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DESCRIPTION JP2015505173

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DESCRIPTION JP2015505173
Abstract: A double backplate microphone is provided that has good signal to noise ratio and can
be manufactured with reduced manufacturing cost. The microphone includes a first back plate
BP1, a second back plate BP2, and a membrane M. The microphone further includes an amplifier
AMP having a single-ended input port. The first back plate BP1 is electrically connected to the
single end input port. [Selected figure] Figure 1
Double backplate MEMS microphone with single ended amplifier input port
[0001]
The present invention relates to a double backplate MEMS microphone with an amplifier having
a single ended input port.
[0002]
A simple MEMS microphone comprises one backplate and one membrane forming a capacitor to
which a bias voltage is applied.
The sound causes the membrane to vibrate. Therefore, the acoustic signal can be converted into
an electrical signal by determining the capacitance of the capacitor. For this purpose, this
membrane or backplate is connected to the amplifier and the other electrode of the capacitor is
electrically connected to a fixed potential. To this end, an amplifier with a single-ended input port
is required.
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1
[0003]
An object of the present invention is to provide a MEMS microphone with an improved signal to
noise ratio. A further object of the invention is to provide a MEMS microphone that can be
manufactured at low manufacturing costs. A third object of the present invention is to provide a
low current consumption MEMS microphone.
[0004]
For this reason, independent claim 1 provides a low current consumption MEMS microphone
that has good signal to noise ratio and can be manufactured with low manufacturing cost.
[0005]
The MEMS microphone comprises a first back plate and a second back plate electrically
connected to ground.
The microphone further comprises a membrane disposed between the first backplate and the
second backplate, and an amplifier having a single-ended input port. The first back plate is
electrically connected to the single-ended input port.
[0006]
In this way a double backplate microphone is formed. A bias voltage may be applied to the
membrane, wherein the first and second backplates are DC biased to a fixed potential. The signal
from the first backplate and the signal from the second backplate both comprise an acoustic
signal converted into the form of an electrical signal, such that there is a better signal-to-noise
ratio. The signals are summed in phase.
[0007]
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However, compared to conventional double backplate microphones, the present invention uses
an amplifier with a single-ended input port to amplify the electrical signal. A conventional double
backplate microphone uses an amplifier with a balanced input port, for example an input port
with two signal connections that receive an electrical signal of opposite polarity but similar
absolute value . An amplifier with a single-ended input port instead of a balanced input port can
be manufactured at low cost. Thus, MEMS microphones with these simple amplifiers can be
manufactured with low manufacturing costs and low current consumption. Such microphones
have lower manufacturing costs compared to conventional double backplate microphones and
also provide better signal to noise ratio as compared to single backplate microphones.
[0008]
The distance between the membrane and the respective backplate may be 2 μm.
[0009]
In one embodiment, this double backplate microphone further comprises a first resistive element
of between 1 GΩ and 1000 GΩ, for example 100 GΩ.
The first resistive element is electrically connected to the first back plate. Through this first
resistive element, the first back plate can be biased relative to the second back plate electrically
connected to ground. The backplate and the membrane form the electrodes of the first capacitor.
The membrane and the second back plate form an electrode of a second capacitor electrically
connected in series to the first capacitor. Thus, the series-connected first and second capacitors
are biased through the first resistance element. The first and second capacitors connected in
series can form a variable capacitance capacitor element. As the capacitance of one capacitor
increases, the capacitance of the other capacitor decreases and vice versa. Thus, the signal
voltage from the first capacitor and the signal voltage from the second capacitor are added in
phase.
[0010]
Only a single-ended output port of this capacitor element is necessary to connect the capacitor
element to an amplifier circuit with an amplifier with a single-ended input port. Thus, the
membrane may be DC connected to a specific potential or may be AC floating.
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[0011]
In one embodiment, the double backplate microphone further comprises a second resistive
element, for example 100 GΩ, between 1 GΩ and 1000 GΩ. The second resistance element is
electrically connected to the membrane. Therefore, the potential of this membrane can be
adjusted independently.
[0012]
The above resistance element may be realized by a diode electrically connected in parallel in
reverse polarity. In the conventional double backplate microphone, three signal ports were
required to electrically connect the capacitor element to the external circuit. That is, the first
backplate is electrically connected to the first input port of the amplifier, the second backplate is
electrically connected to the second balance port of the amplifier, and the membrane is at the
membrane potential. Are electrically connected to a voltage source that supplies However, in this
embodiment of the invention, only two signal ports are needed to connect the capacitor element
to the external circuit.
