U.S. patent application number 15/471502 was filed with the patent office on 2018-08-16 for microphone system having microphone transducer in feedback loop with adjustable frequency -3db point and improved settling speed.
The applicant listed for this patent is Akustica, Inc., Robert Bosch GmbH. Invention is credited to Chinwuba Ezekwe, Saikrishna Ganta.
Application Number | 20180234763 15/471502 |
Document ID | / |
Family ID | 63106467 |
Filed Date | 2018-08-16 |
United States Patent
Application |
20180234763 |
Kind Code |
A1 |
Ganta; Saikrishna ; et
al. |
August 16, 2018 |
MICROPHONE SYSTEM HAVING MICROPHONE TRANSDUCER IN FEEDBACK LOOP
WITH ADJUSTABLE FREQUENCY -3dB POINT AND IMPROVED SETTLING
SPEED
Abstract
A microphone biasing circuit comprises a first amplifier having
an output connected to a first node and an input connected to a
second node; and a first feedback path connected from the first
node to the second node. The first feedback path comprises a
microphone having a first terminal connected to the first node and
a second terminal connected to a third node, the microphone being
configured to provide a sensed voltage at the first node in
response to sound, the third node having a first DC bias voltage;
and a first capacitor connected between the third node and the
second node.
Inventors: |
Ganta; Saikrishna;
(Milpitas, CA) ; Ezekwe; Chinwuba; (Albany,
CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Akustica, Inc.
Robert Bosch GmbH |
Pittsburgh
Stuttgart |
PA |
US
DE |
|
|
Family ID: |
63106467 |
Appl. No.: |
15/471502 |
Filed: |
March 28, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62459836 |
Feb 16, 2017 |
|
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|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H03F 2203/45288
20130101; H03F 2203/45138 20130101; H04R 2201/003 20130101; H04R
3/06 20130101; H03F 3/45475 20130101; H03F 2203/45212 20130101;
H03F 3/45973 20130101; H04R 19/005 20130101 |
International
Class: |
H04R 3/06 20060101
H04R003/06; H03F 3/45 20060101 H03F003/45 |
Claims
1. A microphone biasing circuit comprising: a first amplifier
having an output connected to a first node and an input connected
to a second node; and a first feedback path connected from the
first node to the second node, the first feedback path comprising:
a microphone having a first terminal connected to the first node
and a second terminal connected to a third node, the microphone
being configured to provide a sensed voltage at the first node in
response to sound, the third node having a first DC bias voltage;
and a first capacitor connected between the third node and the
second node.
2. The microphone biasing circuit of claim 1, wherein the first
amplifier is an operational amplifier having an inverting input
connected to the second node and a non-inverting input connected to
a fixed reference voltage.
3. The microphone biasing circuit of claim 1, further comprising: a
charge pump circuit configured to provide the first DC bias
voltage; and a first diode and a second diode, each connected
between the charge pump circuit and the third node, the first diode
and the second diode being connected antiparallel with one
another.
4. The microphone biasing circuit of claim 1, further comprising: a
third diode and a fourth diode, each connected between the second
node and a fourth node, the third diode and the fourth diode being
connected antiparallel with one another, the fourth node having a
second DC bias voltage that is coupled to the second node via the
third diode and the fourth diode.
5. The microphone biasing circuit of claim 4, further comprising: a
second feedback path connected from the first node to the fourth
node, the second feedback path comprising: an offset correction
circuit arranged in the second feedback path and configured to
adjust a DC offset of a feedback voltage at the second node to have
a predetermined magnitude.
6. The microphone biasing circuit of claim 5, the offset correction
circuit comprising: at least one of (i) a low pass filter circuit
and (ii) a proportional-integrator circuit arranged in the second
feedback path and configured to adjust the DC offset of the output
signal at the second node to have the predetermined magnitude.
7. The microphone biasing circuit of claim 6, wherein a gain and
bandwidth of the at least one of the low pass filter and the
proportional-integrator is adjustable to modify at least one of a
bandwidth and a cut-off frequency of the microphone biasing
circuit.
8. The microphone biasing circuit of claim 5, the offset correction
circuit comprising: a resistor and second capacitor connected in
parallel between the first node and a fifth node; a second
amplifier having an output connected to the fourth node and an
input connected to the fifth node; and a third capacitor connected
from the output of the second amplifier to the input of the second
amplifier.
