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JP2001238287

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DESCRIPTION JP2001238287
[0001]
The present invention relates to a microphone device formed in a semiconductor chip and
provided with an element for sensing a sound pressure such as an electret capacitor, and a DC
component unnecessary as an audio signal in an output signal of the microphone device. The
present invention relates to a microphone filter that removes frequency components.
[0002]
2. Description of the Related Art A conventional microphone device and a filter for a microphone
are shown in FIG. In FIG. 4, a microphone device provided with an electret condenser EC as the
microphone device MU2 is illustrated. When the electret capacitor EC receives a sound pressure,
its capacitance value changes, and an input signal Vin is generated between its both electrodes.
The ground potential GND is applied to one end of the electret capacitor EC. Then, an impedance
conversion circuit including diodes D1 and D2, a resistor R1 and N channel MOS transistors T1
and T2 is connected to both ends of the electret capacitor EC. Specifically, the anode of the diode
D1 is connected to one end of the electret capacitor EC, and the cathode is connected to the other
end of the electret capacitor EC. Further, with regard to the diode D2, the anode and the cathode
are reversely connected to the diode D1 and are connected to both ends of the electret capacitor
EC. Furthermore, a resistor R1 is connected in parallel across the electret capacitor EC. The
source of the transistor T1 is connected to one end of the electret capacitor EC, and the gate is
connected to the other end of the electret capacitor EC. The source of the transistor T2 is
connected to the drain of the transistor T1. The power supply potential Vdd is applied to the
drain of the transistor T2, and the constant potential Vref2 is applied to the gate. The ground
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potential GND is applied to the back gates of the transistors T1 and T2.
[0003]
The voltage between the gate and the source of the transistor T1 is maintained at 0 V by the
diodes D1 and D2 and the resistor R1 when the input signal Vin is not given. When the input
signal Vin is applied, the voltage between the gate and the source of the transistor T1 fluctuates.
Then, the current flowing between the drain and the source changes accordingly. The transistor
T1 is a depletion type, and a current flows between the drain and the source even if the gatesource voltage is 0V. By the change of the drain-source current of the transistor T1, the current
flowing between the drain-source of the transistor T2 also changes, and the gate-source voltage
of the transistor T2 fluctuates. Then, the fluctuation of the potential at the source of the
transistor T2 becomes an output signal Vout2.
[0004]
Further, as shown in FIG. 4, the microphone filter FT2 is configured of a CR circuit composed of a
capacitor C1 and a resistor R4. The output signal Vout2 from the microphone device MU2 is
given to one end of the capacitor C1, and one end of the resistor R4 is connected to the other end
of the capacitor C1. Further, the constant potential Vref1 is applied to the other end of the
resistor R4.
[0005]
The microphone filter FT2 removes a direct current component and a low frequency component
included in the output signal Vout2 by outputting a voltage drop at the resistor R4. Since the
output signal Vout2 functions as an audio signal, its frequency range may be approximately 100
Hz to 20 kHz. Therefore, a DC component and a low frequency component unnecessary as an
audio signal are removed from the output signal Vout2.
[0006]
The output of the microphone filter FT2 is input to the amplifier. FIG. 4 exemplifies an amplifier
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including a voltage follower and an inverting amplifier. That is, the output of the microphone
filter FT2 is input to the positive input terminal of the operational amplifier OP1. The output of
the operational amplifier OP1 is input to the negative input terminal of the operational amplifier
OP1, and the operational amplifier OP1 functions as a voltage follower. The output of the
operational amplifier OP1 is input to the negative input terminal of the operational amplifier OP2
via the resistor R2. The output Vout3 of the operational amplifier OP2 is also input to the
negative input terminal of the operational amplifier OP2 via the resistor R3, and the operational
amplifier OP2 functions as an inverting amplifier. The constant potential Vref1 is applied to the
positive input terminal of the operational amplifier OP2.
