JP2016123028A - Capacitor microphone and microphone circuit for use therein - Google Patents

Abstract

PROBLEM TO BE SOLVED: To output a sound signal excellent in reproducibility and even a considerably large sound pressure is not distorted when it is inputted, in a capacitor microphone.SOLUTION: In a capacitor microphone, a microphone element 10 converts a change of capacitance between a movable electrode and a fixed electrode, dependent on the sound pressure, into a voltage signal or an acoustic-electric conversion signal ms. An amplitude control circuit 16 provided in a microphone circuit 12 monitors the amplitude value of a preamplifier output signal MS outputted from a preamplifier 14, and when the amplitude value exceeds a predetermined monitoring value, applies a reverse phase signal cshaving a waveform of reverse phase (reversal of polarity) and a relatively low amplitude for the acoustic-electric conversion signal ms to one side of the microphone element 10.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a condenser microphone that converts voice or sound into an electrical signal by using a change in capacitance, and a microphone circuit used therefor.

  In recent years, MEMS (Micro Electro Mechanical System) microphones monolithically fabricated on a semiconductor substrate using a wafer process have the advantages of miniaturization, thinning, high heat resistance, and mass productivity. It is popular.

  A MEMS microphone is a type of condenser microphone. A movable electrode that vibrates under sound pressure and a fixed electrode that remains stationary even under sound pressure are placed in parallel via a gap (thus forming a capacitor). ), A change in capacitance according to the vibration of the movable electrode is taken out as a voltage signal, that is, an acoustoelectric conversion signal. In general, an acoustoelectric transducer or MEMS microphone element of a MEMS microphone forms a large number of holes (openings) in the fixed electrode on the front side that directly receives sound waves, and the inner movable electrode is formed by the pressure of sound that has passed through these holes. And has a cavity (back chamber) that forms an acoustic resonator behind the movable electrode.

  Unlike an electret condenser microphone (ECM), a MEMS microphone requires application of a DC (direct current) bias voltage in order to store electric charge in a capacitance between electrodes. Usually, a DC bias voltage is applied to the movable electrode, and an acoustoelectric conversion signal is extracted from the fixed electrode. The acoustoelectric conversion signal output from the MEMS microphone element is input to the first stage amplifier or preamplifier, where it undergoes impedance conversion and is amplified with a predetermined gain. In this case, there is a parasitic capacitance between the input terminal of the preamplifier and the ground potential. Since the capacitance of the MEMS microphone element is very small, if the parasitic capacitance is considerably large, the microphone sensitivity may be lowered. For this reason, it is desirable that the parasitic capacitance of the preamplifier is as small as possible.

  On the other hand, as a bias clamp element connected between the input terminal of the preamplifier and the reference potential (normal ground potential) in a MEMS microphone, a high resistance value and stability in a short time are required. -Many parallel diode pairs (APDP) are used. APDP is a circuit in which a pair of diodes are connected in parallel with their polarities opposite to each other. Of these pair of diodes, one diode applied in the reverse direction acts as a high resistance to a signal voltage equal to or lower than the breakdown voltage. The other diode applied in the forward direction also acts as a high resistance against a signal voltage equal to or lower than the ON voltage.

JP2013-85025A

  In general, the major sensitivity of MEMS microphones is around -40 [dBV / Pa]. For example, a microphone sensitivity of -42 dBV means that a signal voltage of -42 dBV = 7.9 mVrms is output from the microphone for a pressure of 1 Pascal sound (assuming that the preamplifier has a gain of 1 (gain 0 dB)). To do). Here, the standard pressure of 1 Pascal corresponds to a sound pressure level of 94 dBVspl, and is a volume when a person speaks loudly. Of course, MEMS microphones are no exception when the basic performance of microphones is to require not only high sensitivity but also good sound quality characteristics.

  However, in conventional condenser microphones (especially MEMS microphones), if a large sound pressure (eg, 124 dBVspl or more) that is not uncommon is input, distortion occurs in the output signal and sound quality characteristics (particularly reproducibility) are significantly reduced. Is a problem.

  The present invention has been made in view of the problems of the prior art, and provides a condenser microphone capable of outputting a sound signal having excellent reproducibility that is not distorted even when a considerably large sound pressure is input, and a microphone circuit used therefor. To do.