9. The microphone biasing circuit of claim 8, wherein at least one
of a resistance of the resistor, a capacitance of the second
capacitor, and a capacitance of the third capacitor is adjustable
to modify at least one of a bandwidth and a cut-off frequency of
the microphone biasing circuit.
10. The microphone biasing circuit of claim 5, the offset
correction circuit comprising: at least one of (i) a digital low
pass filter and (ii) a digital proportional-integrator arranged in
the second feedback path and configured to adjust the DC offset of
the output signal at the second node to have the predetermined
magnitude; an analog-to-digital converter arranged in the second
feedback path between the first node and an input of one of the at
least one of digital low pass filter and digital
proportional-integrator; and a digital-to-analog converter arranged
in the second feedback path between the fourth node and an output
of one of the at least one of the digital low pass filter and
digital proportional-integrator.
11. The microphone biasing circuit of claim 10, wherein a gain and
bandwidth of the at least one of the digital low pass filter and
the digital proportional-integrator is adjustable to modify at
least one of a bandwidth and a cut-off frequency of the microphone
biasing circuit.
12. The microphone biasing circuit of claim 10, the offset
correction circuit comprising: an anti-aliasing filter arranged in
the second feedback path between the first node and the
analog-to-digital converter.
13. The microphone biasing circuit of claim 5, further comprising:
a diode linearization circuit, which includes the third diode and
the fourth diode, arranged in the second feedback path, the diode
linearization circuit configured to have substantially linear
current-voltage characteristics.
14. The microphone biasing circuit of claim 5, further comprising:
a third amplifier arranged in the second feedback path having an
output connected the fourth node and an input connected to a sixth
node; an input resistance arranged in the second feedback path
connected between an output of the offset correction circuit and
the input of the third amplifier; and a fifth diode and a sixth
diode, each connected between the output of the third amplifier and
the input of the third amplifier, the fifth diode and the sixth
diode being connected antiparallel with one another.
15. The microphone biasing circuit of claim 14, wherein the input
resistance has a variable resistance that is adjustable to modify
at least one of a bandwidth and a cut-off frequency of the
microphone biasing circuit.
16. The microphone biasing circuit of claim 5, further comprising:
a variable resistance arranged in the second feedback path
connected between an output of the offset correction circuit and a
sixth node; and a plurality of attenuator circuits arranged in the
second feedback path connected in series between the sixth node and
the fourth node.
17. The microphone biasing circuit of claim 5, further comprising:
a resistance arranged in the second feedback path connected between
an output of the offset correction circuit and a sixth node; and a
plurality of circuits arranged in the second feedback path
connected in series between the sixth node and the fourth node,
each circuit comprising: a third amplifier; a fifth diode and a
sixth diode, each connected between the output of the third
amplifier and an output of the respective circuit, the fifth diode
and the sixth diode being connected antiparallel with one another;
and a seventh diode and an eighth diode, each connected between the
output of the third amplifier and the input of the third amplifier,
the seventh diode and the eighth diode being connected antiparallel
with one another, wherein the third diode and the fourth diode are
the fifth diode and the sixth diode for one of the circuits in the
plurality of circuits.
18. The microphone biasing circuit of claim 1, wherein the
microphone comprises a capacitive transducer.
19. The microphone biasing circuit of claim 1, wherein the
microphone comprises a microelectromechanical systems (MEMS)
transducer.
Description
[0001] This application claims the benefit of priority of U.S.
provisional application Ser. No. 62/459,836, filed on Feb. 16, 2017
the disclosure of which is herein incorporated by reference in its
entirety.
FIELD
[0002] The device disclosed in this document relates to microphone
biasing circuits and, more particularly, to microphone biasing
circuits having a microphone transducer in a feedback loop.
BACKGROUND
[0003] Microphones are transducers that convert sound into an
electrical signal. Microphones are used in a multitude of different
applications, such as sound recording, telephones, hearing aids,
and various sensor systems. Microphones generally operate most
accurately within a particular range of sound levels, depending on
a sensitivity and configuration of the microphone. In very loud
sound environments, the output signal of the microphone will often
become distorted. Particularly, essentially any microphone will
have an acoustical overload point (AOP), which is a level of sound
at which the microphone can no longer effectively distinguish
between the actual sound signal and noise/distortion. For example,
the AOP may be defined as the sound pressure level at which
distortion in the output signal reaches 10%.
