JP2006229336A - Capacitive microphone - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To put into practical use a capacitive microphone employing a micro sound-sensitive capacitor manufactured by an MEMS or the like, not in a research or experimental scale but in an industrial scale suitable for the mass production of the microphone built in a portable apparatus. <P>SOLUTION: A sound-sensitive capacitor Cm and a reference capacitor Cr are charged with potentials +Vc1 and -Vc2 of reverse polarities in order to offset capacitance equivalently, and a microphone output is obtained by converting the amount of charge of differential capacitance (ΔCm) extracted equivalently by an offset into a voltage. A DC component extracted from the voltage conversion output is fed back to at least one of the potentials +Vc1 and -Vc2, thus suppressing the DC component to zero. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an ultra-small capacitive microphone that is particularly effective when used for a microphone mounted on a small portable device such as a cellular phone.

  Capacitance microphones can be classified into two types, DC bias type (DC bias condenser microphone) and electret type (electret condenser microphone), depending on the method of applying a bias electric field to the sound sensing capacitor.

  The DC bias type is configured using a sound sensing capacitor in which a movable electrode forming an acoustic diaphragm (diaphragm) is opposed to a fixed electrode with an air dielectric interposed therebetween. When a DC bias voltage is applied between the electrodes of the sound sensing capacitor via a high impedance (for example, 10 MΩ) resistor in series, a voltage change corresponding to the change in the capacitance of the sound sensing capacitor due to the sound pressure occurs at both ends of the series resistor. appear. This voltage change is separated and extracted from the DC bias voltage by the AC coupling circuit, and amplified while performing impedance conversion in the first stage circuit with high input impedance, so that the microphone output (acoustic signal) that can be used in the subsequent circuit can be extracted. it can.

  In this DC bias type, there is a hassle to apply a high-voltage DC bias voltage from the outside. Therefore, in general, an electret type that does not require the DC bias voltage is often used. This electret type is also mounted on small portable devices such as mobile phones.

  The electret type is a so-called electret in which a large amount of electric charge is accumulated and fixed by polarization or the like between a movable electrode forming an acoustic diaphragm and a fixed electrode facing the movable electrode. A high bias electric field is constantly applied to the space. The movable electrode and the fixed electrode form a sound-sensitive capacitor that changes in capacitance due to sound pressure. Appear in For example, a microphone signal that can be used in a subsequent circuit can be taken out by impedance-converting this voltage change with an FET source follower.

  The electret condenser microphone does not have the trouble of applying a high DC bias voltage from the outside, but there is a problem of performance degradation due to the deterioration of the electret. In the electret, the accumulated electric charge is gradually discharged due to insulation failure of the dielectric itself or the surface thereof, and therefore, the initial high electric field cannot be stably maintained for a long time. So-called deterioration with time occurs. This deterioration (loss of accumulated charge) depends on the environmental conditions, but is particularly noticeable at high temperatures. Therefore, when soldering the electret condenser microphone, it is necessary to pay sufficient attention not to raise the processing temperature.

  However, in recent years, in order to prevent environmental pollution, it is inevitable to use heavy metals such as lead. However, lead-free solder that does not contain lead is at a temperature about 30 ° C higher than that of conventional leaded solder. Need processing. For this reason, there has been a problem that the above-described electret condenser microphone is likely to deteriorate in performance at the stage of assembly.

  In addition, in the above-described conventional capacitance type microphone, both the electret type and the DC bias type are configured by assembling a plurality of parts, each of which is a sound-sensitive capacitor that forms the main part. It was produced by a so-called discrete assembly method.

  On the other hand, there is a strong demand for high performance and miniaturization in portable devices such as mobile phones in which this type of microphone is incorporated, and accordingly, various functional components such as circuits incorporated in the portable device are improved in performance and superb. Miniaturization has been carried out. Microphones are no exception, and their miniaturization is required.

  In recent years, it has become possible to process ultra-small sound-sensitive capacitors on a silicon chip by using micro-electro mechanical systems (MEMS). If an ultra-small sound-sensitive capacitor manufactured by MEMS can be used for an electrostatic capacity-type microphone, an ultra-small electrostatic capacity-type microphone suitable for use in the portable device can be obtained. In this case, a DC bias type condenser microphone is suitable for fabrication by MEMS.

