U.S. patent application number 15/586023 was filed with the patent office on 2018-11-08 for input impedance biasing.
This patent application is currently assigned to Cirrus Logic International Semiconductor Ltd.. The applicant listed for this patent is Cirrus Logic International Semiconductor Ltd.. Invention is credited to Ravi Kummaraguntla, Vivek Saraf, Axel Thomsen, John C. Tucker.
Application Number | 20180324525 15/586023 |
Document ID | / |
Family ID | 62218330 |
Filed Date | 2018-11-08 |
United States Patent
Application |
20180324525 |
Kind Code |
A1 |
Saraf; Vivek ; et
al. |
November 8, 2018 |
INPUT IMPEDANCE BIASING
Abstract
Input impedance biasing may be improved with an
ultra-high-input-impedance biasing circuit having low temperature
variation. The impedance biasing circuit may include a first
transistor coupled to a first power supply and a second transistor
coupled to a second power supply. A gate of the first transistor
may be coupled to a gate of the second transistor at an
intermediate bias node. The first transistor and the second
transistor may provide a selected DC impedance at the intermediate
bias node. The impedance may be used to provide low-pass and or
high-pass filtering of audio signals and/or noise.
Inventors: |
Saraf; Vivek; (Austin,
TX) ; Thomsen; Axel; (Austin, TX) ;
Kummaraguntla; Ravi; (Austin, TX) ; Tucker; John
C.; (Austin, TX) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Cirrus Logic International Semiconductor Ltd. |
Edinburgh |
|
GB |
|
|
Assignee: |
Cirrus Logic International
Semiconductor Ltd.
Edinburgh
GB
|
Family ID: |
62218330 |
Appl. No.: |
15/586023 |
Filed: |
May 3, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H04R 3/00 20130101; H03H
7/40 20130101; H03F 3/185 20130101; H03F 2200/18 20130101; H03F
2200/03 20130101; H04R 3/04 20130101; H04R 2201/003 20130101 |
International
Class: |
H04R 3/04 20060101
H04R003/04; H03H 7/40 20060101 H03H007/40 |
Claims
1. An apparatus, comprising: a bias circuit configured to provide
an impedance, the bias circuit comprising: a first transistor
coupled to a first power supply; a second transistor coupled to a
second power supply, wherein a gate of the first transistor is
coupled to a gate of the second transistor at an intermediate bias
node, wherein the first transistor and the second transistor
provide a selected DC impedance at the intermediate bias node, and
wherein the provided DC impedance is a controlled DC impedance; and
a feedback loop configured to control the provided DC impedance
based, at least in part, on a voltage representative of the voltage
at the intermediate bias node.
2. The apparatus of claim 1, wherein a drain and a source of the
first transistor are coupled together and to the first power
supply, and wherein a drain and a source of the second transistor
are coupled together and to the second power supply.
3. The apparatus of claim 1, wherein the selected DC impedance is
provided by a junction between the gate of the first transistor and
the gate of the second transistor and a source/drain of the first
transistor and a source/drain of the second transistor,
respectively.
4. The apparatus of claim 1, wherein the provided DC impedance is a
controlled DC impedance.
5. (canceled)
6. The apparatus of claim 1, wherein the feedback loop is
configured to detect a voltage representative of the voltage at the
intermediate bias node and control the first power supply to set
the intermediate bias node to an input voltage based on the
detected voltage.
7. The apparatus of claim 1, wherein the feedback loop comprises: a
replica bias circuit comprising: a third transistor coupled to the
first power supply; and a fourth transistor coupled to the second
power supply, wherein a gate of the third transistor is coupled to
a gate of the fourth transistor at a replica bias node; and an
operational amplifier coupled to the replica bias node and
configured to control the first power supply to obtain an input
voltage at the replica bias node.
8. The apparatus of claim 7, wherein the operational amplifier is
further configured to control the second power supply to obtain the
input voltage at the replica bias node.
9. The apparatus of claim 1, wherein the selected DC impedance has
an impedance value that is combined with a capacitance of an
attached device to create a high-pass filter with a corner at a
selected frequency for filtering the input voltage.