[0004] Some types of microphones, such as condenser microphones and
capacitive MEMS (microelectromechanical systems) microphones,
require a DC bias voltage in order to operate. MEMS microphones
additionally require a very high resistance to establish proper DC
biasing. This resistance is on the order of few 100's of Giga
Ohms.
[0005] FIG. 1a shows a microphone circuit 1 for biasing a MEMS
microphone 10. The microphone circuit 1 includes charge pump 5 that
provides a DC bias voltage for the microphone 10. The circuit 1
includes diodes 25 and 35 which are coupled antiparallel to one
another between the charge pump 5 and a node 50. A capacitor 60 is
connected between the node 50 and ground. The microphone 10 is
connected between the node 50 and a node 40. The microphone 10
modulates the voltage at the node 40 to provide a sensed voltage in
response to sound. The circuit 1 further includes diodes 20 and 30
which are coupled antiparallel to one another between the node 40
and ground. Finally, the circuit 1 includes a pre-amplifier 70
having an input connected to the node 40, which provides an output
signal at an output node 80 based on the sensed voltage.
[0006] One disadvantage of the circuit 1 is that the sensed voltage
at the node 40 often has an undesired DC offset. Particularly, due
to parasitic resistance R.sub.parasitic of the microphone 10, a
small leakage current flows from the node 50 to the node 40,
through the microphone 10. The leakage current then flows from the
node 40 to ground, through the diodes 20, 30. As a result of the
leakage current, the sensed voltage may have a shifted DC offset.
For example, the DC offset for the sensed voltage may shift
slightly by approximately 300 mV.
[0007] Another disadvantage of the circuit 1 is that, at high
signal levels, the diodes 20, 30 will clip the sensed voltage,
which greatly reduces the AOP of the circuit. Particularly, each of
the diodes 20, 30 has a forward voltage V.sub.F (e.g., 700 mV) at
which it will turn on. At high signal levels, the diodes 20, 30
start to turn on, which distorts the sensed voltage. When the
sensed voltage falls below -V.sub.F, the diode 20 will turn on and
clip the sensed voltage. Similarly, when the sensed voltage rises
above +V.sub.F, then the diode 30 will turn on and clip the sensed
voltage.
[0008] FIG. 1b shows an exemplary waveform 90 for the sensed
voltage at the node 40 of the circuit 1 in response to microphone
10 being subjected to a high SPL 20 Hz acoustical signal. As can be
seen, the waveform 90 is distorted (clipped) when the signal level
is too high, due to the diodes 20, 30 being turned on. As is
apparent, this clipping effect caused by the turning on of the
diodes 20, 30 greatly limits the AOP of the microphone circuit 1.
FIG. 2 shows a plot illustrating a frequency spectrum 95 of the
waveform 90. As can be seen, the frequency spectrum 95 includes a
spike at 20 Hz, which corresponds to the actual sound (i.e. the 20
Hz acoustical signal). However, as can also be seen, the frequency
spectrum 95 further includes additional large spikes at 40 Hz, 60
Hz, 80 Hz, 100 Hz, 120 Hz, 140 Hz, and 180 Hz, which correspond to
the distortion introduced by the turning on of the diodes 20, 30.
As is apparent, this clipping effect caused by the turning on of
the diodes 20, 30 greatly limits the AOP of the microphone circuit
1.
[0009] One configuration that can reduce the distortion effect
includes arranging series stacks of the diodes 20, 30 to provide
more headroom for the sensed voltage. This modification increases
the AOP of the microphone circuit, but has disadvantages.
Particularly, this configuration provides reduced effectiveness at
higher temperatures (due to a reduction of forward voltage V.sub.F
at higher temperatures) and may cause tones in the output signal at
normal operation. Another configuration that can increases the AOP
of the microphone circuit includes a microphone 10 that is
configured with reduced sensitivity. The circuit employs electronic
gain to compensate for the reduced sensitivity of the microphone.
However, this configuration has the disadvantage of consuming more
power. A further configuration that can increase the AOP of the
microphone circuit is one in which the gain of the microphone is
reduced when high sound levels are detected. However, this
configuration has the disadvantage of creating acoustical
artifacts, such as clicks and pops, in the output signal. Yet
another configuration that can increase the AOP of the microphone
circuit is one in which the microphone has multiple membranes with
differing sensitivity. The circuit switches between multiple
membranes depending on sound levels. However, this configuration
also has the disadvantage of creating acoustical artifacts in the
output signal.