  However, the base capacity (non-changing capacity) of the ultra-small sound sensor made by MEMS or the like is much smaller than that of the conventional discrete assembly system, and is only about 1 to several [pF] at most. Absent. The change in capacity due to sound pressure is less than 1/1000 of the base capacity. As an example, in the case of a sound-sensitive capacitor having a base capacitance of about 1 [pF], the capacitance change due to sound pressure is only about 1 [fF] (1 fF = 1/1000 [pF]).

  In order to detect this very slight change in capacitance using the above-described DC bias voltage and series resistance, a very high DC bias voltage and an extremely high series resistance are required. Is difficult to realize.

  Therefore, the present inventor examined a circuit as shown in FIG. 2 in order to efficiently detect a slight change in capacitance such as 1/1000 of the base capacitance of 1 [pF]. This detection circuit is a capacitance change detection circuit for general measurement disclosed in Patent Document 1 and is not for a capacitive microphone. However, it is considered whether this can be applied to the detection of the capacitance change of the sound sensing capacitor. did.

  The detection circuit shown in FIG. 2 is configured using a switch circuit S1 that performs charge / discharge switching of the detected capacitor Cx and a charge amount / voltage conversion circuit 23. The switch circuit S1 has two selected ports a and b for one common port c and performs a switching operation periodically. For each switching operation, one end of the detected capacitor Cx is alternately connected to the ports a and b. The port a is connected to a predetermined charging potential −Vc, and the port b is connected to the input of the charge amount / voltage conversion circuit 23. The other end of the detected capacitor Cx is connected to a common reference potential (GND).

  The charge / voltage conversion circuit 23 includes an operational amplifier OP1, a feedback capacitor Cf, and a switch circuit S3. The switch circuit S3 operates in synchronization with the switch circuit S1 to periodically discharge and reset the feedback capacitor Cf. The non-inverting input (+) of the operational amplifier OP1 is connected to a common reference potential (Vr = GND). Therefore, the inverting input (−) of the operational amplifier OP1 is virtually short-circuited (active ground) to the common reference potential (Vr) by negative feedback.

  In the above circuit, first, when the ports a-c of S1 are connected, the detected capacitor Cx has a charge corresponding to the product of the capacitor capacitance (Cx) of the detected capacitor Cx and the charging potential -Vc. Charged. At this time, the feedback capacitor Cf is short-circuited by the switch circuit S3 and is in a discharge reset state (no charge state).

  Next, when the ports b-c of S1 are connected, the detected capacitor Cx is connected to the inverting input (-) of the operational amplifier OP1. At the same time, S3 is opened, and a negative feedback capacitor Cf is interposed in the negative feedback control loop of the operational amplifier OP1. Then, the negative feedback operation that forms the virtual short circuit charges the feedback capacitor Cf with a charge corresponding to the charge amount (Vr × Cx) of the detected capacitor Cx. At this time, a voltage for charging the feedback capacitor Cf with the charge amount (Vr × Cx) appears at the output of the operational amplifier OP1.

  In other words, the charge amount / voltage conversion circuit 23 mirror-transfers the charge amount of the detected capacitor Cx to the charge amount of the feedback capacitor Cf at a predetermined magnification, and transfers the charge amount necessary for the mirror transfer to the feedback capacitor Cf. By outputting the voltage Va for charging, the capacitance change of the detected capacitor Cx is converted into a voltage change and output.

  Here, the present inventor has verified whether or not a capacitance change due to sound pressure can be practically detected when the detected capacitor Cx is replaced with the ultra-small sound sensing capacitor. However, as described above, the capacitance change due to the sound pressure of the sound sensing capacitor is extremely small with respect to the base capacitance. For this reason, when detecting the change in capacity, it has been found that a fixed nominal capacity such as a base capacity is dominant and the target capacity change cannot be detected with a high SN ratio.

  In order to perform detection with a high S / N ratio, it is necessary to operate the detection circuit at the center of its input / output dynamic range. The dynamic range becomes narrower as the detection sensitivity increases. In order to detect minute capacitance changes, the dynamic range must be reduced. However, when the dynamic range is reduced, it is difficult to stably keep the operating point of the detection circuit at the center of the input / output dynamic range due to the nominal capacitance. become. In addition, detection at a high S / N ratio is also inhibited by masking a small change in capacitance with a relatively large nominal capacitance. In any case, a nominal capacity that is too large is an obstacle to detection.