[0013]
In one embodiment, with respect to ground potential, the membrane is biased at a voltage
between 5V and 15V, for example 10V.
[0014]
The second back plate is electrically connected to ground.
[0015]
In one embodiment, the first backplate is biased at a voltage between -2V and + 2V.
[0016]
In one embodiment, the amplifier is a low noise amplifier.
[0017]
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In one embodiment, the double backplate microphone further comprises a carrier substrate, a
MEMS chip, and an IC chip.
The first back plate, the membrane, and the second back plate are disposed in the MEMS chip.
The amplifier comprises an amplifier circuit arranged in this IC chip.
The MEMS chip and the IC chip are disposed on a carrier substrate.
[0018]
Since the capacitor element consisting of the first capacitor and the second capacitor is
electrically connected to the amplifier only through the first back plate, the integrated circuit of
the MEMS chip and the amplifier on which these capacitors are mounted is mounted Only a
single signal line is required to electrically connect with the integrated IC chip.
[0019]
In one embodiment, the double backplate microphone of the present invention comprises a first
and a second resistive element, which are mounted as an SMD component arranged on a carrier
substrate or a circuit in an IC chip It is formed as an element.
[0020]
In one embodiment, the microphone comprises a MEMS chip, wherein the first backplate, the
membrane and the second backplate are disposed on the MEMS chip and the amplifier is
disposed on the MEMS chip And an amplifier circuit.
このようなチップはシリコンチップであってよい。
[0021]
In one embodiment, the IC chip is an application-specific integrated circuit (ASIC) chip.
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[0022]
Further description of the basic principles and schematic embodiments of the present invention
is given in the following figures.
[0023]
FIG. 2 shows an equivalent circuit diagram of a basic embodiment.
It is a figure which shows the equivalent circuit schematic of a still more sophisticated MEMS
microphone.
FIG. 5 is an equivalent circuit diagram of a MEMS microphone provided with an amplifier having
a balanced input port.
FIG. 2 shows a double backplate microphone with a carrier substrate carrying a MEMS chip, an
IC chip and two resistive elements.
[0024]
FIG. 1 shows an equivalent circuit diagram of a MEMS microphone DBM provided with a first
back plate BP1 and a second back plate BP2.
The membrane M is disposed between the first back plate BP1 and the second back plate BP2.
The second back plate BP2 is electrically connected to the ground GND. The first back plate BP1
is electrically connected to the single end input port SEIP of the amplifier AMP. The first back
plate BP1 and the membrane M form the electrodes of the first capacitor (C1 in FIG. 1). The
membrane M and the second back plate BP2 form the electrodes of the second capacitor (C2 in
FIG. 1). The series connection of the first capacitor and the second capacitor forms a capacitive
element CE having a variable capacitance, which varies with time depending on the received
sound pressure. Only the single-ended output port SEOP is required to electrically connect the
capacitive element CE to the single-ended input port SEIP of the amplifier AMP. For this purpose,
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signal lines electrically connecting the single-ended output port SEOP and the single-ended input
port SEIP may be provided, for example, as metallization. The first back plate BP1 is biased at a
first voltage V1 via a first voltage source VS1 and a first resistive element R1. Therefore, the first
resistance element R1 is electrically connected to the single end output port SEOP of the
capacitive element CE and the single end input port SEIP of the amplifier AMP.
[0025]
Thus, by virtue of this double backplate structure, a MEMS microphone with good signal-to-noise
ratio is provided, which allows low manufacturing costs by utilizing an amplifier with only a
single-ended input port. .
[0026]
FIG. 2 shows an embodiment of a double backplate MEMS microphone DBM with additional
circuit elements.
The first back plate BP1 and the membrane of FIG. 1 are schematically shown as a first capacitor
C1. The second back plate BP2 and the membrane M are schematically shown as a second
capacitor C2. This membrane is biased by the power supply VS2 through the second resistance
element R2. Therefore, the second resistance element R2 is electrically connected to the port
MBP for biasing the membrane.
[0027]
This voltage source may be realized by a charge pump.
[0028]
The second back plate BP2 is connected to the ground GND, and the first back plate BP1 is
connected to the input of the amplifier.
The signal from the second backplate and the signal from the first backplate are summed in
phase. The membrane is biased via the second resistive element, for example via an ultra-high
impedance network, so that the voltage V2 is not shorted.
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[0029]
In contrast to conventional double backplate microphones, the parasitic capacitance between the
membrane and the ground is no longer relevant to these. Thus, this capacity is minimized.