[0010] Accordingly, what is needed is a microphone biasing circuit
that achieves a high AOP with high energy efficiency and without
introducing acoustical artifacts into the output signal. It would
be further advantageous if the microphone biasing circuit had
adjustable bandwidth and cutoff frequencies, and faster settling
speeds.
SUMMARY
[0011] A microphone biasing circuit is disclosed. The microphone
biasing circuit comprises a first amplifier having an output
connected to a first node and an input connected to a second node;
and a first feedback path connected from the first node to the
second node. The first feedback path comprises a microphone having
a first terminal connected to the first node and a second terminal
connected to a third node, the microphone being configured to
provide a sensed voltage at the first node in response to sound,
the third node having a first DC bias voltage; and a first
capacitor connected between the third node and the second node.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] The foregoing aspects and other features of a microphone
circuit are explained in the following description, taken in
connection with the accompanying drawings.
[0013] FIG. 1a shows a microphone circuit according to the prior
art.
[0014] FIG. 1b shows a plot illustrating an exemplary signal sensed
by the microphone of FIG. 1a in response to a high SPL acoustical
signal.
[0015] FIG. 2 shows a plot illustrating a frequency spectrum of the
waveform of FIG. 1b.
[0016] FIG. 3a shows a microphone circuit that utilizes energy
efficient antiparallel diode biasing but also has a high AOP.
[0017] FIG. 3b shows a plot illustrating an exemplary waveform
sensed by the microphone of FIG. 3a in response to a high SPL
acoustical signal.
[0018] FIG. 4a shows a microphone circuit having an offset
correction feedback loop.
[0019] FIG. 4b shows exemplary frequency planning for the
microphone circuit of FIG. 4a.
[0020] FIG. 5 shows an exemplary diode linearization circuit.
[0021] FIG. 6 shows the diode linearization circuit of FIG. 5
having a variable input resistance.
[0022] FIG. 7 shows a cascaded form of the diode linearization
circuit of FIG. 5.
[0023] FIG. 8 shows a cascaded diode linearization circuit formed
by a series of attenuators.
[0024] FIG. 9 shows a microphone circuit having an exemplary
digital offset correction circuit.
[0025] FIG. 10 shows a microphone circuit having an exemplary
analog offset correction circuit.
[0026] FIG. 11 shows an example of the low frequency -3 dB point
being adjusted by an order of magnitude.
DETAILED DESCRIPTION
[0027] For the purposes of promoting an understanding of the
principles of the disclosure, reference will now be made to the
embodiments illustrated in the drawings and described in the
following written specification. It is understood that no
limitation to the scope of the disclosure is thereby intended. It
is further understood that the present disclosure includes any
alterations and modifications to the illustrated embodiments and
includes further applications of the principles of the disclosure
as would normally occur to one skilled in the art which this
disclosure pertains.
[0028] FIG. 3a shows a microphone circuit 100 that advantageously
utilizes energy efficient diode biasing but also has a high AOP. It
is noted that, although the circuit 100 is shown and described in a
single-ended form, in some embodiments, the circuit 100 is in a
differential form. The microphone circuit 100 includes a microphone
110 connected between a node 150 and a node 140. The microphone 110
is configured to modulate the voltage at the node 140 to provide a
sensed voltage V.sub.sense in response to sound. In at least one
embodiment, the microphone 110 is a MEMS (microelectromechanical
systems) microphone. In some embodiments, some or all of the
components of the microphone circuits described herein are
integrated together on a single chip with the MEMS microphone. In
many embodiments, the operating principle of the microphone 110 is
that of a capacitive sensor having at least one electrically
conductive membrane, diaphragm, or the like that is mechanically
responsive to sound waves. In the illustrations provided herein,
the microphone 110 is depicted schematically as a variable
capacitor.
[0029] The microphone circuit 100 includes a charge pump 105
configured to provide a predetermined DC bias voltage V.sub.DC. The
circuit 100 includes diodes 125 and 135 which are coupled
antiparallel to one another between the charge pump 105 and a node
150. The diodes 125 and 135 operate to couple the predetermined DC
bias voltage V.sub.DC from the charge pump 105 to the node 150. In
one embodiment, the predetermined DC bias voltage V.sub.DC is 20
V.