  Therefore, the present inventor examined the use of a detection circuit as shown in FIG. 3 in order to remove or reduce a fixed nominal capacitance that hinders detection of a minute capacitance change (see Patent Document 1). (See FIG. 6).

  The detection circuit shown in FIG. 3 detects the capacitance change of the detected capacitor Cx by the charge amount / voltage conversion circuit 23 in the same manner as that shown in FIG. A reference capacitor (counter capacitor) Cr for canceling is used. That is, the capacitor Cx and the reference capacitor Cr are charged with potentials −Vc and + Vc having opposite polarities, thereby canceling the capacitance between the capacitors Cx and Cr.

  In FIG. 3, a detected capacitor Cx and a reference capacitor Cr are connected in series to form a capacitive voltage dividing circuit. When charging is performed by connecting both ends of the capacitive voltage dividing circuit to potentials -Vc and + Vc having opposite polarities by the switch circuits S1 and S2, the capacitors Cx and Cr are provided at the voltage dividing point A of the capacitive voltage dividing circuit. Differential capacity (Cx−Cr) appears equivalently.

  If the charge amount of the differential capacity (Cx−Cr) is converted into a voltage, only the capacity change of the detected capacitor Cx can be efficiently detected without being disturbed by the fixed nominal capacity. I can foresee. And if the detection circuit is used, the expectation that the capacity | capacitance change by the sound pressure of the said microminiature sound-sensing capacitor may be detected with high SN also arises.

  However, according to the knowledge of the present inventor, it has been found that even the circuit shown in FIG. 3 cannot detect the capacitance change due to the sound pressure of the ultra-small sound sensing capacitor at a practical level. In other words, it turned out that the above-mentioned foresight was off and expectations were betrayed. This is due to the fact that the capacitance of the ultra-small sound sensor capacitor is too small. As described above, the base capacitance of an ultra-small sound sensor made by MEMS or the like is very small, which is only about 1 to several [pF] at most. Furthermore, the capacity change due to the sound pressure is less than 1/1000.

  In order to detect this extremely small change in capacitance with a practical level of S / N ratio, it is necessary to cancel the capacitance with a reference capacitor (counter capacitor) with very high accuracy. For this purpose, it is necessary to reduce the capacitance error between the sound sensing capacitor and the reference capacitor as much as possible, and it is necessary to make the error smaller than at least the change in capacitance of the sound sensing capacitor. In this case, since the change in capacitance of the sound sensing capacitor is 1/1000 or less of the base capacitance, the capacitance error of the reference capacitor is required to be 1/1000 or less. However, it is actually impossible to realize such a high-accuracy reference capacitor in a minute capacitance range of 1 to several [pF]. In particular, it cannot be realized on an industrial scale where mass productivity and yield are problems.

  In addition to the base capacitance of the sound sensing capacitor, the fixed nominal capacitance includes a capacitance that is difficult to predict, for example, a stray capacitance distributed over a wiring or the like. Even if the stray capacitance is negligibly small, it is a fatal disturbance factor that hinders normal detection in a situation where a capacitance change of 1/1000 or less must be detected in a minute capacitance range of 1 to several [pF]. It becomes.

  Although it has been described that the above-mentioned ultra-small acoustic capacitor can be fabricated on a semiconductor chip by MEMS, in the technical field (semiconductor integrated circuit, etc.), the acoustic capacitor is mounted on the same semiconductor chip together with a circuit unit such as a detection circuit. It was believed that an integrated formation was optimal.

  However, when the present inventor manufactures the above-mentioned ultra-small sound-sensing capacitor on a semiconductor chip by MEMS, the structure of the sound-sensing capacitor and the detection circuit divided into separate chips is more It came to know that it was very advantageous from the viewpoint of yield. This is because the production conditions (for example, process steps) of the two are greatly different. In this case, the sound-sensitive capacitor and the detection circuit which are manufactured separately in different chips are connected by inter-chip wiring, and this wiring also increases the nominal capacitance.

