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DESCRIPTION JP2006229336

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DESCRIPTION JP2006229336
An object of the present invention is to commercialize an electrostatic capacitance type
microphone using a microminiature sensing capacitor manufactured by MEMS or the like on an
industrial scale suitable for mass production, for example, for embedded in a portable device, not
on research and experimental scale. Do. SOLUTION: An equivalent cancellation of capacitance is
performed between the two capacitors by charging the noise-sensitive capacitor Cm and the
reference capacitor Cr with potentials + Vc1 and -Vc2 opposite to each other, and equivalently
extracted by this cancellation. The microphone output is obtained by voltage converting the
charge amount of the differential capacitance (ΔCm), and the DC component is obtained by
feeding back the direct current component extracted from the voltage conversion output to at
least one of the potential + Vc1 and -Vc2 Suppress to zero. [Selected figure] Figure 1
Capacitive microphone
[0001]
The present invention relates to a microminiature electrostatic capacitance type microphone
which is particularly effective when applied to a microphone mounted on a small portable device
such as a portable telephone.
[0002]
Capacitive microphones can be divided into two types, DC bias type (DC bias capacitor
microphone) and electret type (electret capacitor microphone), according to a method of
applying a bias electric field to a sound sensing capacitor.
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[0003]
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 in series via a resistor of high impedance (for example, 10
MΩ) between the electrodes of this noise sensitive capacitor, a voltage change corresponding to
the capacitance change of the sound sensitive capacitor due to the sound pressure is applied
across the series resistance. appear.
This voltage change is separated and extracted from the DC bias voltage by the AC coupling
circuit, and amplified while being impedance converted by the first stage circuit of high input
impedance, so that the microphone output (acoustic signal) available in the subsequent stage
circuit can be extracted. it can.
[0004]
In this DC bias system, there is a need to externally apply a high voltage DC bias voltage.
Therefore, in general, electrets that do not require the DC bias voltage are often used. The
electret type is also mounted on small portable devices such as cellular phones.
[0005]
In the electret type, a dielectric so-called electret in which a large amount of electric charge is
accumulated and fixed by polarization or the like is disposed between a movable electrode
forming an acoustic diaphragm and a fixed electrode opposed to the movable electrode. A high
bias electric field is constantly applied to the space of. The movable electrode and the fixed
electrode form a sound sensing capacitor that causes a capacitance change due to sound
pressure. When the capacitance of the sound sensing capacitor changes due to the sound
pressure, the bias electric field causes a voltage change according to the capacitance change
Appear in A microphone signal that can be used in the subsequent stage circuit can be extracted
by performing impedance conversion of this voltage change using, for example, a source follower
of an FET.
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[0006]
Although the electret type condenser microphone does not have the trouble of applying a high
voltage DC bias voltage from the outside, the performance deterioration due to the deterioration
of the electret becomes a problem. In the electret, the accumulated charge gradually discharges
due to insulation failure of the dielectric itself or the surface thereof, so that the initially high
electric field can not be stably maintained for a long time. It causes so-called deterioration with
time. This deterioration (loss of accumulated charge) appears notably at high temperatures,
although it depends on the environmental conditions. Therefore, when the electret condenser
microphone is soldered, it is necessary to be sufficiently careful not to raise the processing
temperature.
[0007]
However, in recent years it has become inevitable to prohibit the use of heavy metals such as
lead to prevent environmental pollution, but lead-free solder that does not contain lead has a
temperature that is about 30 ° C higher than conventional leaded solder. Require processing in
For this reason, the electret condenser microphone described above has a problem that the
performance thereof is likely to be deteriorated at the assembly stage.
[0008]
Moreover, as for the conventional electrostatic capacitance type microphone mentioned above, as
for the electret system and the DC bias system, the sound sensing capacitor which makes the
main part was comprised by the assembly of the some components produced separately. It was
produced by the so-called discrete assembly method.
[0009]
On the other hand, there is a strong demand for higher performance and smaller size in portable
devices such as mobile phones in which this type of microphone is incorporated, and accordingly,
performance enhancement and superconductivity of various functional components such as
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circuits incorporated in the portable device. Miniaturization is in progress. There is no exception
to microphones, and significant miniaturization is required.