[0030] The circuit 100 avoids the problem of signal clipping at
high signal levels by placing the microphone 110 in a negative
feedback loop and isolating the node 140 from any diodes that could
distort the sensed voltage V.sub.sense. To accomplish this, the
circuit 100 includes an amplifier 170, such as an operational
amplifier or an operational transconductance amplifier. The output
of the amplifier 170 is connected to the node 140. The
non-inverting input of the amplifier 170 is connected to a
reference voltage, which represents a desired DC bias point. A
capacitor 160 is connected between the node 150 and a node 145 at
the inverting input of the amplifier 170. In this way, a feedback
path 190 is defined between the output of the amplifier 170 and the
inverting input of the amplifier 170. The microphone 110 and the
capacitor 160 are arranged within the feedback path 190.
[0031] The circuit 100 further includes diodes 120 and 130 which
are coupled antiparallel to one another between the node 145 and a
node 185. The node 185 is connected to a corrective DC bias voltage
V.sub.bias. The diodes 120 and 130 operate to couple the DC bias
voltage V.sub.bias from the node 185 to the node 145. In one
embodiment, V.sub.bias is selected such that the voltage at the
node 145 is the same as the reference voltage that is connected to
the non-inverting input of the amplifier 170. The value of the DC
bias voltage V.sub.bias at the node 185 can be selected to
counteract the undesired DC shift from the desired DC bias point at
the node 145 due to leakage currents in the feedback path 190.
[0032] In the embodiment shown, the amplifier 170 serves the
purpose of a pre-amplifier having high input impedance and low
output impedance. However, in some embodiments, the circuit 100 may
include separate a pre-amplifier (not shown) having an input
connected to the node 140 and configured to provide an output
signal V.sub.out at an output node 180 based on the sensed voltage
V.sub.sense at the node 140.
[0033] It should be noted that, although the node 150 carries the
predetermined DC bias voltage V.sub.DC provided from the charge
pump 105, it is advantageously a virtual AC ground. Accordingly,
the diodes 125, 135 are not turned on during operation and do not
clip or distort any signal in the feedback path 190. Additionally,
the capacitor 160 isolates the node 145 from the predetermined DC
bias voltage V.sub.DC at the node 150. In this way, the node 145 is
a virtual ground, which experiences no substantial AC or DC
voltages. As a result, the diodes 120, 130 are also not turned on
during operation and do not clip or distort any feedback signals.
Furthermore, the amplifier 170 does not experience any high
voltages and can, therefore, be designed using low voltage
transistors. FIG. 3b shows an exemplary waveform 102 for the sensed
voltage V.sub.sense at the node 140 of the circuit 100 in response
to microphone 110 being subjected to a loud 20 Hz tone. As can be
seen, the waveform 102 is not distorted at high signal levels.
[0034] As discussed above, in practical implementations of the
microphone circuits described herein, the feedback voltage at the
node 145, which is input into the inverting input of the amplifier
170, may be shifted slightly from a desired DC bias point due to
leakage currents flowing through the capacitor 160 and/or the
diodes 120, 130. The DC bias voltage V.sub.bias can be selected to
counteract the undesired DC shift at the node 145. In some
embodiments, the DC bias voltage V.sub.bias is provided dynamically
by means of an offset correction feedback loop.
[0035] FIG. 4a shows a microphone circuit 300 having an offset
correction feedback loop. The circuit 300 is similar to the circuit
100 shown in FIG. 3a and like elements are identified to with
common reference labels and not described again in detail. In
addition to the components of the circuit 100, the circuit 300
includes a further feedback path 310 between the node 140 and the
node 145 at the inverting input of the amplifier 170. The feedback
path 310 at least includes an offset correction circuit 320 and the
diodes 120, 130. In many embodiments, the feedback path 310 further
includes a diode linearization circuit 330. Additionally, although
the circuit 300 is shown and described in a single-ended form, in
some embodiments, the circuit 300 is in a differential form.
[0036] The offset correction circuit 320 is connected between the
node 140 and the diode linearization circuit 330 and is configured
to adjust or correct the DC offset present in the feedback voltage
at the node 145, such that it is equal to a desired DC bias point.
In one embodiment, the offset correction circuit 320 is configured
to provide the DC bias voltage V.sub.bias such that the inverting
and non-inverting inputs of the amplifier 170 are equal to each
other. In some embodiments, the offset correction circuit 320
includes a proportional-integrator circuit and/or a low pass filter
circuit. The offset correction circuit 320 can be implemented using
analog, digital, or mixed components.