The present invention has been made under the technical background as described above. The purpose of the present invention is to develop a capacitive microphone using an ultra-small sound-sensitive capacitor manufactured by MEMS or the like on a research / experimental scale. For example, it is to make it practical on an industrial scale suitable for mass production for use in mobile devices.
JP 2004-184307 A

The capacitive microphone according to the present invention is basically specified by the following items (1) to (10).
(1) A sound sensor capacitor, a reference capacitor, a first switch, a second switch, a switch control circuit, a reference voltage generation circuit, a calibration voltage generation circuit, a charge / voltage conversion circuit, and a detection circuit (2) One end of the sound sensor capacitor and one end of the reference capacitor are connected, and the connection point is connected to the input of the charge amount / voltage conversion circuit. (3) The sound sensor capacitor The other end of the reference capacitor is switched and connected to the output of the reference voltage generating circuit and the ground potential point by the first switch. (4) The other end of the reference capacitor is connected to the output of the calibration voltage generating circuit by the second switch. (5) The reference voltage generation circuit outputs a constant voltage. (6) The calibration voltage generation circuit has a polarity opposite to that of the output voltage of the reference voltage generation circuit and passes through a low frequency range. Phi (7) The switch control circuit includes a first connection state in which the first switch and the second switch are simultaneously connected to the ground potential point, and the first switch as a reference voltage generation circuit. And a second connection state in which the second switch is simultaneously connected to the output of the calibration voltage generation circuit at a constant cycle and at a high speed. (8) The charge amount / voltage conversion circuit includes the first switch, Synchronizing with the second switch and outputting a voltage proportional to the amount of charge input in the first connection state (9) The detection circuit detects an audio signal component from the output of the charge amount / voltage conversion circuit ( 10) For the low-pass filter, extract the low-frequency component of the detector circuit output and input it to the calibration voltage generator circuit.

The capacitance type microphone according to the present invention is also specified by the following items (21) to (25).
(21) A sound-sensing capacitor that generates a change in capacitance due to sound pressure and a fixed-capacitance reference capacitor are connected in series to form a capacitance voltage dividing circuit.
(22) A first connection mode in which both ends of the capacitive voltage dividing circuit are connected to a common reference potential, and both ends of the capacitive voltage dividing circuit are connected to a first potential and a second potential having opposite polarities with respect to the common reference potential. A charge / discharge switch circuit that alternately switches and sets the second connection mode, and switching control means for performing a switching operation of the charge / discharge switch circuit at a frequency sufficiently higher than the frequency region of the acoustic signal.
(23) The differential capacitor that operates synchronously with the switching of the charge / discharge switch circuit and that appears equivalently between the voltage dividing point of the capacitor voltage dividing circuit and the common reference potential is set every time the first connection mode is set. A charge amount / voltage conversion circuit that converts the charge amount into a pulse voltage and outputs the pulse voltage is provided.
(24) A detection circuit for extracting a signal in a frequency domain from the output of the charge amount / voltage conversion circuit to the acoustic frequency domain is provided.
(25) having a low-pass filter that extracts a direct current component from the output of the detection circuit, and variably controlling at least one of the first potential or the second potential using the output of the low-pass filter Thus, a negative feedback control loop for suppressing the DC component to zero is formed.

In the invention specified by the above items (21) to (25), the following items (A) to (E) can be appropriately selected and implemented.
(A) The first potential and / or the second potential is supplied by a variable voltage generation circuit whose output voltage is variably controlled by voltage, and the variable voltage generation circuit is controlled by the output of the low-pass filter. (B) The low-pass filter is configured by using a switched capacitor in a time constant circuit (C) The charge amount / voltage conversion circuit is In addition, negative feedback is applied by a feedback capacitor having a predetermined capacity, and the feedback capacitor is configured using an operational amplifier that discharges and resets for each setting of the first connection mode. By mirror-transferring the charged charge amount of the feedback capacitor to the charged charge amount of the feedback capacitor, for each setting of the second connection mode. Output a voltage pulse corresponding to the amount of electric charge (D) The detection circuit holds and outputs the pulse voltage output from the charge amount / voltage conversion circuit while sequentially updating the analog sample hold (E) An intermediate amplifier circuit that amplifies the output of the charge amount / voltage conversion circuit and transmits the output to the detection circuit is provided.

The capacitance type microphone according to the present invention is also specified by the following items (81) to (85).
(81) A sound-sensitive capacitor that generates a change in capacitance due to sound pressure, and a fixed-capacitance reference capacitor.
(82) The sound-sensing capacitor and the reference capacitor are charged with potentials of opposite polarities, thereby equivalently canceling the capacitance between the two capacitors.
(83) A charge amount / voltage conversion circuit that converts the charge amount of the differential capacitance that is equivalently extracted by the cancellation to voltage conversion is provided.
(84) An acoustic signal as a microphone output is extracted from the voltage conversion output, and a direct current component included in the voltage conversion output is extracted.
(85) A negative feedback control loop is formed in which the DC component is zero-suppressed by feeding back the DC component to the charging potential of at least one of the reference capacitor or the sound sensing capacitor.