[0010]
In recent years, micro-machining technology of silicon (MEMS: Micro Electro Mechanical
Systems) has made it possible to process an ultra-small sound-sensitive capacitor on a silicon
chip. If the microminiature sound sensing capacitor manufactured by this MEMS can be used for
a capacitance type microphone, a microminiature capacitance type microphone suitable for builtin use in the above-mentioned portable device becomes possible. In this case, a DC-biased
condenser microphone is suitable for MEMS fabrication.
[0011]
However, the base capacitance (unchanged capacitance) of the microminiature sensing capacitor
manufactured by MEMS etc. is significantly smaller than that of the conventional discrete
assembly type sensing capacitor, and it is only about 1 to several [pF]. Absent. The volume
change due to sound pressure is less than one thousandth of the base volume. As an example, in
the case of a sound sensing capacitor having a base capacitance of about 1 [pF], the capacitance
change due to the sound pressure is only about 1 [fF] (1 fF = 1/1000 [pF]).
[0012]
In order to detect this very slight capacitance change using the DC bias voltage and the series
resistance described above, a very high DC bias voltage and a series resistance of an extremely
high resistance value are required, and at least in the above-mentioned portable device. Is
difficult to achieve.
[0013]
Therefore, the inventor examined a circuit as shown in FIG. 2 in order to efficiently detect a slight
change in capacitance, for example, one thousandth of a base capacitance of 1 [pF].
Although this detection circuit is a capacitance change detection circuit for general measurement
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disclosed in Patent Document 1 and is not for a capacitive microphone, it is examined whether
this detection circuit can be applied to the capacitance change detection of the above-mentioned
sound sensing capacitor. did.
[0014]
The detection circuit shown in FIG. 2 is configured using a switch circuit S1 that performs
charging / discharging 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 switching operation periodically. One end of the detected capacitor Cx is
alternately connected to the ports a and b for each switching operation. 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. Further, the other end of the detected capacitor
Cx is connected to the common reference potential (GND).
[0015]
The charge amount / voltage conversion circuit 23 is configured of 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
noninverting input (+) of the operational amplifier OP1 is connected to the common reference
potential (Vr = GND). Therefore, the inverting input (-) of the operational amplifier OP1 is
virtually shorted (actively grounded) to the common reference potential (Vr) by negative
feedback.
[0016]
In the above circuit, first, when the port ac of S1 is connected, a charge corresponding to the
product of the capacitor capacitance (Cx) of the detected capacitor Cx and the charging potential
-Vc is connected to the detected capacitor Cx. Be charged. At this time, the feedback capacitor Cf
is short-circuited by the switch circuit S3 and is in the discharge reset state (non-charge state).
[0017]
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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 opens, and the negative
feedback capacitor Cf intervenes in the negative feedback control loop of the operational
amplifier OP1. Then, a charge corresponding to the charge amount (Vr × Cx) of the detected
capacitor Cx is charged in the feedback capacitor Cf by the negative feedback operation forming
the virtual short circuit. At this time, a voltage for charging the charge amount (Vr × Cx) to the
feedback capacitor Cf appears at the output of the operational amplifier OP1.
[0018]
That is, in the charge amount / voltage conversion circuit 23, the charge amount of the detected
capacitor Cx is mirror-transferred to the charge amount of the feedback capacitor Cf at a
predetermined magnification, and the charge amount necessary for the mirror transfer is used as
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.
[0019]
Here, the present inventor verified whether the capacitance change due to the sound pressure
can be practically detected when the detected capacitor Cx is replaced with the microminiature
sound sensing capacitor.
However, as described above, the capacitance change due to the sound pressure of the sound
sensing capacitor is extremely smaller than the base capacitance. For this reason, it has been
found that fixed nominal capacity such as base capacity is dominant in detection of the capacity
change, and it is not possible to detect the target capacity change with a high SN ratio.