10. The apparatus of claim 1, further comprising a feedback loop
configured to control an intermediate bias node voltage at the
intermediate bias node, wherein the feedback loop is configured to
detect a voltage representative of the voltage at the intermediate
bias node and control the first power supply to set the
intermediate bias node to an input voltage based on the detected
voltage.
11. The apparatus of claim 1, wherein an input voltage is received
from a MEMS microphone, and wherein the selected DC impedance has
an impedance value that when combined with a capacitance of an
attached device creates a high-pass filter with a corner at an
audible frequency for filtering an input audio signal from the MEMS
microphone.
12. A method, comprising: providing an impedance at an intermediate
bias node, wherein the impedance is provided with a bias circuit
that comprises: a first transistor coupled to a first power supply;
and a second transistor coupled to a second power supply, wherein a
gate of the first transistor is coupled to a gate of the second
transistor at the intermediate bias node, and wherein the first
transistor and the second transistor provide a selected DC
impedance at the intermediate bias node between the first power
supply and the second power supply; low-pass filtering, with the
impedance provided at the intermediate bias node, noise from a
terminal at which a direct current (DC) signal is applied; and
high-pass filtering, with the impedance provided at the
intermediate bias node, a signal from a terminal at which an
alternating current (AC) signal is applied.
13. The method of claim 12, further comprising controlling the
provided DC impedance with a feedback loop.
14. The method of claim 13, wherein the step of controlling the
provided DC impedance comprises: replicating the process of
providing the selected DC impedance with a third transistor and a
fourth transistor coupled to a replica bias node; and controlling
the first power supply to control the selected DC impedance based,
at least in part, on a voltage at the replica bias node.
15. The method of claim 12, further comprising receiving an input
audio signal from a MEMS microphone, wherein the step of providing
the impedance creates a high-pass filter with a corner at an
audible frequency for filtering the input audio signal from the
MEMS microphone.
16. An apparatus, comprising: an audio controller configured to
perform steps comprising: providing an impedance at an intermediate
bias node, wherein the impedance is provided with a bias circuit
that comprises: a first transistor coupled to a first power supply;
and a second transistor coupled to a second power supply, wherein a
gate of the first transistor is coupled to a gate of the second
transistor at the intermediate bias node, and wherein the first
transistor and the second transistor provide a selected DC
impedance at the intermediate bias node between the first power
supply and the second power supply; low-pass filtering, with the
impedance provided at the intermediate bias node, noise from a
terminal at which a direct current (DC) signal is applied; and
high-pass filtering, with the impedance provided at the
intermediate bias node, a signal from a terminal at which an
alternating current (AC) signal is applied.
17. The apparatus of claim 16, wherein the controller is further
configured to perform the step of controlling the provided DC
impedance with a feedback loop.
18. The apparatus of claim 17, wherein the step of controlling the
provided DC impedance comprises: replicating the process of
providing the selected DC impedance with a third transistor and a
fourth transistor coupled to a replica bias node; and controlling
the first power supply to control the selected DC impedance based,
at least in part, on a voltage at the replica bias node.
19. The apparatus of claim 16, wherein the controller is further
configured to perform the step of receiving an input audio signal
from a MEMS microphone, wherein the step of providing the impedance
creates a high-pass filter with a corner at an audible frequency
for filtering the input audio signal from the MEMS microphone.
Description
FIELD OF THE DISCLOSURE
[0001] The instant disclosure relates to input impedance biasing.
More specifically, portions of this disclosure relate to high input
impedance biasing with low temperate variation.
BACKGROUND
[0002] The input and/or output impedances of integrated circuits
typically need to be regulated to achieve a desired performance
from the integrated circuits without signal degradation. For
example, FIG. 1 is a schematic block diagram illustrating a
conventional input impedance biasing circuit according to the prior
art. In FIG. 1, a capacitor 102 may represent a portion of a
microphone and be coupled to circuitry of an audio controller. A
high impedance may be desired at input node 106 of additional
circuitry 104. As illustrated in FIG. 1, a conventional system to
provide high impedance at input node 106 includes an impedance
string 110 that consists of a complementary string of polysilicon
diodes connected in anti-series. Depending on the impedance
provided by impedance string 110, a start-up switch 112 may be
included to initialize the input node 106 to a desired voltage,
such as VCM.sub.in.