  In the invention specified by the above items (81) to (82), preferably, the sound-sensing capacitor and the reference capacitor are connected in series to form a capacitive voltage dividing circuit, and both ends of the capacitive voltage dividing circuit are shared. By connecting and charging the first potential and the second potential having opposite polarities with respect to the reference potential, the capacitance difference between both capacitors appears equivalently at the voltage dividing point of the capacitance voltage dividing circuit. The quantity / voltage conversion circuit converts the charge of the capacity difference into a voltage.

  It is possible to avoid the influence of a fixed nominal capacitance that prevents the detection of a very small capacitance change due to the sound pressure of the ultra-small sound sensor capacitor. Capacitive microphones can be put to practical use on an industrial scale suitable for mass production, for example, for use in mobile devices, rather than on a research / experimental scale.

  FIG. 1 is a circuit diagram showing the main part of a capacitive microphone according to an embodiment of the present invention. The capacitance type microphone shown in the figure is of the DC bias type, and the sound of the ultra-small sound sensing capacitor Cm in which the capacitance changes due to the sound pressure, the fixed capacitor reference capacitor Cr, and the sound sensing capacitor Cm. And a detection circuit unit 20 that detects a minute capacitance change (ΔCm) due to pressure and outputs an acoustic signal (microphone output).

  The ultra-small acoustic capacitor Cm is formed on a dedicated semiconductor chip (silicon) IC 1 by MEMS. The reference capacitor Cr and the detection circuit unit 20 are formed on a semiconductor chip IC2 different from the sound sensing capacitor Cm. This is because, when the ultra-small sound-sensing capacitor Cm is fabricated on a semiconductor chip by MEMS, the sound-sensing capacitor Cm and the detection circuit unit 20 are configured separately in separate chips IC1 and IC2, thereby reducing the number of semiconductor manufacturing process steps. It is very advantageous in reducing and improving mass productivity and yield. The sound sensing capacitor Cm and the detection circuit unit 20 are connected by interchip wiring.

  A reference capacitor Cr having a fixed capacity is connected in series to the ultra-small sound-sensing capacitor Cm. As a result, both capacitors Cm and Cr form a capacitive voltage dividing circuit. The capacitive voltage dividing circuit is configured by the charge / discharge switch circuits S1 and S2 of the detection circuit unit 20 to have a common reference potential (GND) and a first potential + Vc1 and a second potential −Vc2 having opposite polarities with respect to the common reference potential. Are connected to each other alternately. In this case, the first potential + Vc1 is a variable potential provided by a variable voltage generation circuit 21 described later, and the second potential -Vc2 is a constant fixed potential.

  The detection circuit unit 20 includes charge / discharge switch circuits S1 and S2, a variable voltage generation circuit 21, a charge / voltage conversion circuit 23, an intermediate amplification circuit 24, an SH circuit (analog sample / hold circuit) 25 as a detection circuit, a low A band amplifier circuit 26, an LPF (low-pass filter) 27, a switching control circuit 31 and the like are included.

  The variable voltage generation circuit 21 outputs the first potential + Vc1 and variably controls the output potential + Vc according to a control voltage given from the outside.

  The charge / discharge switch circuits S1 and S2 are two-select switches each having two selected ports a and b for one common port c, and operate in synchronization with each other. When the ports a-c of S1 and S2 are connected, a first connection mode (discharge mode) is established in which both ends of the capacitive voltage dividing circuit (Cm, Cr series circuit) are connected to a common reference potential (GND). The Further, when the ports b-c of S1 and S2 are connected, a first connection mode (charging mode) in which both ends of the capacitance voltage dividing circuit (Cm, Cr) are connected to the first potential + Vc1 and the second potential -Vc2. ) Is set.

The switching operation of the charge / discharge switch circuits S1 and S2 is controlled by the switching control circuit 31. The switching control circuit 31 switches the switching circuits S1 and S2 at a frequency (high frequency) sufficiently higher than the frequency range of the acoustic signal.

  The charge amount / voltage conversion circuit 23 includes an operational amplifier OP1, a feedback capacitor C1 having a predetermined capacity, and a switch circuit S3 that discharges and resets the feedback capacitor C1 for each setting of the first connection mode.