[0037] The diode linearization circuit 330 is configured such that,
the diodes 120, 130 in combination with the diode linearization
circuit 330 have more linear current-voltage characteristics and
can be modeled as resistor. The linearization of the diodes 120,
130 enables greater degrees of freedom in stabilizing the feedback
loops of the microphone circuit 300. Additionally, the
linearization of the diodes 120, 130 enables transient response
and/or settling times of the feedback loops of the microphone
circuit 300 to be much faster than with non-linearized diodes.
[0038] The pole frequency due to diodes and MEMs capacitance is
inversely proportional to product of R.sub.diode*C.sub.mems, where
R.sub.diode is a resistance of the diodes 120, 130 and C.sub.mems
is a capacitance of the microphone 110. This pole is highly
dependent on the leakage currents of the microphone 110, leakage
currents of the capacitor 160, and temperature, which makes it
difficult to stabilize the system for different microphones and
diodes having different intrinsic characteristics. Without the
diode linearization circuit 330, the offset correction circuit 320
generally would need to be designed with a very narrow bandwidth,
resulting in very slow settling response. The diode linearization
circuit 330 enables the feedback loop 310 to stabilize more
quickly, and generally provides more degrees of freedom in the
design of the offset correction circuit 320. Additionally, the
circuit 300 retains the noise advantages of diode biasing and
avoids the severe noise penalties associated with low value linear
resistors.
[0039] The amplifier 170, the microphone 110, the capacitor 160,
the diodes 120, 130, and the diode linearization circuit 330
essentially operate as an analog integrator circuit 340.
Particularly, the microphone 110 and capacitor 160 operate as a
feedback capacitance for the analog integrator circuit 340.
Similarly, the diodes 120, 130, in combination with the diode
linearization circuit 330 operate as an input resistance for the
analog integrator circuit 340. The cut-off frequencies of the
analog integrator circuit 340 are in part defined by the effective
resistance of the diodes 120, 130 in combination with the diode
linearization circuit 330. For the purpose of frequency planning,
the circuit 300 can be divided in two primary circuits, the analog
integrator circuit 340 and the offset correction circuit 320.
Accordingly, the overall bandwidth of the system and the cut-off
frequencies for the system are defined by the analog integrator
circuit 340 and the offset correction circuit 320. FIG. 4b shows
exemplary frequency planning for the analog integrator circuit 340
and the offset correction circuit 320 in order to achieve a desired
overall performance for the circuit 300. As will be discussed in
greater detail, the frequencies responses of the analog integrator
circuit 340 and the offset correction circuit 320 can be designed
and adjusted to shape the frequency response of the overall
system.
[0040] FIG. 5 shows a diode linearization circuit 400, which is an
exemplary embodiment of the diode linearization circuit 330. The
diode linearization circuit 400 includes an amplifier 410, such as
an operational amplifier. An input resistor 420 is connected
between a node 415 having the DC bias voltage V.sub.bias, which is
received from the offset correction circuit 320, and the inverting
input of the amplifier 410. The non-inverting input of the
amplifier 410 is connected to a fixed common voltage, such as
ground. The circuit 400 includes diodes 425, 435 that are coupled
antiparallel to one another between the output of the amplifier 410
and the inverting input of the amplifier 410. Finally, the diodes
120, 130 are coupled antiparallel to one another between the node
145 and an output of the amplifier 410. It is noted that that the
diodes 120, 130 and the node 145 correspond to those of the circuit
300 shown in FIG. 4a. In one embodiment, the diodes 425, 435 are N
times larger than the diodes 120, 130. The noise of the input
resistor 420 is attenuated by a factor of N.sup.2.
[0041] FIG. 6 shows a diode linearization circuit 500, which is
similar to the diode linearization circuit 400, except that is has
an adjustable input resistance. Particularly, the diode
linearization circuit 500 includes variable resistance circuit 510
in place of the input resistor 420. As shown, the variable
resistance circuit 510 includes a plurality of resistors connected
in parallel with one another and having different resistance
values. In one embodiment, the resistances of each resistor
progressively vary by a predetermined factor from one another
(e.g., 2.times., 5.times., 10.times., 100.times.). The variable
resistance circuit 510 includes switches connected in series with
each resistor. The switches can be closed or opened to establish
different current paths through individual or parallel combinations
of the resistors, thereby providing adjustable input resistance for
the diode linearization circuit 500. In other embodiments, the
variable resistance circuit 510 may instead comprise a
potentiometer or any other type of variable resistance. As
discussed in greater detail below, the variable resistance can be
used to adjust the bandwidth and cutoff frequencies of the analog
integrator circuit 340.