  The microphone circuit of the present invention has a movable electrode and a fixed electrode that are arranged to face each other with a gap therebetween, and changes the capacitance between the movable electrode and the fixed electrode in accordance with the input sound pressure according to the first change. A microphone circuit that is electrically connected to a capacitor-type microphone element that converts to a signal and performs a certain signal processing on the first signal, the input terminal of which is connected to the output terminal of the microphone element 1 and an amplitude value of a second signal output from the first amplifier circuit, and when the amplitude value of the second signal exceeds a predetermined monitoring value, the first signal is changed to the first signal. On the other hand, an amplitude control circuit that applies a third signal having a waveform having a relatively low amplitude value and an opposite phase to either the movable electrode or the fixed electrode.

  In addition, the microphone of the present invention includes a movable electrode and a fixed electrode that are arranged to face each other via a gap, and a first change in capacitance between the movable electrode and the fixed electrode according to an input sound pressure is detected. A capacitor-type microphone element for converting the signal into a first signal, a first amplifier circuit whose input terminal is connected to the output terminal of the microphone element, and an amplitude value of the second signal output from the first amplifier circuit. And when the amplitude value of the second signal exceeds a predetermined monitoring value, a third signal having a waveform having a relatively low amplitude value and an antiphase with respect to the first signal is transmitted to the movable electrode or An amplitude control circuit applied to any one of the fixed electrodes.

  In the microphone circuit or microphone of the present invention, when the amplitude value of the second signal output from the first amplifier circuit does not exceed the monitoring value, the third signal is not supplied from the amplitude control circuit to the microphone element. . As a result, the first signal generated by the acoustoelectric conversion by the microphone element is directly input to the first amplifier circuit. However, when the amplitude value of the second signal exceeds the monitored value, the sum of the first signal generated by the acoustoelectric conversion generated by the microphone element and the third signal from the amplitude control circuit is added. A signal is input to the first amplifier circuit.

  In a preferred aspect of the present invention, the input terminal of the first amplifier circuit is connected to either the movable electrode or the fixed electrode of the microphone element, and the third signal from the amplitude control circuit is connected to the other side of the coupling capacitor. Applied.

  In another preferred embodiment, a bias clamping element such as an anti-parallel diode pair is connected between the input terminal of the first amplifier circuit and the reference potential. In this case, the monitoring value is set to a level at which the amplitude value of the second signal reaches before the amplitude value of the signal input to the first amplifier circuit reaches the on-voltage of the anti-parallel diode pair. .

  In another preferred aspect, the amplitude control circuit includes a second amplifier circuit that amplifies the second signal output from the first amplifier circuit with a positive or negative gain.

  According to the condenser microphone or the microphone circuit of the present invention, it is possible to output an audio signal that is not distorted even if a considerably large sound pressure is input by the configuration and operation as described above, and has excellent reproducibility. Sound quality characteristics can be obtained.

It is a block diagram which shows the basic composition of this invention. It is a block diagram which shows the structure of the capacitor | condenser microphone in one Embodiment of this invention. It is a figure which shows typically the structure of the principal part of the MEMS microphone element used for the said capacitor | condenser microphone. It is a figure which shows typically the waveform of the acoustoelectric signal output from the said MEMS microphone element. It is a block diagram which shows one structural example of the control part contained in the said capacitor | condenser microphone. It is a figure which shows the gain characteristic of the variable attenuator and inverting amplifier which are contained in the said capacitor | condenser microphone. It is a figure for demonstrating typically the main effect | action in the said capacitor | condenser microphone. It is a figure for demonstrating typically the main effect | action in the said capacitor | condenser microphone. It is a figure for demonstrating typically the main effect | action in the said capacitor | condenser microphone. It is a figure which shows the waveform of each part obtained by simulation in the said capacitor | condenser microphone. It is a block diagram which shows the structure of the principal part of the amplitude control circuit in one modification. It is a block diagram which shows the structure of the principal part of the amplitude control circuit in another modification.

[Basic configuration]
As shown in FIG. 1, the condenser microphone of the present invention includes a capacitor-type microphone element 10 and a microphone circuit 12 electrically connected to the microphone element 10 as basic components.

