and one input terminal of the first operational amplifier and a second operational amplifier.
A first capacitor provided between the output terminal of the first operational amplifier and a signal line having one end connected to the input terminal of the first operational amplifier and the other end connected to the acoustic wave sensor; A second capacitor provided between the output terminal of the operational amplifier and the signal line, and a signal output terminal connected to the output terminal of the second operational amplifier. A microphone device wherein a predetermined current is supplied from an output terminal of an amplifier via a second capacitor. 3. A sound wave sensor for changing impedance according to an input sound wave signal, a first operational amplifier provided with a feedback network for performing signal amplification between input and output terminals, a second operational amplifier, A first operational amplifier provided between one input terminal of the first operational amplifier and an output terminal of the second operational amplifier.
And a signal line having one end connected to the input terminal of the first operational amplifier and the other end connected to the acoustic wave sensor, a current path having one end connected to the signal line, and a second operational amplifier. And a signal output terminal connected to an output terminal of an amplifier, wherein a predetermined current is supplied to the acoustic wave sensor from the current path. 4. A sound wave sensor for changing a capacitance in accordance with an input sound wave signal, a first operational amplifier provided with a feedback network for amplifying a signal between input and output terminals, and a second operational amplifier. And one input terminal of the first operational amplifier and the second
A first capacitor provided between the output terminal of the operational amplifier and a signal line having one end connected to the input terminal of the first operational amplifier and the other end connected to the acoustic wave sensor;
A current path having one end connected to the signal line and a signal output terminal connected to the output terminal of the second operational amplifier are provided, and a predetermined current is supplied to the acoustic wave sensor from the current path. A microphone device characterized by the above-mentioned. 5. A shield means for electrically shielding at least a part of the signal line, and a shield voltage applying means for applying a shield voltage based on the other input terminal voltage of the first operational amplifier to the shield means. The microphone device according to any one of claims 1 to 4, further comprising: