JP2008153981A - Capacitance change detection circuit and condenser microphone device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve the problem that it takes time to charge a microcapacitor on start-up of a condenser microphone device using an MEMS (Micro Electro Mechanical Systems) microphone. <P>SOLUTION: A lowpass filter (LPF) 26 comprising a high resistor R1 and a capacitance C1 is provided between one terminal of a condenser Cm constituting the MEMS microphone and a charge pump for charging Cm. The LPF 26 is provided with a diode D1 to be a forward bias when starting charging in parallel. A capacitance C2 and a high resistor R2 are connected in parallel to a feedback route that connects an inverted input terminal of an operational amplifier 28 connected to the other terminal of Cm and an output terminal, and a diode D2 is connected to C2 and R2 in parallel and in the direction of being a forward direction bias when starting charging. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a capacitance change detection circuit that detects a change in capacitance of a capacitor in response to sound, and a capacitor microphone device, and more particularly to noise reduction and rise time reduction of a capacitor microphone.

  In recent years, MEMS microphones have attracted attention as a type of condenser microphone. The basic structure of the MEMS microphone is a capacitor composed of two electrode plates, a diaphragm and a back plate, arranged in close proximity to each other. The structure is formed on a silicon substrate using MEMS (Micro Electro Mechanical Systems) technology. It is formed. The MEMS microphone can withstand the temperatures of standard solder reflow processes and can be soldered with other components on a printed circuit board, for example. Further, the MEMS microphone can be formed smaller than a general electret condenser microphone (ECM). In this respect, the device including the MEMS microphone can be downsized by increasing the mounting density.

  The ECM uses an electret element that holds a charge semipermanently, and thus has a structure that does not require a bias voltage, whereas a MEMS microphone requires a relatively high DC bias voltage for operation. By applying the bias voltage, the capacitor constituting the MEMS microphone is charged with a constant charge Q. When the diaphragm vibrates due to the sound pressure in this state, the capacitance C of the capacitor changes, causing a change in the voltage V between the terminals. This change in voltage V is output as an audio signal.

  FIG. 4 is a circuit diagram of a condenser microphone device using a conventional MEMS microphone. The apparatus includes a capacitor Cm which is a MEMS microphone, a bias power supply unit 2 and an output unit 4.

  The bias power supply unit 2 includes a charge pump circuit 10, a capacitor C1, and a resistor R1, and supplies a bias voltage to a capacitor Cm connected to the terminal VB.

  The high voltage generated by the charge pump circuit 10 is supplied to the capacitor Cm via a CR low-pass filter (LPF) 12 composed of C1 and R1. The LPF 12 is provided to smooth the output signal of the charge pump circuit 10 and remove high-band noise that may be included in the output signal. In particular, it is desirable to suitably remove noise included in the output signal of the charge pump circuit 10 when acquiring a high-quality sound signal. On the other hand, since the main function of the charge pump circuit 10 is to charge the capacitor Cm and maintain a constant bias voltage, the LPF 12 basically only needs to be able to transmit direct current. From such a point of view, the resistor R1 is set to a high resistance that can suppress the cut-off frequency of the LPF 12 and sufficiently prevent noise from passing therethrough.

  The output unit 4 includes an operational amplifier 14, a capacitor C2, and a resistor R2. The output unit 4 has a capacitor Cm connected to the input terminal IN, takes in the potential change generated by the capacitor Cm according to the sound from the input terminal IN, amplifies it, and outputs it from the output terminal OUT.

The operational amplifier 14 is connected to Cm at the inverting input terminal and C2 to the feedback path, thereby constituting an inverting amplifier circuit. In order to set the gain of the inverting amplifier circuit determined according to Cm / C2 to 1 or more, C2 is set to a value smaller than Cm. Here, Cm can be a minute value of about several pF, for example, and C2 is set to a very small value accordingly. The inverting input terminal is floating when only Cm and C2 are connected, and since the capacitance is very small, the smoothing effect cannot be expected, so the potential of the inverting input terminal becomes unstable. Therefore, the resistor R2 is connected in parallel to the feedback path to which the capacitor C2 is connected, so that the potential of the inverting input terminal is stabilized. On the other hand, R2 is required not to pass the potential fluctuation generated at the inverting input terminal according to the sound to the output terminal via R2, and to maintain a high output impedance for the capacitor Cm. Therefore, R2 is set to a sufficiently large value, and the inverting input terminal and the output terminal are simply connected in a direct current manner.
Japanese Patent Laid-Open No. 11-317996 JP 2003-243944 A

  When starting the condenser microphone device using the MEMS microphone, it is necessary to charge the capacitor Cm with the current from the charge pump circuit 10. However, since the resistance R1 is made high to reduce noise, there is a problem that the charging time constant becomes long and it takes time until a sufficient bias voltage is applied to the capacitor Cm. When the bias voltage is low, the voltage change generated between the terminals of the capacitor Cm in accordance with the sound becomes small, causing a problem that the microphone sensitivity is lowered. Therefore, the conventional condenser microphone device that charges and uses the capacitor Cm has a problem that it takes a long time until a good audio signal is obtained after activation.

  The present invention has been made in order to solve the above-mentioned problems. A condenser microphone device capable of obtaining a good audio signal in a short time from the start of charging of the capacitor Cm and a capacitance change while realizing low noise. An object is to provide a detection circuit.

  A capacitance change detection circuit according to the present invention is connected to a capacitor unit constituting a capacitor microphone, and detects a change in capacitance of the capacitor unit according to sound as an electric signal. A charge pump for charging the capacitor unit, a low-pass filter inserted between the charge pump and the capacitor unit for smoothing an output signal of the charge pump and removing noise, and the low-pass filter , A first rectifier circuit in which the bias state at the start of charging of the capacitor unit is a forward bias, an operational amplifier in which the other of the capacitor unit is connected to an inverting input terminal, and a feedback path of the operational amplifier And an inverting amplifier circuit including a feedback capacitor connected in series with the feedback capacitor. Provided in parallel with the feedback path, connected in parallel to the feedback path and the feedback resistor including the feedback capacitor, a feedback resistor connecting the output terminal of the operational amplifier and the inverting input terminal, and the operational amplifier A second rectifier circuit connecting the output terminal and the inverting input terminal in a direction in which a bias state at the start of charging of the capacitor unit is a forward bias. The first rectifier circuit or the second rectifier circuit can be formed of a diode.

  A capacitor microphone device according to the present invention detects a change in capacitance of a capacitor unit that generates a change in capacitance according to sound, and is connected to the capacitor unit as an electric signal. A capacitance change detection circuit, wherein the capacitance change detection circuit is connected to one of the capacitor units and charges the capacitor unit; the charge pump and the capacitor unit; A low-pass filter that smoothes the output signal of the charge pump and removes noise, and is connected in parallel to the low-pass filter, and the bias state at the start of charging of the capacitor unit is a forward bias. A rectifier circuit, an operational amplifier having the other input terminal connected to the inverting input terminal, and the operational amplifier An inverting amplifier circuit including a feedback capacitor connected in series to a feedback path; a feedback resistor provided in parallel to the feedback path including the feedback capacitor; and connecting the output terminal of the operational amplifier and the inverting input terminal; Provided in parallel with the feedback path including the feedback capacitor and the feedback resistor, the bias state at the start of charging of the capacitor unit is forward biased between the output terminal and the inverting input terminal of the operational amplifier. And a second rectifier circuit connected in a direction to be. The first rectifier circuit or the second rectifier circuit can be formed of a diode.

  According to the present invention, during startup, a current flows between the charge pump and one side of the capacitor unit via the first rectifier circuit, and the potential on one side of the capacitor unit is quickly set to the normal operation state. At this time, due to capacitive coupling, the potential on the other side of the capacitor portion also changes following one side, and it is possible that the bias voltage is not effectively applied to the capacitor portion. According to the present invention, such a potential change on the other side of the capacitor unit is suppressed by a current flowing between the other side of the capacitor unit and the output terminal of the operational amplifier via the second rectifier circuit, Or it is quickly resolved. Therefore, a sufficient bias voltage is quickly applied to the capacitor unit at the time of startup, and a good audio signal can be obtained in a short time.

  Hereinafter, embodiments of the present invention (hereinafter referred to as embodiments) will be described with reference to the drawings.