  The microphone element 10 is configured as a capacitor-type acoustoelectric conversion element, and a change in capacitance between the movable electrode and the fixed electrode according to the sound pressure is expressed as a voltage signal or an acoustoelectric conversion signal (first signal) ms. Convert to The microphone circuit 12 performs certain signal processing on the acoustoelectric conversion signal ms generated by the microphone element 10 (for example, one or more of impedance conversion, amplification, amplitude control, analog-digital conversion, noise removal processing). I do. The microphone circuit 12 includes at least a first-stage amplifier circuit or preamplifier 14 and an amplitude control circuit 16.

  The preamplifier 14 has an input terminal connected to the output terminal of the microphone element 10, performs impedance conversion from a high impedance to a low impedance for the voltage signal from the microphone element 10 or the preamplifier input signal AS, and has a constant gain. Signal amplification is performed (usually 0 dB or more).

The amplitude control circuit 16 monitors the amplitude value of the voltage signal output from the preamplifier 14 or the preamplifier output signal (second signal) MS, and when the amplitude value exceeds a predetermined monitoring value K, the acoustoelectric conversion signal ms. A voltage signal or an anti-phase signal (third signal) cs ₋ having a waveform with a relatively low amplitude and anti-phase (polarity inversion) with respect to the electrode on one side of the microphone element 10 (preferably an electrode on the output terminal side) The electrode is applied to the opposite electrode).

More specifically, when the amplitude value of the preamplifier output signal MS does not exceed the monitoring value K, the negative phase signal cs ₋ is not supplied from the amplitude control circuit 16 to the microphone element 10. Therefore, the acoustoelectric conversion signal ms generated by the microphone element 10 is directly input to the preamplifier 14 as the preamplifier input signal AS. However, when the amplitude value of the preamplifier output signal MS exceeds the monitoring value K, a sum signal (the sum signal (ac-electric conversion signal ms generated by the microphone element 10) and the reverse phase signal cs ₋ from the amplitude control circuit 16 is added. ms + cs ₋ ) is input to the preamplifier 14 as the preamplifier input signal AS.

According to the basic configuration of the present invention, even if the microphone element 10 receives a considerably large sound pressure (for example, 124 dBVspl or more) and generates the acoustoelectric conversion signal ms having a considerably large amplitude corresponding to the sound pressure, The reverse phase signal cs ₋ from the amplitude control circuit 16 is added to the acoustoelectric conversion signal ms in or around the element 10 in the form of subtraction, so that the waveform is almost distorted with the same phase as the acoustoelectric conversion signal ms. First, a preamplifier input signal AS whose amplitude value remains below a certain level is obtained. Thereby, the waveform of the preamplifier output signal AS output from the preamplifier 14 is not distorted.

When the microphone element 10 does not have an electret (permanent charge storage member), the bias power supply circuit 18 applies a DC bias voltage BV for storing a constant charge between the electrodes to one of the electrodes of the microphone element 10. Apply. When the microphone element 10 has an electret, the power supply circuit 18 can be omitted.

[Configuration of the embodiment]

  FIG. 2 shows the configuration of a condenser microphone in a preferred embodiment of the present invention. The microphone element 10 in this embodiment is a MEMS microphone element 20, and includes a movable electrode 20a and a fixed electrode 20b that are monolithically formed on a semiconductor substrate using a wafer process.

  The bias power supply circuit 18 has a charge pump circuit 22 and normally applies a DC bias voltage BV to the movable electrode 20a. In general, a charge pump circuit is a small and inexpensive DC / DC converter that boosts an input voltage by switching the connection state (series / parallel) of a plurality of capacitors that store charges at a constant cycle, but switching to an output voltage. It is easy to include ripples. This voltage ripple is easily transmitted to the preamplifier 14 via the MEMS microphone element 20. In this embodiment, the output voltage of the charge pump circuit 22 is passed through a low-pass filter 28 composed of a resistor 24 and a capacitor 26 to remove voltage ripple from the DC bias voltage BV.