  The operational amplifier OP1 virtually short-circuits (active ground) the inverting input (−) to the same potential as the non-inverting input (+) by capacitive negative feedback by the feedback capacitor C1. The charge amount of the differential capacitance that appears equivalently at the voltage dividing point A of (Cm, Cr) is mirror-transferred to the charge amount of the feedback capacitor C1 at a predetermined magnification. The feedback capacitor C1 is discharged and reset every time the first connection mode is set. Therefore, the mirror transfer is performed every time the first connection mode is set. Each time, a voltage pulse corresponding to the charge amount of the differential capacitor is output from the operational amplifier OP1.

  When the capacitive voltage dividing circuit (Cm, Cr) is connected between the first potential + Vc1 and the second potential -Vc2 via the switch circuits S1 and S2, the reference capacitor Cr has the capacitor capacitance (Cr) and the first capacitance. The charge corresponding to the product of the potential + Vc1 is charged to the sound sensing capacitor Cm, and the charge corresponding to the product of the capacitor capacity (Cm) × the second potential −Vc2 is charged.

  At this time, since the first potential + Vc1 and the second potential -Vc2 are opposite in polarity to the common reference potential (Vr = GND), both the capacitors Cr and Cm, that is, the voltage dividing point A has both capacitors Cr. , Cm, the difference capacity for charging the difference between the charge charges of Cm appears equivalently. That is, capacitance cancellation is equivalently performed between both capacitors Cr and Cm. The charge of the differential capacity is converted into voltage and output by the charge amount / voltage conversion circuit 23 every time the first connection mode (charge mode) is set.

  The charge amount / voltage conversion circuit 23 converts the charge amount of the equivalent differential capacitance observed at the voltage dividing point A into a voltage and outputs the voltage every time the first connection mode is set. The output voltage appears in a pulse shape for each operation of the charge / discharge switch circuits S1 and S2. Therefore, the charge amount / conversion circuit 23 outputs a continuous pulse signal (a high frequency pulse signal sufficiently higher than the acoustic frequency region) that is amplitude-modulated by the charge amount. This continuous pulse signal is input to the SH circuit 25 via the intermediate amplifier circuit 24.

  The intermediate amplifier circuit 24 is composed of an operational amplifier OP2, resistors R1 to R3, and DC blocking capacitors C2 and C3. The input and output of the intermediate amplifier circuit 24 are DC blocked by the capacitors C2 and C3, so that the pulse signal is AC amplified. To the next stage. The intermediate amplifier circuit 24 performs voltage amplification with a predetermined gain while faithfully holding the amplitude information (amplitude waveform) of the continuous pulse signal.

  The SH circuit 25 as a detection circuit includes sampling switch circuits S4 and S5, a voltage storage capacitor C4, an operational amplifier OP3, gain setting resistors R4 and R5, a phase adjustment feedback capacitor C5, and the like. The pulse amplitude of the signal is sampled and held and output. As a result, the SH circuit 25 operates as a kind of envelope detection circuit for the continuous pulse signal. The detection circuit operates as a synchronous detection circuit when the sampling switch circuits S4 and S5 operate in synchronization with the charge / discharge switch circuits S1 and S2. By this synchronous envelope detection, signals in the frequency domain up to the acoustic frequency domain are discriminated and output.

  The detection output of the SH circuit 25 is amplified and transmitted while the residual high frequency component is completely removed by the low-frequency amplifier circuit 26, and is derived as a microphone output Vao of a predetermined level. The low-frequency amplifier circuit 26 is configured to block high-frequency components exceeding the acoustic frequency region by the operational amplifier OP4, resistors R6 to R9, and capacitors C7 and C8. The low-frequency amplifier circuit 26 has a transmission band from the direct current range to the acoustic frequency range. Therefore, the microphone output Vao is used after the direct current is cut off by the alternating current coupling by the capacitor.

  The LPF 27 extracts a direct current component (ultra-low frequency signal) included in the output Vao of the low-frequency amplifier circuit 26. The extracted DC component is given to the variable voltage generation circuit 21 as a voltage control signal. As a result, a negative feedback control loop is formed for negative feedback control of the first potential + Vc1 so as to suppress the DC component to zero. By this negative feedback control, the direct current component appearing at the voltage dividing point A of the capacitance voltage dividing circuit (Cr, Cm) is also zero-suppressed.