[0007]
If the capacitance value of the capacitor C1 is C and the resistance value of the resistor R4 is R,
the microphone filter FT2 has a DC component or a low value at the cutoff frequency f = 1 /
(2πCR). The frequency component is removed from the output signal Vout2. In order to remove
low frequency signals and direct current components of about 100 Hz or less from the output
signal Vout2, the product of the capacitance value C and the resistance value R, that is, the time
constant must be a large value. A combination of a 6 kΩ resistance or a 100 pF capacitance and
a 16 MΩ resistance is required. If a combination of such a large resistance and capacitance is to
be formed in one semiconductor chip, the chip area becomes large, which hinders the
miniaturization and cost reduction of the semiconductor chip. Therefore, the conventional
microphone filter FT2 can not be contained in the semiconductor chip in which the microphone
device MU2 is formed, and has to be configured using capacitors and resistors of individual
components.
[0008]
However, even if capacitors and resistors of individual parts are used, the cost and cost of the
parts are increased, the number of processing steps is increased, and because they can not be
accommodated in the semiconductor chip on which the microphone device is formed, etc. It was
difficult to reduce it. Also, along with this, the amplifier was not housed in the semiconductor
chip in which the microphone device was formed.
[0009]
Therefore, the object of the present invention is to realize a microphone filter which can be
formed in the same semiconductor chip as the microphone device and has a large time constant
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having resistance and capacitance values as components, and consequently, the miniaturization
and cost of the microphone device It is about trying to reduce.
[0010]
According to the present invention, a capacitor having one end and the other end to which an
output of a microphone is input, and a first current electrode connected to the one end of the
capacitor are fixed. A first transistor having a second current electrode to which a potential is
applied and a control electrode; a second current electrode connected to the first current
electrode and the second current electrode of the first transistor; and the first transistor A filter
for a microphone, comprising: a second transistor having a control electrode connected to a
control electrode; and a constant current source connected to the first current electrode of the
second transistor and the control electrode.
[0011]
The invention according to claim 2 is a microphone formed in a semiconductor chip, and the
filter for a microphone according to claim 1, formed in the semiconductor chip, and the output of
the microphone is inputted to the other end of the capacitor. And a microphone device.
[0012]
The invention according to claim 3 is the microphone device according to claim 2, wherein an
input end formed in the semiconductor chip and connected to the first current electrode of the
first transistor of the filter for the microphone A microphone device further comprising an
amplifier having
[0013]
The invention according to claim 4 is a microphone device comprising a microphone formed in a
semiconductor chip, and an amplifier formed in the semiconductor chip and having an input end
to which an output of the microphone is given.
[0014]
DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a microphone device MU1
according to an embodiment of the present invention.
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Also in FIG. 1, as in the case of the microphone device MU2 shown in FIG. 4, a microphone device
provided with an electret capacitor EC is shown as an example.
That is, the ground potential GND is applied to one end of the electret capacitor EC, and the
capacitance value of the electret capacitor EC changes when it receives a sound pressure, and an
input signal Vin is generated between the both electrodes.
The anode of the diode D1 is connected to one end of the electret capacitor EC, and the cathode
is connected to the other end of the electret capacitor EC.
Further, with regard to the diode D2, the anode and the cathode are reversely connected to the
diode D1 and are connected to both ends of the electret capacitor EC.
Furthermore, a resistor R1 is connected in parallel across the electret capacitor EC. The source of
the transistor T1 is connected to one end of the electret capacitor EC, and the gate is connected
to the other end of the electret capacitor EC. The source of the transistor T2 is connected to the
drain of the transistor T1. The power supply potential Vdd is applied to the drain of the
transistor T2, and the constant potential Vref2 is applied to the gate. The ground potential GND
is applied to the back gates of the transistors T1 and T2.
[0015]
The operation of the impedance conversion circuit including the electret capacitor EC and the
diodes D1 and D2, the resistor R1 and the transistors T1 and T2 is the same as that of the
microphone device MU2, and thus the description thereof is omitted.
[0016]
Now, although the microphone device MU1 according to the embodiment of the present
invention is formed in a semiconductor chip, a microphone filter FT1 and an amplifier are also
formed in the semiconductor chip.
[0017]
The microphone filter FT1 is basically the same CR circuit as the conventional microphone filter
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FT2, but a transistor of a current mirror circuit is used for its resistance.
That is, the microphone filter FT1 includes a capacitor C1, N channel MOS type transistors T3
and T4, and a constant current source IS, and the transistors T3 and T4 and the constant current
source IS constitute a current mirror circuit.