  FIG. 1 is a schematic circuit diagram of a condenser microphone device according to an embodiment. This device uses a MEMS microphone as a condenser microphone, and further includes a capacitance change detection circuit that is connected to the MEMS microphone and detects a change in capacitance of the MEMS microphone according to sound as an electrical signal. . In FIG. 1, a capacitor Cm is a capacitor unit configured as a MEMS microphone, one terminal of which is connected to the output terminal VB of the bias power supply unit 20 of the capacitance change detection circuit, and the other terminal detects capacitance change. It is connected to the input terminal IN of the output part 22 of the circuit.

  The capacitance change detection circuit can be formed as an integrated circuit (IC) on a silicon semiconductor substrate, for example. When configured as an IC, the capacitance change detection circuit can be integrated on a semiconductor chip on which the MEMS microphone is formed, or can be formed on a semiconductor chip different from the MEMS microphone.

  The bias power supply unit 20 includes a charge pump circuit 24, a capacitor C1, a resistor R1, and a diode D1, and supplies a bias voltage to the capacitor Cm connected to the terminal VB.

  The charge pump circuit 24 boosts a relatively low voltage of the reference power source to generate a high voltage necessary for driving the MEMS microphone, and charges the capacitor Cm with the high voltage. In this apparatus, the charge pump circuit 24 outputs a positive voltage, and the charging current flows toward the capacitor Cm.

  The capacitor C1 is connected between the terminal VB and the ground potential. The resistor R1 is connected between the terminal VB and the output terminal of the charge pump circuit 24. C1 and R1 constitute an LPF 26, which smoothes the output signal of the charge pump circuit 24 and removes high-band noise that can be included in the output signal. For example, the LPF 26 removes switching noise, clock noise, thermal noise, and the like in the charge pump circuit 24.

  In particular, it is desirable to suitably remove noise included in the output signal of the charge pump circuit 24 when acquiring a high-quality sound signal. On the other hand, the main function of the charge pump circuit 24 is to charge the capacitor Cm and keep it at a constant bias voltage, so basically the LPF 26 only needs to be able to transmit direct current. From this point of view, the resistor R1 is set to a high resistance that can sufficiently suppress the cut-off frequency of the LPF 26 and sufficiently block the passage of noise.

  The diode D1 is arranged in parallel with the resistor R1, and is connected in parallel with the LPF 26 between the output terminal of the charge pump circuit 24 and the terminal VB of the capacitor Cm. The diode D1 has an anode connected to the output terminal of the charge pump circuit 24, a cathode connected to the terminal VB, and constitutes a rectifier circuit that allows current to flow in one direction from the charge pump circuit 24 toward the capacitor Cm.

  The output unit 22 includes an operational amplifier 28, a capacitor C2, a resistor R2, and a diode D2. The output unit 22 has a capacitor Cm connected to the input terminal IN. The capacitor Cm captures a potential change generated according to the sound from the input terminal IN, amplifies it, and outputs it from the output terminal OUT.

  The operational amplifier 28 forms an inverting amplifier circuit in which an inverting input terminal is connected to an input terminal IN, and an output terminal thereof is connected to the output terminal OUT of the output unit 22. The operational amplifier 28 amplifies the potential fluctuation of the inverting input terminal with a gain according to the capacitor Cm connected to the inverting input terminal and the capacitor C2 connected in series to the feedback path connecting the output terminal and the inverting input terminal. Output. The operational amplifier 28 performs impedance conversion with the output terminal OUT being a low output impedance while maintaining a high input impedance for the capacitor Cm.

  The capacitor C2 adjusts the gain of the inverting amplifier circuit. The gain is basically given by Cm / C2, and C2 is set to a value smaller than Cm in order to obtain a gain of 1 or more. Here, Cm can be a minute value of about several pF, for example, and C2 is set to a very small value accordingly.

  The resistor R2 is arranged in parallel with the feedback path to which the capacitor C2 is connected, and connects the output terminal and the inverting input terminal of the operational amplifier 28. The resistor R2 is provided to stabilize the potential of the inverting input terminal that is in a floating state when only Cm and C2 are connected. On the other hand, R2 is required not to pass the potential fluctuation generated at the inverting input terminal according to the sound to the output terminal via R2, and to maintain a high output impedance for the capacitor Cm. Therefore, R2 is set to a sufficiently large value, and the inverting input terminal and the output terminal are simply connected in a direct current manner.