  Although the LPF 27 is shown in a simplified manner in FIG. 1, an LPF in an extremely low frequency region is realized even with a very small capacitor Ct by using a capacitor Ct as a time constant circuit and a switched capacitor by a switch circuit S6. can do. As a result, it is not necessary to attach a large-capacitance capacitor for obtaining a large time constant, and the entire microphone including the detection circuit unit 20 can be further downsized.

  The differential capacitance that appears equivalently at the voltage dividing point A is a capacitance that appears equivalently due to the cancellation of the sound sensing capacitor Cm and the reference capacitor Cr. Using the capacitance (Cr) of the reference capacitor Cr and the first potential + Vc1 as parameters, the higher the first potential + Vc1, the greater the equivalent value of the counter capacitance, and vice versa. That is, the counter capacitance is variably controlled by the first potential + Vc1. Therefore, by controlling the first potential + Vc1 by the negative feedback control loop, it is possible to cancel out only the fixed nominal capacity mainly including the base capacity of the sound sensing capacitor Cm.

  As a result, even if the base capacitance of the sound-sensitive capacitor Cm is 1 to several [pF] and the capacitance change due to the sound pressure is as extremely small as 1/1000 or less of the capacitance, it is fixed. Only the nominal capacity can be canceled out, and the extremely small capacity change can be detected with a high S / N ratio. By this detection operation, the sound sensed by the sound sensing capacitor Cm as a very small capacitance change can be converted into a voltage.

  As described above, it is possible to avoid the influence of a fixed nominal capacitance that hinders detection of an extremely small change in capacitance due to the sound pressure of the ultra-small acoustic sensor capacitor. Capacitive microphones using sound capacitors can be put to practical use not on a research / experimental scale but on an industrial scale suitable for mass production, for example, for use in mobile devices.

  As described above, the present invention has been described based on the typical embodiments. However, the present invention can have various modes other than those described above. For example, the negative feedback control may be performed on the second potential −Vc2 or both the first and second potentials + Vc1 and −Vc2. The negative feedback control is also performed by, for example, amplifying a DC component extracted by the LPF 27 and superimposing a voltage obtained by the amplification on the first potential + Vc1 and / or the second potential -Vc2. It is possible.

  Capacitive microphones using ultra-small sound-sensitive capacitors manufactured by MEMS or the like may be put into practical use on an industrial scale suitable for mass production, for example, for mobile device incorporation, rather than on a research / experimental scale It becomes possible.

It is a circuit diagram which shows the principal part of the electrostatic capacitance type microphone which makes one Embodiment of this invention. It is a circuit diagram which shows an example of the conventional capacity | capacitance change detection circuit developed for the use different from an electrostatic capacitance type microphone. It is a circuit diagram which shows another example of the conventional capacity | capacitance change detection circuit developed for the use different from an electrostatic capacitance type microphone.

Explanation of symbols

Cm Ultra-small sound sensor capacitor Cr standard capacitor (counter capacity)
+ Vc1 1st potential -Vc2 2nd potential IC1 Semiconductor chip (ultra-small sound sensing capacitor)
IC2 Semiconductor chip (detection circuit part)
S1, S2 charge / discharge switch circuit 20 detection circuit section 21 variable voltage generation circuit 23 charge amount / voltage conversion circuit 24 intermediate amplification circuit 25 SH circuit (detection circuit)
26 Low frequency amplifier 27 LPF
31 Switching control circuit

Claims (9)