  In FIG. 3, the structure of the principal part of the MEMS microphone element 20 is shown. As shown in the figure, the MEMS microphone element 20 has a film-like movable electrode 20a that vibrates when subjected to sound pressure and a rigid fixed electrode 20b that does not vibrate in parallel via an air gap to form a capacitor. . A large number of holes 28 are formed in the fixed electrode 20b on the front side that directly receives sound waves, and the inner movable electrode 20a vibrates due to the pressure of the sound that has passed through the holes 28, and the change in capacitance is a voltage signal, that is, an acoustic signal. It is converted into an electrical conversion signal ms.

  More specifically, when the movable electrode 20a is swung in the direction approaching the fixed electrode 20b, the capacitance is changed in the increasing direction, and the signal voltage of the acoustoelectric conversion signal ms is changed in the decreasing direction. When the approach of the movable electrode 20a reaches the peak value, the signal voltage of the acoustoelectric conversion signal ms is minimized. Immediately after that, when the movable electrode 20a is swung away from the fixed electrode 20b, the capacitance is changed in a decreasing direction, and the signal voltage of the acoustoelectric conversion signal ms is increased. When the separation of the movable electrode 20a reaches the peak value, the signal voltage of the acoustoelectric conversion signal ms is maximized. Thereafter, the above-described acoustoelectric conversion cycle is repeated according to the waveform of the input sound pressure.

  Normally, a clamp element 30 having a high resistance value is connected between the fixed electrode 20b and the ground potential. The acoustoelectric conversion signal ms generated in the MEMS microphone element 20 as described above is taken out from the node N between the movable electrode 20b and the clamp element 30 as an AC signal based on zero volts.

Figure 4 shows the waveforms of the acoustoelectric conversion signal ms obtained when entering the sound from a certain point of time t _s to the MEMS microphone element 20 to the node N schematically. The node N before time t _s is held (clamped) at a potential near zero volts.

3 and 4, it is assumed that the anti-phase signal cs ₋ from the amplitude control circuit 16 is not supplied to the MEMS microphone element 20. Reverse phase signal from the amplitude control circuit 16 cs _- If is supplied to the MEMS microphone element 20, the reverse phase signal to the acousto-electric conversion signals ms cs _- are added together in time.

  Referring back to FIG. 2, the fixed electrode 20b on the output terminal side of the MEMS microphone element 20 is connected to the input terminal of the preamplifier 14 formed of, for example, an operational amplifier or a MOS transistor. APDP is connected as a clamp element 30 between the input terminal of the preamplifier 14 and the ground potential. The APDP 30 includes a pair of diodes 30a and 30b that are connected in parallel with their polarities reversed.

The voltage of the preamplifier input signal AS from the MEMS microphone element 20 is applied to both the diodes 30a and 30b. Here, when the polarity of the preamplifier input signal AS is positive, the voltage of the preamplifier input signal AS is applied in the forward direction to one diode 30a, and the voltage of the preamplifier input signal AS is applied in the reverse direction to the other diode 30b. The In this case, the diode 30a works as a high resistance against a signal voltage of an on voltage V _F (usually several hundred millivolts) or less, and the diode 30b acts as a high resistance to a signal voltage of a breakdown voltage V _A (usually several volts) or less. work. Conversely, when the polarity of the preamplifier input signal AS is negative, the voltage of the preamplifier input signal AS is applied in the reverse direction to one diode 30a, and the voltage of the preamplifier input signal AS is applied in the forward direction to the other diode 30b. The In this case, the diode 30a serves as a high resistance to breakdown voltage V _A or less of the signal voltage, the diode 30b acts as a high resistance to following the signal voltage on the voltage V _F.

Viewed another way, when the voltage of the preamplifier input signal AS exceeds the ON voltage V _F a positive polarity, the diode 30a becomes conductive, the preamplifier input signal AS is clipped on-voltage V _F. Further, the absolute value voltage of the preamplifier input signal AS is negative polarity when exceeding the ON voltage V _F, the diode 30b becomes conductive, the preamplifier input signal AS is clipped on-voltage V _F. At that time, the waveform of the preamplifier input signal AS is distorted, and the waveform of the preamplifier output signal MS generated from the preamplifier 14 is also distorted. In particular, in the case of the MEMS microphone element 20, since the capacitor capacity is extremely small on the order of pF, if even a slight current leak occurs in the APDP 30, the signal waveform is greatly distorted.