[0003] Numerous drawbacks are associated with conventional input
impedance biasing circuits, such as impedance string 110. For
example, the impedance provided at input node 106 by impedance
string 110 may vary significantly across operating temperatures.
The variation may make it difficult for audio controller 100 to
meet different specifications, such as linearity and signal to
noise ratio (SNR).
[0004] Shortcomings mentioned here are only representative and are
included simply to highlight that a need exists for improved
electrical components, particularly for improved input impedance
biasing circuits. Embodiments described herein address certain
shortcomings but not necessarily each and every one described here
or known in the art.
SUMMARY
[0005] Input impedance biasing may be improved through use of an
impedance source having low temperature variation. A high impedance
may be created with at least two transistors coupled together. The
gate current of each of the transistors may be controlled to use
the transistors as resistors. Specifically, the leakage current of
the transistors may be controlled to obtain a gate current that
allows the transistors to be used as resistors. In one example, one
terminal of a transistor resistor may be the gate of the transistor
and the other terminal of the transistor resistor may be the
coupled-together drain and source terminals of the transistors. The
provided impedance may be configured by coupling devices in series
or parallel to obtain different resistance values. The impedance
may also be configured by adjusting the design parameters of the
transistor in the structure, such as their lengths, widths, or
electrochemical properties to obtain different resistance values.
In addition, the voltages applied to the terminals of the
transistors may also be modified to obtain different resistance
values.
[0006] The gate current variation exhibited by transistors across a
particular temperature range may be designed to be low. For
example, the gate current variation exhibited by transistors
between a "hot" temperature of 125.degree. C. and a "cold"
temperature of -40.degree. C. may be less than or equal to 50
percent. Therefore, the variation of the resistance provided by
transistor resistors that rely on gate current to create the
resistance may also be less than or equal to 50 percent across a
"hot" temperature of 125.degree. C. and a "cold" temperature of
-40.degree. C.
[0007] According to one embodiment, an apparatus may include a bias
circuit configured to provide an impedance. The bias circuit may
include a first transistor coupled to a first power supply and a
second transistor coupled to a second power supply. A gate of the
first transistor may be coupled to a gate of the second transistor
at an intermediate bias node. The first transistor and the second
transistor may provide a selected DC impedance at the intermediate
bias node.
[0008] In another embodiment, a method includes: providing an
impedance at an intermediate bias node; low-pass filtering, with
the impedance provided at the intermediate bias node, noise from a
terminal at which a direct current (DC) signal is applied; and
high-pass filtering, with the impedance provided at the
intermediate bias node, a signal from a terminal at which an
alternating current (AC) signal is applied. The impedance may be
provided with a bias circuit that includes a first transistor
coupled to a first power supply and a second transistor coupled to
a second power supply. A gate of the first transistor may be
coupled to a gate of the second transistor at an intermediate bias
node. The first transistor and the second transistor may provide a
selected DC impedance at the intermediate bias node. A controller
may be configured in software or hardware to perform this method or
similar methods.
[0009] In yet another embodiment, an apparatus may include an audio
controller configured to perform steps that include: providing an
impedance at an intermediate bias node; low-pass filtering, with
the impedance provided at the intermediate bias node, noise from a
terminal at which a direct current (DC) signal is applied; and
high-pass filtering, with the impedance provided at the
intermediate bias node, a signal from a terminal at which an
alternating current (AC) signal is applied. The impedance may be
provided with a bias circuit that includes a first transistor
coupled to a first power supply and a second transistor coupled to
a second power supply. A gate of the first transistor may be
coupled to a gate of the second transistor at an intermediate bias
node. The first transistor and the second transistor may provide a
selected DC impedance at the intermediate bias node. The controller
may be configured in software or hardware to perform this method or
similar methods.
[0010] The foregoing has outlined rather broadly certain features
and technical advantages of embodiments of the present invention in
order that the detailed description that follows may be better
understood. Additional features and advantages will be described
hereinafter that form the subject of the claims of the invention.
It should be appreciated by those having ordinary skill in the art
that the conception and specific embodiment disclosed may be
readily utilized as a basis for modifying or designing other
structures for carrying out the same or similar purposes. It should
also be realized by those having ordinary skill in the art that
such equivalent constructions do not depart from the spirit and
scope of the invention as set forth in the appended claims.