  The diode D2 is disposed in parallel with the feedback path to which the capacitor C2 is connected and the resistor R2. The diode D2 forms a rectifier circuit that allows current to flow in one direction from the inverting input terminal of the operational amplifier 28 to the output terminal.

  Next, the operation of this condenser microphone device will be described. FIG. 2 is a schematic graph showing a voltage change in the bias power supply unit 20, where the horizontal axis represents time t and the vertical axis represents the voltage V. Before the start-up of this apparatus (t <t0), the capacitor Cm is basically discharged, and for example, the terminal VB becomes 0V.

  When this apparatus is activated at time t0, the charge pump circuit 24 starts boosting. The voltage Vcp at the output terminal of the charge pump circuit 24 quickly reaches the target value Vm as shown by the dotted line 30 in FIG. The voltage Vb at the terminal VB to which the capacitor Cm is connected rises as the charging of the capacitor Cm proceeds by the current Icp from the charge pump circuit 24. The rise time becomes shorter as Icp increases. In FIG. 2, a solid line 32 indicates a change in Vb in this apparatus. In the present apparatus, since the diode D1 is provided, this Icp becomes larger than the conventional one and the rise time is shortened. Specifically, at the start of charging, the potential difference between Vcp and Vb is relatively large, and the diode D1 is forward biased and becomes conductive. As a result, the output terminal of the charge pump circuit 24 and the terminal VB are connected with a low resistance, and much larger Icp than that flowing through the resistor R1 is supplied to the capacitor Cm.

  The conduction by the forward bias of the diode D1 basically continues until the potential difference between the two ends decreases and reaches a predetermined forward voltage Vf. In the case of a silicon diode, this forward voltage Vf is about 0.6V. Since the normal set value of Vm is set to a value at least several times greater than Vf, the diode D1 can significantly reduce the startup time of the apparatus. In FIG. 2, for comparison, a change in Vb in a conventional circuit in which charging is performed via R1 from the beginning of rising is indicated by a dotted line 34.

  When Vcp-Vb falls below Vf, the diode D1 is turned off, and Vb gradually approaches Vm due to charging via R1 as in the conventional circuit shown in FIG. 4, so that the apparatus shifts to a steady drive state. In this steady driving state, the capacitor Cm is charged via R1 and maintains the bias state at the voltage Vm, and the LPF 26 composed of R1 and C1 suppresses the fluctuation of Vm due to noise, and the sound signal with good sound quality. Can be generated.

  FIG. 3 is a schematic graph showing a change in the voltage Vin at the input terminal IN of the output unit 22. The horizontal axis represents time t and the vertical axis represents the voltage V, as in FIG. When charging starts at time t0 and the potential Vb of the terminal VB rises, the potential Vin of the terminal IN can also rise from time t0 due to capacitive coupling by the capacitor Cm. At this time, as described above, in order to stabilize Vin, the output section 22 is provided with a resistor R2 as in the conventional circuit shown in FIG. 4, and this R2 basically depends on Vin according to the potential Vout of the output terminal OUT. Try to keep the ground state.

  However, since R2 has a high resistance, it is not possible to suppress an increase in Vin with respect to a sudden change in Vb caused by providing the diode D1, and to quickly restore the increased Vin to the ground state. I can't. In FIG. 3, the dotted line 36 shows the rise of Vin up to the vicinity of Vm at the start-up of the present apparatus and how the raised Vin is gradually restored to the ground state by the resistor R2. As described above, when the potential Vin of the terminal IN is increased, the bias voltage with respect to the capacitor Cm is decreased, and the microphone sensitivity is decreased.

  In response to this problem, the present apparatus is provided with a diode D2 at the output section 22. The diode D2 is forward biased and becomes conductive when the potential Vin at the terminal IN is higher than the forward voltage Vf by the potential Vout at the output terminal OUT. In this state, the diode D2 can connect a current between the terminal IN and the output terminal OUT with a low resistance, and can pass a much larger current than that flowing through the resistor R2. This current suppresses or reduces the potential increase of Vin induced by the rapid increase in potential Vb, and when Vb reaches the vicinity of Vm and the change becomes gentle, Vin is rapidly lowered toward the ground state. As a result, as shown by a solid line 38 in FIG. 3, this apparatus can suppress the peak of the rise of Vin that can be induced at the time of startup, and can shorten the return time from the startup time t0 to the ground state. it can. Therefore, a decrease in microphone sensitivity at the time of activation is suppressed, and acquisition of a high-quality sound signal is quickly realized.