  In this embodiment, as will be described in detail later, the MEMS microphone element 20 inputs a considerably large sound pressure (for example, 124 dBVspl or more), and generates an acoustoelectric conversion signal ms having a considerably large amplitude value corresponding to the sound pressure. Even if it is generated, the function of the amplitude control circuit 16 prevents the APDP 30 from conducting, so that the waveform of the preamplifier input signal AS or the preamplifier output signal MS is not distorted.

  The amplitude control circuit 16 in this embodiment includes a variable attenuator 32, an inverting amplifier 34, and a switch 36, and a control unit 38 that controls their gain (or attenuation rate) or on / off state.

The variable attenuator 32 receives the preamplifier output signal MS from the preamplifier 14, attenuates the preamplifier output signal MS with a variable attenuation factor instructed by the control unit 38 by, for example, a voltage control method or a digital method, and outputs the preamplifier output signal. An in-phase attenuation signal cs ₊ having a relatively low amplitude value with respect to the MS is generated. The inverting amplifier 34 includes, for example, an operational amplifier or a CMOS inverter, and amplifies the attenuation signal cs ₊ from the variable attenuator 32 with a variable gain or a fixed gain of 0 dB or more, which is normally instructed by the control unit 38, and also an attenuation signal. The polarity of cs ₊ is inverted to generate a reverse phase signal cs ₋ . The switch 36 is composed of, for example, a transistor or a transmission gate. When the amplitude value (peak value) of the preamplifier output signal MS is less than or equal to the monitoring value K under the control of the control unit 38, the switch 36 is kept off and the amplitude of the preamplifier output signal MS is maintained. When the value (peak value) exceeds the monitoring value K, it is switched to the on state.

As shown in FIG. 5, the control unit 38 includes an envelope circuit 41, an error amplifier 42, a reference voltage generation circuit 44, a gain control unit 46, and an output control unit 48 as a preferred configuration example. The envelope circuit 41 receives the preamplifier output signal MS from the preamplifier 14 and generates a top envelope signal TEM representing, for example, a top envelope (upper envelope) connecting the upper peak values of the signal _MS . Error amplifier 42 receives a reference voltage V _K corresponding to monitoring value K from the top envelope signal TE _MS and the reference voltage generating circuit 44 from the envelope circuit 41 generates an error signal ER that represents the difference or error .

Reference voltage V _K supplied from the reference voltage generating circuit 44 is set to a value associated with the on-voltage V _F of APDP30. That is, from the amplitude value of the acoustoelectric conversion signal ms produced in MEMS microphone element 20 when to increase to a level that exceeds the ON voltage V _F, the amplitude value of the preamplifier input signal AS reaches the ON voltage V _F of APDP30 Also, the amplitude value of the preamplifier output signal MS is set so as to reach the reference voltage V _K at the previous stage. As an example, the amplification factor of the preamplifier 14 is 1 (0 dB), when the on-voltage V _F of APDP30 is 300mVrms may be set to V _K = 240~260mVrms.

As another example, with although not shown, it constitutes an envelope circuit 41 to produce a bottom envelope signal BE _MS representing the bottom envelope (lower envelope) connecting the lower peak value of the preamplifier output signal MS, negative A similar error signal ER can be obtained even if the reference voltage generating circuit 44 is configured so as to generate the reference voltage -V _K.

Gain control section 46, polarity and absolute value of the error signal ER outputted from the error amplifier 42 (error amount) is monitored, when the polarity of the error signal ER is positive, i.e. the voltage level of the top envelope signal TE _MS reference When the voltage is equal to or lower than V _{K, the} gain control for the variable attenuator 32 and the inverting amplifier 34 is suspended.

However, the polarity of the error signal ER is negative, that is, when the voltage level of the top envelope signal TE _MS exceeds the reference voltage V _K, the gain control unit 46 is operated, the variable attenuator 32 and / or inverting amplifiers 34 In contrast, the gain control of negative feedback is performed in the direction in which the excess amount approaches zero.