An output signal Vout2 from the microphone unit MU1 is given to one end of the capacitor C1.
The drain of the transistor T4 is connected to the other end of the capacitor C1. The constant
potential Vref1 is applied to the source of the transistor T4. The source of the transistor T3 is
connected to the source of the transistor T4, and the gate of the transistor T4 is connected to the
gate of the transistor T3. The drain of the transistor T3 is connected to one end of a constant
current source IS, and the gate of the transistor T4 is shorted. The other end of constant current
source IS is supplied with power supply potential Vdd. The ground potential GND is also applied
to the back gates of the transistors T3 and T4.
[0018]
The microphone filter FT1 outputs a voltage drop between the drain and source of the transistor
T4 to remove a direct current component and a low frequency component contained in an output
signal at the source of the transistor T2.
[0019]
The output of the microphone filter FT1 is input to the amplifier.
Also in FIG. 1, as in FIG. 4, an amplifier including a voltage follower and an inverting amplifier is
illustrated. That is, the output of the microphone filter FT1 is input to the positive input terminal
of the operational amplifier OP1. The output of the operational amplifier OP1 is input to the
negative input terminal of the operational amplifier OP1, and the operational amplifier OP1
functions as a voltage follower. The output of the operational amplifier OP1 is input to the
negative input terminal of the operational amplifier OP2 via the resistor R2. The output Vout3 of
the operational amplifier OP2 is also input to the negative input terminal of the operational
amplifier OP2 via the resistor R3, and the operational amplifier OP2 functions as an inverting
amplifier. The constant potential Vref1 is applied to the positive input terminal of the operational
amplifier OP2.
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[0020]
The reason why the transistor of the current mirror circuit is used as a resistor in the
microphone filter FT1 will be described below.
[0021]
For example, in the case of a MOS transistor, the relationship between the drain-source current
IDS and the drain-source voltage VDS, that is, the current-voltage characteristics, generally
increases the drain-source current IDS with the increase of the drain-source voltage VDS. It is
divided into a resistive region and a constant current region in which the drain-source current
IDS does not increase beyond a predetermined value even if the drain-source voltage VDS
increases.
However, in fact, as shown in FIG. 2, a phenomenon in which the drain-to-source current IDS
slightly increases with the increase of the drain-to-source voltage VDS also in the constant
current region can be seen. This phenomenon is considered to result from the fact that the
depletion layer generated at the drain end changes the effective channel length, and is referred
to as a channel length modulation effect. The channel length modulation effect is expressed by
the following equations 1 and 2.
[0024] Here, VGS is the gate-source voltage of the MOS transistor, VT is the threshold voltage of
the MOS transistor, λ is the channel length modulation coefficient, β is the gain constant, W is
the channel width, L is the channel length, and μ is the channel The carrier mobility on the
surface and COX each represent the capacitance value of the gate insulating film per unit area.
[0025] As shown in FIG. 2, it is known that convergence of each current-voltage characteristic in
a constant current region taking into account the channel length modulation effect toward one
VDS axis toward one VDS axis is possible. The absolute value of the voltage up to this
intersection is referred to as the Early voltage VA, and its value is about 50 to 100 V in the
transistor on the integrated circuit.
[0026] This channel length modulation effect can be considered as a phenomenon in which the
drain-source voltage VDS largely changes due to a slight change in the drain-source current IDS,
from another point of view. That is, it can be seen that the MOS transistor in the constant current
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region has a large value of resistance (differential resistance).
[0027] Therefore, if this is used, a large value of resistance can be provided on the
semiconductor chip without using the resistance of the individual parts. If the resistance value is
large, the capacitance value in the microphone filter FT1 does not have to be a large value. This
is the reason why a transistor is used as a resistor in the microphone filter FT1.
[0028] However, it may be a problem to provide a constant gate-source voltage to one MOS
transistor and use the drain-source at that time as a resistor. For example, it is conceivable that
the current-voltage characteristic changes due to a temperature change, and the value as the
resistance in the microphone filter FT1 fluctuates. In that case, since the value of the cutoff
frequency f is greatly influenced, the function as a filter for audio signals may be impaired.