[0020]
In order for detection to be performed at a high signal-to-noise ratio, it is necessary to operate
the detection circuit in the middle of its input / output dynamic range. The dynamic range
becomes narrower as the detection sensitivity is increased. Although it is necessary to reduce the
dynamic range in order to detect a minute capacitance change, when the dynamic range is
reduced, it is difficult to stably keep the operating point of the detection circuit in the center of
the input / output dynamic range due to the above nominal capacitance. become. In addition, the
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detection at a high SN ratio is also hindered by the fact that a minute volume change is masked
by a relatively large nominal volume. In any case, the relatively large nominal capacity is an
impediment to detection.
[0021]
Therefore, in order to remove or reduce the fixed nominal capacitance which hinders detection of
a minute capacitance change, the present inventor examined the use of a detection circuit as
shown in FIG. See Figure 6).
[0022]
The detection circuit shown in FIG. 3 detects a change in capacitance of the detected capacitor Cx
by the charge amount / voltage conversion circuit 23 as in the case shown in FIG. 2 but
equivalently uses a fixed nominal capacitance. A reference capacitor (counter capacitor) Cr is
used to cancel out.
That is, by charging the detection capacitor Cx and the reference capacitor Cr with the potentials
-Vc and + Vc having opposite polarities to each other, capacitance cancellation is performed
between the two capacitors Cx and Cr.
[0023]
In FIG. 3, the detection capacitor Cx and the reference capacitor Cr are connected in series to
form a capacitive voltage dividing circuit. When charging is performed by connecting both ends
of this capacitive voltage dividing circuit to potentials −Vc and + Vc opposite in polarity to each
other by switch circuits S1 and S2, both capacitors Cx and Cr are connected to voltage dividing
point A of the capacitive voltage dividing circuit. The differential capacitance (Cx-Cr) of L appears
equivalently.
[0024]
By converting the charge amount of the difference capacitance (Cx-Cr) into a voltage, it is
possible to efficiently detect only the change in capacitance of the detection capacitor Cx without
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being hindered by the fixed nominal capacitance. It is foreseeable. 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 also be detected by high SN will arise.
[0025]
However, according to the knowledge of the inventor of the present invention, it has been found
that even with the circuit shown in FIG. 3, the capacitance change due to the sound pressure of
the microminiature sound-sensing capacitor can not be detected at a practical level. That is, it
turned out that the forecast mentioned above was deviated and the expectation was betrayed.
This is due to the fact that the capacitance of the microminiature sound sensing capacitor is too
small. As described above, the base capacitance of the microminiature noise-sensitive capacitor
manufactured by MEMS or the like is very small, at most about 1 to several [pF]. Furthermore,
the change in volume due to sound pressure is only one thousandth or less.
[0026]
In order to detect this extremely minute change in capacitance at a practical level of SN ratio, it is
necessary to perform capacitance cancellation with a reference capacitor (counter capacitor) with
extremely high accuracy. For this purpose, it is necessary to minimize the capacitance error
between the sensor capacitor and the reference capacitor, and it is necessary to make the above
error smaller than at least the change in capacitance of the sensor capacitor. In this case, since
the capacitance change of the sound sensing capacitor is equal to or less than 1/1000 of the
base capacitance, the capacitance error of the reference capacitor is required to be equal to or
less than 1/1000. However, it is practically impossible to realize such a high precision reference
capacitor in a small capacitance range of 1 to several [pF]. In particular, it can not be realized on
an industrial scale where mass productivity and yield become problems.
[0027]
In addition to the base capacitance of the sound sensing capacitor, the fixed nominal capacitance
also includes a difficult-to-predict capacitance such as stray capacitance distributed in wiring or
the like. Even in the case of stray capacitance that is normally negligible, a critical disturbance
factor that prevents normal detection in situations where it is necessary to detect a capacitance
change of less than 1000 in a small capacitance range of 1 to several [pF] It becomes.
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[0028]
It has been described that the microminiature sound sensing capacitor can be fabricated on a
semiconductor chip by MEMS, but in the technical field (semiconductor integrated circuit etc.),
the sound sensing capacitor and the circuit part such as a detection circuit are on the same
semiconductor chip It was believed to be optimal to form an accumulation.