In this gain control, the gain control unit 46 determines that the error (excess) amount | ER | is within a certain range (that is, the amplitude value of the acoustoelectric conversion signal ms is less than or equal to twice the monitored value K). The attenuation factor of the variable attenuator 32 is variably controlled while the gain of the inverting amplifier 34 is fixed. In this case, as shown in FIG. 6A, as the error (excess) amount | ER | is larger, the attenuation rate of the variable attenuator 32 is decreased in inverse proportion (the amplitude of the attenuation signal cs _{+ is} thereby reduced). Gain control is performed.

Then, even when the attenuation factor of the variable attenuator 32 is lowered to zero, when gain control of negative feedback is further necessary (that is, when the amplitude value of the acoustoelectric conversion signal ms exceeds twice the monitored value K), The gain of the inverting amplifier 34 is variably controlled within a positive range while the attenuation rate of the variable attenuator 32 is kept at zero. In this case, as shown in FIG. 6B, as the error (excess) amount | ER | is larger, the amplification factor of the inverting amplifier 34 is increased in proportion thereto (thereby, the amplitude of the attenuation signal cs ₊ is increased). Gain control is performed.

The output control unit 48, when the polarity of the error signal ER is positive, i.e. when the voltage level of the envelope signal TE _MS is less than or equal to the reference voltage V _K controls the switch 36 to the OFF state, the polarity of the error signal ER negative when, that is, when the top envelope signal TE _MS exceeds the reference voltage V _K, it controls the switch 36 to the oN state. The anti-phase signal cs ₋ output from the amplitude control circuit 16 through the switch 36 in the ON state is applied to the movable electrode 20 a of the MEMS microphone element 20 through the coupling capacitor 40. The coupling capacitor 40 blocks the amplitude control circuit 16 from the MEMS microphone element 20 and the bias power supply circuit 18 in terms of DC.

Note that a configuration in which the negative phase signal cs ₋ from the amplitude control circuit 16 is applied to the electrode (fixed electrode) 20b on the output terminal (node N) side of the MEMS microphone element 20 is also possible. However, since the capacitor capacity of the MEMS microphone element 20 is extremely small on the order of pF, the acoustoelectric conversion signal is affected by the coupling capacitor 40 or the like when the antiphase signal cs ₋ is superimposed on the acoustoelectric conversion signal ms on the output terminal side. There is a possibility that the waveform of the ms or preamplifier input signal AS may be disturbed. Therefore, it is preferable that the reverse phase signal cs ₋ supplied from the amplitude control circuit 16 to the MEMS microphone element 20 is applied to the electrode 20a facing the electrode 20b on the output terminal (node N) side as described above.

[Operation of the embodiment]

  With reference to FIGS. 7A to 7C, main operations (particularly, operations of the amplitude control circuit 16) in this embodiment will be described. 7A to 7C, it is assumed that the amplification factor of the preamplifier 14 is 1 (gain 0 dB). Therefore, the preamplifier input signal AS (not shown) has substantially the same amplitude and waveform as the preamplifier output signal MS.

When the sound pressure received by the MEMS microphone element 20 is not so large, as shown in FIG. 7A, the level of the amplitude value or the top envelope signal TE _MS preamplifier output signal MS is seated at lower levels V _F of the monitoring value K ing. In this case, the negative phase signal cs ₋ is not output from the amplitude control circuit 16. In the amplitude control circuit 16, since the polarity of the error signal ER output from the error amplifier 42 is positive, gain control (amplitude control) is not performed, and the switch 36 is held in the off state.

At this time, the acoustoelectric conversion signal ms corresponding to the input sound pressure is output as it is from the MEMS microphone element 20. The acoustoelectric conversion signal ms is input to the preamplifier 14 as it is as the preamplifier input signal AS (ms). In this case, since the amplitude value of the pre-amplifier input signal AS (ms) is sufficiently lower than the ON voltage V _F of APDP30, will not be clipped. Therefore, the preamplifier output signal MS without distortion is obtained at the output terminal of the preamplifier 14.