[0029] Therefore, the transistor of the current mirror circuit is adopted as a resistor in the
microphone filter FT1. A current mirror circuit is resistant to characteristic variations due to
temperature changes, and can be formed in a semiconductor chip without increasing the area as
much.
[0030] In the microphone filter FT1, a constant current having the same value as the current
output from the constant current source IS flows between any drain and source of the transistors
T3 and T4. However, since the channel length modulation effect exists as described above, when
there is a change in the drain-source voltage VDS of the transistor T4, the drain-source current
IDS of the transistor T4 slightly changes in a linear characteristic. Do. That is, the transistor T4
functions as a large value resistor.
[0031] As can be seen from the current-voltage characteristics of FIG. 2, the channel length
modulation effect is weaker as the gate-source voltage VGS is lower, that is, as the value of the
drain-source current IDS in the constant current region is lower. , The value of resistance
(differential resistance in the constant current region) approaches infinity. Therefore, in order to
increase the resistance value, the current output from the constant current source IS is narrowed
to keep the drain-source current IDS of the transistor T4 at a small value, and the drain-source
voltage of the transistor T4 is reduced. The variation of the drain-source current IDS may be
reduced with respect to the variation of VDS.
[0032] The channel length modulation effect is also weakened by the increase of the value of the
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gain constant β in Equation 2. This is because, as shown in FIG. 3, when the value of the gain
constant β is different, the value of the early voltage VA changes. In FIG. 3, β1> β2 and VA1>
VA2. Therefore, in order to increase the resistance value, the value of the gain constant β may
be increased. For that purpose, as can be seen from Equation 2, the channel length L of the
transistor T4 may be designed to be short, or the channel width W may be designed to be large.
Of course, these transistor sizes are determined by performing simulations or making and
evaluating prototypes in consideration of other factors such as chip area.
[0033] Although the channel length modulation effect of the MOS transistor is used in this
embodiment, the same effect can be obtained by using the Early effect of the bipolar transistor.
When a bipolar transistor is used for the transistors T3 and T4, the above-mentioned gate, drain
and source may be replaced with base, collector and emitter, respectively.
[0034] Further, in the present embodiment, although the microphone device has been described
as an example having an electret capacitor, the present invention can be applied to any other
microphone device which can be formed in a semiconductor chip. . As such an example, for
example, a piezoelectric microphone element formed in a semiconductor chip can be considered.
[0035] By using the microphone device MU1 according to the embodiment of the present
invention, the differential resistance due to the channel length modulation effect in the currentvoltage characteristic between the drain and the source of the transistor T4 can be used as a
resistor, and the resistance value and the capacitance value are components. A microphone filter
having a large time constant can be realized. Further, since the transistors T3 and T4 and the
constant current source IS constitute a current mirror circuit, they are resistant to variations in
the current-voltage characteristics of the transistor T4 due to temperature changes, and are
formed in the semiconductor chip without increasing the area so much. can do.
[0036] In addition, since the microphone filter and the microphone are provided in the same
semiconductor chip, downsizing and cost reduction of the microphone device can be achieved.
[0037] In addition, since the amplifier is also provided in the same semiconductor chip, it is
possible to further reduce the size and cost of the microphone device.
[0038] According to the present invention, the differential resistance due to the channel length
modulation effect or the Early effect in the current-voltage characteristic between the first and
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second current electrodes of the first transistor can be used as a resistor. Thus, it is possible to
realize a microphone filter having a large time constant having resistance and capacitance values
as components. In addition, since the first and second transistors and the constant current source
constitute a current mirror circuit, there is resistance to variations in the current-voltage
characteristics of the first transistor due to temperature changes, and without increasing the area
so much in the semiconductor chip Can be formed.
[0039] According to the second aspect of the present invention, since the microphone filter and
the microphone according to the first aspect are provided in the same semiconductor chip,
downsizing and cost reduction of the microphone device can be achieved.
[0040] According to the third aspect of the present invention, since the amplifier is also
provided, the miniaturization and cost reduction of the microphone device can be further
achieved.
[0041] According to the fourth aspect of the present invention, since the amplifier and the
microphone are provided in the same semiconductor chip, the microphone device can be
miniaturized and the cost can be reduced.
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