  Note that, by configuring the diode D2 so that the on-current of the diode D2 becomes a large value, the height of the peak of Vin appearing on the solid line 38 can be lowered to Vf in principle. For example, the current can be increased by increasing the channel cross-sectional area of the diode D2.

  As described above, this apparatus can be quickly started up by the diode D1 provided in the bias power supply unit 20 and the diode D2 provided in the output unit 22.

  Incidentally, the fluctuation of the voltage between the terminals of the capacitor Cm generated according to the voice is basically small compared to the forward voltage Vf of the diode, so that the diodes D1 and D2 are not turned on by the voltage fluctuation, and the voice signal Basically there is no loss of dynamic range.

  In the above-described embodiment, the configuration using the diodes D1 and D2 is shown, but other rectifiers, rectifier circuits, switch elements, and switch circuits may be used instead of the diodes.

1 is a schematic circuit diagram of a condenser microphone device according to an embodiment of the present invention. It is a typical graph which shows the voltage change in a bias power supply part. It is a typical graph which shows the change of the input terminal voltage of an output part. It is a circuit diagram of the capacitor | condenser microphone apparatus using the conventional MEMS microphone.

Explanation of symbols

  20 bias power supply unit, 22 output unit, 24 charge pump circuit, 26 low-pass filter (LPF), 28 operational amplifier, Cm MEMS microphone capacitor, C1, C2 capacitance, D1, D2 diode, R1, R2 resistance.

Claims (4)

A capacitance change detection circuit that is connected to a capacitor portion constituting a capacitor microphone and detects a change in capacitance of the capacitor portion according to sound as an electric signal,
A charge pump connected to one of the capacitor units and charging the capacitor unit;
A low-pass filter inserted between the charge pump and the capacitor unit to smooth the output signal of the charge pump and remove noise;
A first rectifier circuit connected in parallel to the low-pass filter, wherein the bias state at the start of charging of the capacitor unit is a forward bias;
An inverting amplifier circuit including an operational amplifier connected to the other inverting input terminal of the capacitor unit and a feedback capacitor connected in series to the feedback path of the operational amplifier;
A feedback resistor provided in parallel with the feedback path including the feedback capacitor, and connecting the output terminal of the operational amplifier and the inverting input terminal;
Provided in parallel with the feedback path including the feedback capacitor and the feedback resistor, the bias state at the start of charging of the capacitor unit is forward biased between the output terminal and the inverting input terminal of the operational amplifier. A second rectifier circuit connected in the direction of
A capacitance change detection circuit comprising: The capacitance change detection circuit according to claim 1,
The capacitance change detection circuit, wherein the first rectifier circuit or the second rectifier circuit is a diode. A capacitor unit that generates a change in capacitance according to sound; and a capacitance change detection circuit that is connected to the capacitor unit and detects a change in capacitance of the capacitor unit according to sound as an electrical signal. A condenser microphone device,
The capacitance change detection circuit is:
A charge pump connected to one of the capacitor units and charging the capacitor unit;
A low-pass filter inserted between the charge pump and the capacitor unit to smooth the output signal of the charge pump and remove noise;
A first rectifier circuit connected in parallel to the low-pass filter, wherein the bias state at the start of charging of the capacitor unit is a forward bias;
An inverting amplifier circuit including an operational amplifier connected to the other inverting input terminal of the capacitor unit and a feedback capacitor connected in series to the feedback path of the operational amplifier;
A feedback resistor provided in parallel with the feedback path including the feedback capacitor, and connecting the output terminal of the operational amplifier and the inverting input terminal;
Provided in parallel with the feedback path including the feedback capacitor and the feedback resistor, the bias state at the start of charging of the capacitor unit is forward biased between the output terminal and the inverting input terminal of the operational amplifier. A second rectifier circuit connected in the direction of
A condenser microphone device comprising: In the condenser microphone device according to claim 3,
The capacitor microphone device, wherein the first rectifier circuit or the second rectifier circuit is a diode.

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