[0042] FIG. 7 shows a cascaded diode linearization circuit 600. The
cascaded diode linearization circuit 600 is similar to the diode
linearization circuit 400. However, the diode linearization circuit
600 includes a plurality of stages 402. Each stage 402 comprises an
amplifier 410 having diodes 425, 435 connected antiparallel to one
another between the output of the respective amplifier 410 and the
inverting input of the respective amplifier 410. The non-inverting
input of each amplifier is connected to a fixed common voltage,
such as ground. Diodes 420, 430 are coupled antiparallel to one
another between each amplifier 410. In at least the last stage 402,
the diodes 420, 430 are one and the same as the diodes 120, 130,
discussed above. Alternatively, in other embodiments, the stages
402 may be replaced by attenuator circuits 404 cascaded in series,
as shown in FIG. 8.
[0043] FIG. 9 shows a microphone circuit 700, which is a preferred
embodiment of the microphone circuit 300 of FIG. 4a, which includes
a digital implementation of the offset correction circuit 320. The
microphone circuit 700 includes the feedback path 310, which
includes the offset correction circuit 320, the diode linearization
circuit 330, and the diodes 120, 130. The diode linearization
circuit 330 is embodied as described with respect to the diode
linearization circuit 400 of FIG. 5, except that the input resistor
420 is illustrated as a variable resistance. The offset correction
circuit 320 is embodied as a digital offset correction circuit 710.
The digital offset correction circuit 710 is configured to adjust
or correct the DC offset present in the feedback voltage at the
node 145, such that it is equal to a desired DC bias point. In one
embodiment, the digital offset correction circuit 710 is configured
to provide the DC bias voltage V.sub.bias such that the inverting
and non-inverting inputs of the amplifier 170 are equal to each
other.
[0044] The digital offset correction circuit 710 includes an
analog-to-digital converter (ADC) 715 connected to the node 140 and
configured to digitize the output signal at the node 140 to provide
a digital feedback signal. The digital offset correction circuit
710 further includes a digital-to-analog converter (DAC) 720
connected to the node 415 of the diode linearization circuit 400
and configured to convert the digital feedback signal back to an
analog voltage. In one embodiment, the ADC 715 provides a digital
output V.sub.out-d. In one embodiment, the ADC 715 is a delta-sigma
based converter, which may comprise a delta-sigma modulator and a
digital filter. In one embodiment, the DAC 720 is a delta-sigma
based converter, which may comprise a delta-sigma modulator and an
analog filter.
[0045] In one embodiment, the digital offset correction circuit 710
includes an anti-aliasing filter 725 is connected between the node
140 and the ADC 715. The anti-aliasing filter 725 is configured
constrain the bandwidth of the output signal to prevent aliasing
when digitized by the ADC 715. Particularly, the anti-aliasing
filter 725 is at least configured to remove or attenuate
alternating components from the output signal that have frequencies
greater than half the sampling rate of the ADC 715.
[0046] In one embodiment, the digital offset correction circuit 710
further includes a digital filter 730 and a proportional integrator
735 configured to adjust or correct the DC offset present in the
feedback voltage at the node 145, such that it is equal to a
desired DC bias point. In one embodiment, digital filter 730 and a
proportional integrator 735 are configured to provide the DC bias
voltage V.sub.bias such that the inverting and non-inverting inputs
of the amplifier 170 are equal to each other. In one embodiment,
the digital filter 730 is a low-pass filter. In one embodiment, the
digital filter 730 comprises a cascaded integrator-comb (CIC)
filter comprising one or more integrator and comb filter pairs. In
one embodiment, the digital filter 730 is configured to slow a roll
off from 200 Hz to 20 KHz. In the embodiment shown, the
proportional integrator 735 comprises a proportional path 740 and
an integration path 745, which are combined by an adder element
750.
[0047] In some embodiments having a delta-sigma based ADC 715
and/or DAC 720, the distortion performance of the ADC 715 and/or
DAC 720 can be affected by the DC offset in the output signal at
the node 140. In one embodiment, the digital offset correction
circuit 710 further includes an ADC/DAC tone controller 755
configured to provide an offset signal that reduces a distortion in
the ADC 715 and/or DAC 720. In one embodiment, ADC/DAC tone
controller 755 is connected to the adder element 750 to inject the
offset signal into the digital feedback signal after the
proportional integrator 735. In other embodiments, the ADC/DAC tone
controller 755 is connected connect to some other adder element to
inject the offset signal into the digital feedback signal elsewhere
in the feedback path 310.