[0029]
However, in the case of fabricating the microminiature noise sensing capacitor on a
semiconductor chip by MEMS, however, the present inventor is more likely to divide the noise
sensing capacitor and the detection circuit into separate chips in terms of the number of process
steps of semiconductor manufacturing and We have come to know that it is very advantageous
from the standpoint of yield etc.
This is because the preparation conditions (for example, process and the like) of the two are
largely different. In this case, although the sensing capacitor and the sensing circuit which are
separately manufactured on separate chips are connected by the inter-chip wiring, this wiring is
also a factor to increase the above-mentioned nominal capacitance.
[0030]
The present invention has been made under the technical background as described above, and
the purpose thereof is to research and experimental scale an electrostatic capacitance type
microphone using a microminiature sound sensing capacitor manufactured by MEMS or the like.
For example, it is to make it practical on an industrial scale suitable for mass production as
embedded in portable devices. Patent document 1: JP-A-2004-184307
[0031]
The capacitive microphone according to the present invention is basically identified by the
following items (1) to (10). (1) Sound sensing capacitor, reference capacitor, first switch, second
switch, switch control circuit, reference voltage generation circuit, calibration voltage generation
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circuit, charge amount / voltage conversion circuit, detection circuit , Low-pass filter (2) one end
of the sound sensing capacitor is connected to one end of the reference capacitor, and the
connection point is connected to the input of the charge amount / voltage conversion circuit (3)
sound sensing capacitor The other end of the reference capacitor is switched between the output
of the reference voltage generation circuit and the ground potential point by the first switch. (4)
The other end of the reference capacitor is the output of the calibration voltage generation
circuit with the second switch. Switch to ground potential point (5) Reference voltage generation
circuit outputs fixed voltage (6) Calibration voltage generation circuit has reverse polarity to the
output voltage of reference voltage generation circuit, and low pass Output a voltage that
changes according to the output of the filter. (7) The switch control circuit simultaneously
connects the first switch and the second switch to the ground potential point, and the reference
voltage generating circuit. And a second connection state in which the second switch is
connected to the output of the calibration voltage generation circuit at the same time, and is
repeated at high speed in a fixed cycle (8) The charge amount / voltage conversion circuit is the
first switch Operate in synchronization with the second switch and output 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)
The low pass filter should extract the low frequency component of the output of the detection
circuit and input it to the calibration voltage generation circuit
[0032]
The capacitive microphone according to the present invention is also specified by the following
items (21) to (25). (21)A sound sensing capacitor that produces a capacitance change due to
sound pressure and a reference capacitor of fixed capacitance 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 a second
connection mode in which both ends of the capacitive voltage dividing circuit are connected to
first and second potentials opposite to each other with respect to the common reference
potential. The charge / discharge switch circuit alternately switches between the connection
mode and the switching mode, and switching control means for switching the charge / discharge
switch circuit at a frequency sufficiently higher than the frequency range of the acoustic signal.
(23)It operates in synchronization with the switching of the charge / discharge switch circuit,
and for each setting of the first connection mode, the charge amount of the differential
capacitance which appears equivalent between the voltage dividing point of the capacity voltage
dividing circuit and the common reference potential A charge amount / voltage conversion
circuit is provided which converts into a pulse voltage and outputs it. (24)A detection circuit is
provided which extracts a signal in a frequency range from the output of the charge amount /
voltage conversion circuit to an acoustic frequency range. (25)A low pass filter for extracting
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a direct current component from the output of the detection circuit is provided, and the output of
the low pass filter is used to variably control at least one of the first potential and the second
potential. A negative feedback control loop is formed which suppresses the minutes to zero.
[0033]
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 a
voltage, and the variable voltage generation circuit is controlled by the output of a low pass filter.