A capacitive microphone specified by the following items (1) to (10).
(1) A sound sensor capacitor, a reference capacitor, a first switch, a second switch, a switch control circuit, a reference voltage generation circuit, a calibration voltage generation circuit, a charge / voltage conversion circuit, and a detection circuit (2) One end of the sound sensor capacitor and one end of the reference capacitor are connected, and the connection point is connected to the input of the charge amount / voltage conversion circuit. (3) The sound sensor capacitor The other end of the reference capacitor is switched and connected to the output of the reference voltage generating circuit and the ground potential point by the first switch. (4) The other end of the reference capacitor is connected to the output of the calibration voltage generating circuit by the second switch. (5) The reference voltage generation circuit outputs a constant voltage. (6) The calibration voltage generation circuit has a polarity opposite to that of the output voltage of the reference voltage generation circuit and passes through a low frequency range. Phi (7) The switch control circuit includes a first connection state in which the first switch and the second switch are simultaneously connected to the ground potential point, and the first switch as a reference voltage generation circuit. And a second connection state in which the second switch is simultaneously connected to the output of the calibration voltage generation circuit at a constant cycle and at a high speed. (8) The charge amount / voltage conversion circuit includes the first switch, Synchronizing with the second switch and outputting a voltage proportional to the amount of charge input in the first connection state (9) The detection circuit detects an audio signal component from the output of the charge amount / voltage conversion circuit ( 10) For the low-pass filter, extract the low-frequency component of the detector circuit output and input it to the calibration voltage generator circuit. Capacitance type microphone specified by the following items (21) to (25).
(21) A sound-sensing capacitor that generates a change in capacitance due to sound pressure and a fixed-capacitance reference capacitor are connected in series to form a capacitance voltage dividing circuit.
(22) A first connection mode in which both ends of the capacitive voltage dividing circuit are connected to a common reference potential, and both ends of the capacitive voltage dividing circuit are connected to a first potential and a second potential having opposite polarities with respect to the common reference potential. A charge / discharge switch circuit that alternately switches and sets the second connection mode, and switching control means for performing a switching operation of the charge / discharge switch circuit at a frequency sufficiently higher than the frequency region of the acoustic signal.
(23) The differential capacitor that operates synchronously with the switching of the charge / discharge switch circuit and that appears equivalently between the voltage dividing point of the capacitor voltage dividing circuit and the common reference potential is set every time the first connection mode is set. A charge amount / voltage conversion circuit that converts the charge amount into a pulse voltage and outputs the pulse voltage is provided.
(24) A detection circuit for extracting a signal in a frequency domain from the output of the charge amount / voltage conversion circuit to the acoustic frequency domain is provided.
(25) having a low-pass filter that extracts a direct current component from the output of the detection circuit, and variably controlling at least one of the first potential or the second potential using the output of the low-pass filter Thus, a negative feedback control loop for suppressing the DC component to zero is formed.   3. The first potential and / or the second potential is supplied from a variable voltage generation circuit whose output voltage is variably controlled by voltage, and the variable voltage generation circuit is controlled by an output of a low-pass filter. Thus, a negative feedback control for suppressing the direct current component to zero is performed.   4. The capacitive microphone according to claim 2, wherein the low-pass filter is configured using a switched capacitor in a time constant circuit.   5. The calculation according to claim 2, wherein the charge amount / voltage conversion circuit is subjected to negative capacitance feedback by a feedback capacitor having a predetermined capacity, and discharge resetting the feedback capacitor every time the first connection mode is set. The charge charge amount of the differential capacitor is mirror-transferred to the charge amount of charge of the feedback capacitor by a negative feedback operation of the operational amplifier, and the charge charge amount is set for each setting of the second connection mode. A capacitance type microphone that outputs a pulse of a voltage corresponding to.   6. The detection circuit according to claim 2, wherein the detection circuit includes an analog sample / hold circuit that holds and outputs a voltage of a pulse output from the charge amount / voltage conversion circuit while sequentially updating the voltage. Capacitance type microphone characterized by     7. The capacitive microphone according to claim 2, further comprising an intermediate amplifier circuit that amplifies an output of the charge amount / voltage conversion circuit and transmits the output to the detection circuit. The capacitance type microphone specified by the following items (81) to (85).
(81) A sound-sensitive capacitor that generates a change in capacitance due to sound pressure, and a fixed-capacitance reference capacitor.
(82) The sound-sensing capacitor and the reference capacitor are charged with potentials of opposite polarities, thereby equivalently canceling the capacitance between the two capacitors.
(83) A charge amount / voltage conversion circuit that converts the charge amount of the differential capacitance that is equivalently extracted by the cancellation to voltage conversion is provided.
(84) An acoustic signal as a microphone output is extracted from the voltage conversion output, and a direct current component included in the voltage conversion output is extracted.
(85) A negative feedback control loop is formed in which the DC component is zero-suppressed by feeding back the DC component to the charging potential of at least one of the reference capacitor or the sound sensing capacitor. 9. The capacitive voltage dividing circuit according to claim 8, wherein the sound sensing capacitor and the reference capacitor are connected in series to form a capacitive voltage dividing circuit. By connecting to the potential and charging, the capacitance difference of both capacitors appears equivalently at the voltage dividing point of the capacitance voltage dividing circuit, and the charge amount / voltage conversion circuit converts the charge of the capacitance difference into voltage. Capacitance type microphone characterized by that.

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