[0048] In one embodiment, the digital offset correction circuit 710
further includes a startup accelerator (not shown) arranged in the
feedback path 310. The startup accelerator is configured to store
predefined or measured startup values for digital feedback signal
in memory, which serve as initial conditions during startup of the
digital feedback loop. In this way, the digital feedback loop is
able startup faster.
[0049] FIG. 10 shows a microphone circuit 800, which is another
embodiment of the microphone circuit 300 of FIG. 4a, which includes
an analog implementation of the offset correction circuit 320. The
microphone circuit 800 includes the feedback path 310, which
includes the offset correction circuit 320, the diode linearization
circuit 330, and the diodes 120, 130. The diode linearization
circuit 330 is embodied as described with respect to the diode
linearization circuit 400 of FIG. 5, except that the input resistor
420 is illustrated as a variable resistance. The offset correction
circuit 320 is embodied as an analog offset correction circuit 810.
The analog offset correction circuit 810 is configured to adjust or
correct the DC offset present in the feedback voltage at the node
145, such that it is equal to a desired DC bias point. In one
embodiment, the analog offset correction circuit 810 is configured
to provide the DC bias voltage V.sub.bias such that the inverting
and non-inverting inputs of the amplifier 170 are equal to each
other.
[0050] The analog offset correction circuit 810 includes a resistor
815 and a capacitor 820 connected in parallel with one another
between the node 140 and a node 825. The node 825 is connected to
an inverting input of an operational amplifier 830. A non-inverting
input of the operational amplifier 830 is connected reference
voltage, which represents the desired DC bias point. The output of
the operational amplifier 830 is connected to the node 415 of the
diode linearization circuit 400. The analog offset correction
circuit 810 further includes a capacitor 835 connected between the
output of the operational amplifier 830 and the inverting input of
the operational amplifier 830. In at least one embodiment, the
resistor 815 has a variable/adjustable resistance. In at least one
embodiment, the capacitor 820 and/or the capacitor 835 have
variable/adjustable capacitance.
[0051] The microphone circuits 700, 800 enable the bandwidth and -3
dB cut-off frequencies to be flexibly adjusted while maintaining
feedback stability and fast settling times. Particularly, in both
of the microphone circuits 700, 800, the resistance of the input
resistor 420 can be adjusted to modify the -3 dB cutoff frequencies
and bandwidth of the analog integrator circuit 340. For the
microphone circuit 700, a proportionality constant of the
proportional path 740 and an integrator gain bandwidth of the
integration path 745 can be adjusted to modify the -3 dB cutoff
frequencies and bandwidth of digital offset correction circuit 710.
Similarly, for the microphone circuit 800, the resistance of the
resistor 815, the capacitance of the capacitor 820, and/or the
capacitance of the capacitor 835 can be adjusted to modify the -3
dB cutoff frequencies and bandwidth of the analog offset correction
circuit 810. As discussed above, the response characteristics of
the analog integrator circuit 340 and the offset correction circuit
320, 710, 810 together define the overall response of the
microphone circuits 700, 800. Additionally, we note that the low
frequency -3 dB point is adjustable using a feedback loop that
already performs DC offset correction and that greatly improves
transient setting speeds. Other methods for adjust -3 dB cutoff
frequencies and bandwidth generally introduce noise and require
separate feedback loops for multiple functionalities.
[0052] FIG. 11 shows an example of the low frequency -3 dB point
being adjusted by an order of magnitude. Due to the linearized
current-voltages characteristics of the diodes 120, 130 in
combination with the diode linearization circuit 400, the -3 dB
cutoff frequencies and bandwidth of the circuit 700 or the circuit
800 can be adjusted without affecting stability. The adjustability
of the low frequency -3 dB point has advantages such being able to
filter out wind noise. Although not illustrated, it is noted that
the microphone circuits 700, 800 also enable adjustability of the
high frequency -3 dB point.
[0053] While the disclosure has been illustrated and described in
detail in the drawings and foregoing description, the same should
be considered as illustrative and not restrictive in character. It
is understood that only the preferred embodiments have been
presented and that all changes, modifications and further
applications that come within the spirit of the disclosure are
desired to be protected.
* * * * *