When the sound pressure received by the MEMS microphone element 20 is large, the amplitude value of the acoustoelectric conversion signal ms generated by the MEMS microphone element 20 also increases as shown in FIG. 7B. As a result, the amplitude value of the preamplifier input signal AS (ms) increases, and the amplitude value of the preamplifier output signal MS also increases. The considerable (e.g. more 124DBVspl) sound pressure is input to the MEMS microphone element 20, acoustoelectric conversion signal ms clogging preamp input signal AS low constant value amplitude value is higher than the ON voltage V _F of APDP30 of (ms) ( when If the amplification factor of the preamplifier 14 is 1 greater than the monitoring value K), the amplitude value of the preamplifier output signal MS exceeds the level V _F of the monitoring value K.

Then, in the amplitude control circuit 16, the polarity of the error signal ER output from the error amplifier 42 changes from positive to negative, negative feedback gain control (amplitude control) is started, and the switch 36 is turned on. That is, due to the functions of the gain control unit 46, the variable attenuator 32, and the inverting amplifier 34 as described above, an antiphase signal having a waveform with a relatively low amplitude and antiphase (polarity inversion) with respect to the acoustoelectric conversion signal ms. cs ₋ is generated, and the negative phase signal cs ₋ is supplied to the MEMS microphone element 20 through the switch 36 and the coupling capacitor 40 in the ON state. Thereby, in the MEMS microphone element 20, the opposite phase signal cs ₋ from the amplitude control circuit 16 overlaps the acoustoelectric conversion signal ms generated in accordance with the input sound pressure or change in capacitance. Thus, the output terminal of the MEMS microphone element 20 or the node N corresponds to the sum of the acoustoelectric conversion signal ms and the reverse phase signal cs ₋ , that is, the subtraction of the attenuation signal cs ₊ from the acoustoelectric conversion signal ms. A voltage signal (ms + cs ₋ ) is obtained as the preamplifier input signal AS. Also in this case, since the amplitude value of the preamplifier input signal AS (ms + cs ₋ ) is lower than the ON voltage V _{F of the} APDP 30, it is not clipped. Therefore, the preamplifier output signal MS without distortion is obtained at the output terminal of the preamplifier 14.

As shown in FIGS. 7B and 7C, when the amplitude value of the preamplifier output signal MS is greater than the level V _F of the monitoring value K, greater amplitude of acoustoelectric conversion signal ms produced by the MEMS microphone element 20 as becomes, the phase-inverted signal cs _- amplitude becomes large, the preamplifier input signal as a result aS (ms + cs _-) amplitude value level V-on voltage _F and stopped a little more than the level APDP30 V _F of the monitoring value K of It is controlled not to reach.

FIG. 8 shows signal waveforms of the acoustoelectric conversion signal ms, the reverse phase signal cs _−, and the preamplifier input signal AS obtained by the simulation performed by the present inventor for the above embodiment. In this simulation, the amplification factor of the preamplifier 14 is set to 1 (gain 0 dB), the monitoring value K (V _K ) in the amplitude control circuit 16 is set to 0.2 volts, and the amplitude value of the acoustoelectric conversion signal ms is set to zero (zero). Each waveform was observed with an oscilloscope when the voltage was gradually increased to a level of 0.4 volts or more with a constant gradient. A sine wave was used as the signal waveform.

As shown in the figure, the amplitude of the reverse phase signal cs ₋ rises from the time (about 4.2 msec) when the amplitude value of the preamplifier input signal AS (that is, the preamplifier output signal MS) reaches the monitoring value K (0.2 volts). The amplitude value of the reverse phase signal cs ₋ increases in proportion to the amplitude value of the acoustoelectric conversion signal ms with time. As a result, even if the amplitude value of the acoustoelectric conversion signal ms further increases, the amplitude value of the preamplifier input signal AS (and thus the preamplifier output signal MS) is suppressed to the vicinity of the monitoring value K (0.2 volts).

[Other Embodiments or Modifications]

The amplitude control circuit 16 in the above-described embodiment includes the individual variable attenuator 32 and the inverting amplifier 34. However, it is also possible to replace the variable attenuator 32 and the inverting amplifier 34 with one inverting amplifier 55 using an operational amplifier 50 as shown in FIG. In this case, the gain of the inverting amplifier 55 is given by the ratio between the resistance value R ₅₂ of the input resistor 52 and the resistance value R ₅₄ of the negative feedback resistor _{54 (R 54 / R 52)} . For at least one of the input resistor 52 and the negative feedback resistor 54, a variable resistor whose resistance value can be variably controlled by, for example, a voltage control method or a digital method may be used.