Performing negative feedback control to suppress the DC component to zero (B) the low pass
filter is configured using a switched capacitor in a time constant circuit (C) the charge amount /
voltage conversion circuit An operational amplifier is used which is given capacitive negative
feedback by a feedback capacitor of a predetermined capacitance and discharges and resets the
feedback capacitor at each setting of the first connection mode, and the differential capacitance
is given by the negative feedback operation of the operational amplifier By mirror-transferring
the charge amount of the charge to the charge amount of the charge of the feedback capacitor,
the Output a pulse of a voltage corresponding to the charge amount (D) The detection circuit
holds and outputs the voltage of the pulse output from the charge amount / voltage conversion
circuit while sequentially updating and outputs an analog sample hold (E) comprising an
intermediate amplification circuit for AC amplifying the output of the charge amount / voltage
conversion circuit and transmitting it to the detection circuit
[0034]
The capacitive microphone according to the present invention is also specified by the following
items (81) to (85). (81)A sound sensing capacitor that produces a capacitance change due to
sound pressure and a reference capacitor of a fixed capacitance are provided. (82)By
equivalently charging the noise-sensitive capacitor and the reference capacitor with potentials of
opposite polarities, the capacitances are equivalently canceled out. (83)A charge amount /
voltage conversion circuit is provided for converting the charge amount of the differential
capacitance which is equivalently extracted by the above-mentioned offsetting. (84)An
acoustic signal as a microphone output is taken out from the voltage conversion output, and a DC
component contained in the voltage conversion output is extracted. (85)A negative feedback
control loop is formed in which the DC component is suppressed to zero by feeding back the DC
component to the charging potential of at least one of the reference capacitor or the sound
sensing capacitor.
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[0035]
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 common. By connecting to the first
potential and the second potential opposite to each other with respect to the reference potential
and charging, the capacitance difference between both capacitors appears equivalently at the
voltage dividing point of the capacity voltage dividing circuit, The quantity / voltage conversion
circuit converts the charge of the above-mentioned capacity difference into a voltage.
[0036]
It is possible to avoid the influence of a fixed nominal capacitance that interferes with the
detection of an extremely small capacitance change due to the sound pressure of the
microminiature sensor capacitor, and thereby, the static using the microminiature sensor
capacitor manufactured by MEMS etc. Capacitive microphones can be put to practical use on
industrial scale suitable for mass production, for example, for embedded in portable devices, not
on research and experimental scale.
[0037]
FIG. 1 is a circuit diagram showing the main part of a capacitive microphone according to an
embodiment of the present invention.
The electrostatic capacitance type microphone shown in the figure is of the DC bias type, and the
sound of the microminiature sound sensing capacitor Cm which causes a capacitance change due
to sound pressure, the reference capacitor Cr of a fixed capacitance, and the sound sensing
capacitor Cm. And a detection circuit unit 20 for detecting a slight change in capacitance (ΔCm)
due to pressure and outputting an acoustic signal (microphone output).
[0038]
The microminiature noise sensing capacitor Cm is formed on a dedicated semiconductor chip
(silicon) IC 1 by MEMS.
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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 microminiature
sound sensing capacitor Cm is fabricated on a semiconductor chip by MEMS, it is necessary to
divide the sound sensing capacitor Cm and the detection circuit unit 20 into separate chips IC1
and IC2, which means the number of process steps of semiconductor manufacturing It is because
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 an interchip wire.
[0039]
A reference capacitor Cr having a fixed capacitance is connected in series to the microminiature
sound sensing capacitor Cm. Thus, both capacitors Cm and Cr form a capacitive voltage dividing
circuit. The capacitance voltage dividing circuit is controlled by the charge / discharge switch
circuits S1 and S2 of the detection circuit unit 20 to a common reference potential (GND) and a
first potential + Vc1 and a second potential -Vc2 which have mutually opposite polarities with
respect to the common reference potential. And are alternately connected and connected. In this
case, the first potential + Vc1 is a variable potential supplied by a variable voltage generation
circuit 21 described later, and the second potential -Vc2 is a fixed potential.
[0040]
The detection circuit unit 20 includes charge / discharge switch circuits S1 and S2, a variable
voltage generation circuit 21, a charge amount / voltage conversion circuit 23, an intermediate
amplification circuit 24, an SH circuit (analog sample hold circuit) 25 as a detection circuit, It
comprises a band amplification circuit 26, an LPF (low pass filter) 27, a switching control circuit
31, and the like.