[0030]
The parasitic capacitance generated between the first back plate BP1 and the ground is shown as
Cp1. The parasitic capacitance generated between the membrane M and the ground is indicated
by the symbol Cm. The parasitic capacitance generated between the second back plate BP2 and
the ground is indicated by the symbol Cp1. In the balanced state, ie no acoustic signal is received
at all, the first capacitor C1 and the second capacitor C2 have a capacitance of between 4 pF and
8 pF, for example of 6 pF. The parasitic capacitance Cp1 between the first back plate BP1 and the
ground may have a value of 0.1 * C1. The parasitic capacitance Cp2 between the second back
plate BP1 and the ground may have a value of 0.5 * C1. The parasitic capacitance Cm between
the membrane M and the ground may have a value of 0.5 * C1. The detection voltage Vsens is
defined by the sum of V1 and V2. The effective detection voltage in which the parasitic
capacitance is considered is expressed by the following equation.
Vsenseff=(C2/(C2+Cm)*V1+V2)*(C1*(C2+Cm))/(C
1*(C2+Cm)+(C2+C1+Cm)*Cp1)... (1)したがって、
Vsenseff=0.714*Vsensとなる。 The effective detection voltage is reduced
by a factor of 0.714.
[0031]
FIG. 3 shows a double backplate microphone DBM with an amplifier AMP with two balanced
input ports, a first balanced input port BIP1 and a second balanced input port BIP2. The first
balance input port BIP1 is electrically connected to the first back plate BP1 of the first capacitive
element C1. The second balanced input port BIP2 is electrically connected to the second back
plate BP2 of the second capacitive element C2. The membrane M is biased through the
membrane input port. Since both backplates of capacitive element CE are electrically connected
to amplifier AMP, in addition to membrane bias port MBP, this capacitive element CE has the first
backplate output port BOP1 and the second backplate output port Requires BOP2.
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[0032]
Assuming that the capacitance of the capacitor and the parasitic capacitance are equal to the
respective capacitances of the embodiment of FIG. 2, the differential effective detection voltage is
given by the following equation.
Vdiff=V2*C2/(C2+Cp2)+V1*C1/(C1+Cp1)... (2)した
がってVdiff=0.788*Vsensとなる。
[0033]
Thus, the detection efficiency of a microphone with an amplifier with single-ended input (see
equation (1)) is reduced by 0.714 / 0.788 = 0.9 relative to a double backplate microphone with a
balanced amplifier input Do.
[0034]
However, detection efficiency is improved compared to a single backplate microphone, and
manufacturing cost and current consumption are reduced compared to a microphone with an
amplifier with a balanced input port.
[0035]
FIG. 4 shows an embodiment of a double backplate microphone DBM, wherein the carrier
substrate CS carries the MEMS chip MC, the resistive elements R1 and R2, and the IC chip IC.
The mechanical parts, in particular the backplates BP1, BP2, the membrane M and the back
volume (back volume) are arranged in the MEMS chip MC.
The circuit elements of the amplifier are integrated in an IC chip. このICチップはASICチッ
プであってよい。
[0036]
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The double backplate MEMS microphone is not limited to the embodiments described herein or
in these figures. Additional backplates, membranes, members such as capacitive or resistive
elements, or amplifiers, or combinations thereof may be included in the present invention. While
a high bias voltage is applied to the membrane, both the potentiostat-side backplate and the highpotential side backplate are biased at a common mode voltage through resistive elements such as
ultra-high impedance networks. This bias voltage is chosen to be the input bias point compatible
with the amplifier. Thus, the microphone is biased with the effective bias voltage V2-V1. With
respect to sound pressure, opposite phase signals are generated at the balance output ports
BOP1 and BOP2, respectively. This difference signal is amplified by the amplifier to become a
single-ended output voltage.
[0037]
AMP: amplifier BIP1: first balance input port BIP2: second balance input port BOP1: first balance
output port BOP2: second balance output port BP1, BP2: first, second back plates C1, C2 First
and second capacitors CE: Capacitive element CM of variable capacitance (in time): Parasitic
capacitance CP1 between membrane and ground: Parasitic capacitance CP2 between first
capacitor and ground: second Parasitic capacitance between the capacitor C2 and the ground CS:
carrier substrate DBM: double back plate microphone GND: ground IC: IC chip M: membrane
MBP: membrane bias port MC: MEMS chip R1: first resistance element R2: first 2 resistance
element SEIP: single-ended input port of amplifier SEOP: thin Ended output port VS1: first voltage
source VS2: second voltage source
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