  As another modified example, as shown in FIG. 10, when the preamplifier 14 is an inverting amplifier circuit, a non-inverting amplifier 56 may be provided after the variable attenuator 32. A configuration in which the non-inverting amplifier 56 is provided in front of the variable attenuator 32 is also possible. Similarly, in the circuit of FIG. 5, a configuration in which the inverting amplifier 34 is provided in front of the variable attenuator 32, or the switch 36 or a function corresponding thereto is provided between the variable attenuator 32 and the inverting amplifier 34 or the preamplifier 14 and the variable attenuator. It is also possible to have a configuration provided between the two.

  The present invention can be suitably applied to a MEMS microphone and a microphone circuit used therefor as in the above-described embodiment, but is widely applicable to any type of condenser microphone and a microphone circuit used therefor.

10 Capacitor-type microphone element 12 Microphone circuit 14 First stage amplifier (preamplifier)
16 Amplitude control circuit 18 Bias power supply circuit 20 MEMS microphone element 28 Low-pass filter 30 Anti-parallel diode pair (APDP)
32 Variable Attenuator 34 Inverting Amplifier 36 Switch 38 Control Unit 40 Coupling Capacitor

Claims (9)

A capacitor type having a movable electrode and a fixed electrode arranged to face each other through a gap, and converting a change in capacitance between the movable electrode and the fixed electrode according to an input sound pressure into a first signal A microphone circuit that is electrically connected to a microphone element and that performs constant signal processing on the first signal,
A first amplifier circuit having an input terminal connected to the output terminal of the microphone element;
The amplitude value of the second signal output from the first amplifier circuit is monitored, and when the amplitude value of the second signal exceeds a predetermined monitoring value, it is relatively low with respect to the first signal A microphone circuit comprising: an amplitude control circuit that applies a third signal having a waveform having an opposite phase to the amplitude value to either the movable electrode or the fixed electrode.   The input terminal of the first amplifier circuit is connected to either the movable electrode or the fixed electrode, and the third signal from the amplitude control circuit is applied to the other via a coupling capacitor. Item 2. The microphone circuit according to Item 1.   The microphone circuit according to claim 1, wherein a clamp element for bias is connected between an input terminal of the first amplifier circuit and a reference potential.   The clamp element is an anti-parallel diode pair, and the monitoring value is obtained before the amplitude value of the signal input to the first amplifier circuit reaches the ON voltage of the anti-parallel diode pair. The microphone circuit according to claim 3, wherein the microphone circuit is set to a level at which an amplitude value of the second signal is reached.   5. The amplitude control circuit according to claim 1, wherein the amplitude control circuit includes a second amplification circuit that amplifies the second signal output from the first amplification circuit with a positive or negative gain. 6. Microphone circuit. A capacitor type having a movable electrode and a fixed electrode arranged to face each other through a gap, and converting a change in capacitance between the movable electrode and the fixed electrode according to an input sound pressure into a first signal A microphone element;
A first amplifier circuit having an input terminal connected to the output terminal of the microphone element;
The amplitude value of the second signal output from the first amplifier circuit is monitored, and when the amplitude value of the second signal exceeds a predetermined monitoring value, it is relatively low with respect to the first signal A microphone having an amplitude control circuit that applies a third signal having a waveform having an opposite phase to the amplitude value to either the movable electrode or the fixed electrode.   The input terminal of the first amplifier circuit is connected to either the movable electrode or the fixed electrode, and the third signal from the amplitude control circuit is applied to the other via a coupling capacitor. Item 7. The microphone according to item 6.   The microphone according to claim 6 or 7, wherein a clamp circuit for bias is connected between an input terminal of the first amplifier circuit and a reference potential.   The said amplitude control circuit has a 2nd amplifier circuit which amplifies the said 2nd signal output from the said 1st amplifier circuit by a positive or negative gain, The method of any one of Claims 6-8. Microphone.

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