[0041]
The variable voltage generation circuit 21 outputs the first potential + Vc1 and variably controls
the output potential + Vc in accordance with a control voltage supplied from the outside.
[0042]
The charge / discharge switch circuits S1 and S2 are two selection switches each having two
selected ports a and b for one common port c, and operate in synchronization with each other.
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When ports a and c of S1 and S2 are connected, a first connection mode (discharge mode) is set
in which both ends of the capacitive voltage divider circuit (Cm, Cr series circuit) are connected
to a common reference potential (GND) Ru.
In addition, when the ports b and c of S1 and S2 are connected, the 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.
[0043]
The switching control circuit 31 controls the switching operation of the charge / discharge
switch circuits S1 and S2. The switching control circuit 31 switches the switch circuits S1 and S2
at a frequency (high frequency) sufficiently higher than the frequency range of the acoustic
signal.
[0044]
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 for discharging
and resetting the feedback capacitor C1 for each setting of the first connection mode.
[0045]
The operational amplifier OP1 virtually shorts (active grounds) the inverting input (-) to the same
potential as the non-inverting input (+) by capacitive negative feedback by the feedback capacitor
C1, but by the negative feedback operation, the capacitive voltage divider circuit The charge
amount of charge of the differential capacitance equivalently appearing at the voltage dividing
point A of (Cm, Cr) is mirror-transferred to the charge amount of charge of the feedback
capacitor C1 at a predetermined magnification.
The feedback capacitor C1 is discharged and reset each time the first connection mode is set.
Therefore, the mirror transfer is performed each time the first connection mode is set. Then, each
time, a pulse of a voltage corresponding to the charge amount of the differential capacity is
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output from the operational amplifier OP1.
[0046]
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
and the first capacitor capacitance Cr The charge corresponding to the product of the potential +
Vc1 is charged, and the charge corresponding to the product of the capacitor capacitance (Cm)
× the second potential -Vc2 is charged to the sound sensing capacitor Cm.
[0047]
At this time, since the first potential + Vc1 and the second potential -Vc2 have opposite polarities
to each other with respect to the common reference potential (Vr = GND), both capacitors Cr and
Cm are connected to each other, that is, both capacitors Cr , And Cm appear equivalently as a
differential capacitance for charging the difference between the charge charges.
That is, capacitance cancellation is equivalently performed between the two capacitors Cr and
Cm. The charge of the difference capacity is voltage-converted by the charge amount / voltage
conversion circuit 23 every setting of the first connection mode (charging mode) and output.
[0048]
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 it for
each setting of the first connection mode. The output voltage appears in the form of a pulse for
each operation of the charge and 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. The continuous pulse signal is input to the SH circuit 25 through the intermediate
amplification circuit 24.
[0049]
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The intermediate amplification circuit 24 is composed of an operational amplifier OP2, resistors
R1 to R3 and DC blocking capacitors C2 and C3, and the input and output thereof are DC blocked
by the capacitors C2 and C3 so that the pulse signal is AC amplified. And transmit to the next
stage. The intermediate amplification circuit 24 performs voltage amplification with a
predetermined gain while faithfully holding the amplitude information (amplitude waveform) of
the continuous pulse signal.
[0050]
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, etc. It samples and holds the pulse amplitude of the signal and
outputs it. Thus, the SH circuit 25 operates as a kind of envelope detection circuit for the
continuous pulse signal. Further, this detection circuit operates as a synchronous detection
circuit as 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.
[0051]
The detection output of the SH circuit 25 is amplified and transmitted while the low frequency
amplification circuit 26 completely removes the residual high frequency component, and is
derived as a microphone output Vao of a predetermined level. The low band amplification circuit
26 is configured to cut off high frequency components exceeding the acoustic frequency range
by the operational amplifier OP4, the resistors R6 to R9, and the capacitors C7 and C8. The low
band amplification circuit 26 has a transmission band from the direct current range to the
acoustic frequency range. Therefore, the microphone output Vao is used after DC blocking by AC
coupling with a capacitor.
[0052]
The LPF 27 extracts a direct current component (ultra low frequency signal) contained in the
output Vao of the low band amplification circuit 26. The extracted DC component is supplied to
the variable voltage generation circuit 21 as a voltage control signal. As a result, a negative
feedback control loop is formed which performs negative feedback control of the first potential +
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Vc1 so as to suppress the DC component to zero. By this negative feedback control, the DC
component appearing at the voltage dividing point A of the capacitive voltage dividing circuit (Cr,
Cm) is also suppressed to zero.
[0053]
The LPF 27 is illustrated in a simplified manner in FIG. 1, but the use of a switched capacitor of a
capacitor Ct and a switch circuit S6 as a time constant circuit realizes an LPF in an ultra low
frequency region even with a very small capacitor Ct. can do. As a result, it becomes unnecessary
to externally attach a large capacity capacitor for obtaining a large time constant, and the entire
microphone including the detection circuit unit 20 can be further miniaturized.
[0054]
The differential capacitance that appears equivalently at the voltage dividing point A is a
capacitance that appears equivalently by the cancellation of the sound sensing capacitor Cm and
the reference capacitor Cr, but the capacitance so-called counter capacitance that is equivalently
offset by the reference capacitor Cr is The capacitance (Cr) of the reference capacitor Cr and the
first potential + Vc1 are used as parameters, and the equivalent value of the counter capacitance
increases as the first potential + Vc1 increases, and vice versa if the potential is lower. That is, the
counter capacitance is variably controlled by the first potential + Vc1. Therefore, by controlling
the first electric potential + Vc1 with the negative feedback control loop, it is possible to offset
only the fixed nominal capacitance mainly composed of the base capacitance of the sound
sensing capacitor Cm without excess or deficiency.
[0055]
As a result, even if the base capacitance of the sound sensing capacitor Cm is a minute
capacitance of 1 to several [pF] and the capacitance change due to the sound pressure is an
extremely minute of 1/1000 or less of the minute capacitance, It is possible to detect only the
nominal capacitance and detect an extremely small change in capacitance with a high SN ratio.
By this detection operation, it is possible to convert the voltage of the sound that the sound
sensing capacitor Cm has captured as a very small change in capacitance.
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[0056]
As described above, it is possible to avoid the influence of the fixed nominal capacitance that
interferes with the detection of a very small capacitance change due to the sound pressure of the
microminiature sound sensing capacitor, thereby making the microminiature sensor
manufactured by MEMS etc. Capacitive microphones using a sound capacitor can be put to
practical use on an industrial scale suitable for mass production, for example, as embedded in a
portable device, not on research and experimental scale.
[0057]
Although the present invention has been described above based on the representative
embodiments, various aspects of the present invention are possible 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, for example, by amplifying a direct current component extracted by the LPF 27 and
superimposing a voltage obtained by this amplification on the first potential + Vc1 and / or the
second potential -Vc2. It is possible.
[0058]
It is possible to commercialize, on research scale and industrial scale, for example, an
electrostatic scale type microphone using a microminiature sense-sensitive capacitor
manufactured by MEMS etc., not on research and experimental scale, for example, for embedded
in portable devices. It will be possible.
[0059]
It is a circuit diagram showing an important section of an electric capacity type microphone
which constitutes one embodiment of the present invention.
FIG. 16 is a circuit diagram showing an example of a conventional capacitance change detection
circuit developed for applications other than capacitive microphones. FIG. 16 is a circuit diagram
showing another example of a conventional capacitance change detection circuit developed for
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applications other than capacitive microphones.
Explanation of sign
[0060]
Cm Microminiature sound sensing capacitor Cr Reference capacitor (counter capacitance) + Vc1
1st potential-Vc2 2nd potential IC 1 Semiconductor chip (microminiature sound sensing
capacitor) IC 2 Semiconductor chip (detection circuit section) 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 amplification circuit 27 LPF 31 Switching control circuit
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