Additional features will be better understood from the following
description when considered in connection with the accompanying
figures. It is to be expressly understood, however, that each of
the figures is provided for the purpose of illustration and
description only and is not intended to limit the present
invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] For a more complete understanding of the disclosed systems
and methods, reference is now made to the following descriptions
taken in conjunction with the accompanying drawings. In the
appended figures, similar components or features may have the same
reference label. Further, various components of the same type may
be distinguished by following the reference label with a second
label that distinguishes among the similar components. If just the
first reference label is used in the specification, the description
is applicable to any one of the similar components having the same
first reference label irrespective of the second reference
label.
[0012] FIG. 1 is a schematic block diagram illustrating a
conventional input impedance biasing circuit according to the prior
art.
[0013] FIG. 2 is a schematic block diagram illustrating an example
high impedance biasing circuit according to some embodiments of the
disclosure.
[0014] FIG. 3 is a schematic block diagram illustrating an example
high impedance biasing circuit with voltage and/or impedance
control according to some embodiments of the disclosure.
[0015] FIG. 4 is a plot illustrating the relationship between
resistance and voltage in an example high impedance biasing circuit
according to some embodiments of the disclosure.
[0016] FIG. 5 is a schematic block diagram illustrating another
example high impedance biasing circuit with voltage and/or
impedance control according to some embodiments of the
disclosure.
[0017] FIG. 6 is a schematic block diagram illustrating an example
high impedance biasing circuit with voltage control and impedance
control according to some embodiments of the disclosure.
[0018] FIG. 7 is a schematic block diagram illustrating another
example high impedance biasing circuit with voltage control and
impedance control according to some embodiments of the
disclosure.
[0019] FIG. 8 is a flow chart illustrating an example method for
providing a high impedance according to one embodiment of the
disclosure.
DETAILED DESCRIPTION
[0020] FIG. 2 is a schematic block diagram illustrating an example
high impedance biasing circuit 200 according to some embodiments of
the disclosure. The high impedance biasing circuit 200 may be
implemented as (or within) a controller integrated circuit (IC),
such as with an audio coder/decoder (CODEC) within a personal
device. As illustrated in FIG. 2, an input signal 260 may be
received at input node 250. The input signal 260 may be a voltage
signal received from a microphone either directly or after some
processing by other components. Additional circuitry 270 may also
be included within an audio CODEC to process the input signal
present at input node 250. Additional circuitry 270 may include
circuitry that processes audio signals.
[0021] Bias circuit 200 may be configured to provide an impedance
at input node 250. Bias circuit 200 may include a first transistor
210 coupled to a first power supply 212. Bias circuit 200 may also
include a second transistor 220 coupled to a second power supply
222. The first transistor 210 may be a p-channel transistor ("PMOS
transistor") or an n-channel transistor ("NMOS transistor").
Similarly, second transistor 220 may be a PMOS transistor or a NMOS
transistor. In one embodiment, first transistor 210 is a PMOS
transistor and second transistor 220 is a NMOS transistor. In some
embodiments, each of first power supply 212 and second power supply
222 may be a terminal through which energy may be transferred from
other components, such as a controller power supply or a
battery.
[0022] In the bias circuit 200, a gate 214 of the first transistor
210 may be coupled to a gate 224 of the second transistor 220 at an
intermediate bias node, such as input node 250. The first
transistor 210 and the second transistor 220 may provide a selected
direct-current (DC) impedance at the intermediate bias node 250.
The selected DC impedance provided by bias circuit 200 may range
from as low as mega-ohms to as high as tera-ohms or exa-ohms.
[0023] The terminals of a drain 216 and a source 218 of the first
transistor 210 may be coupled together and to the first power
supply 212. Similarly, a drain 226 and a source 228 of the second
transistor 220 may be coupled together and to the second power
supply 222. A selected DC impedance may be provided at a junction
between the gate 214 of the first transistor 210 and the gate 224
of the second transistor 220 and a source/drain 216/218 of the
first transistor 210 and a source/drain 226/228 of the second
transistor 220, respectively. The selected DC impedance may be
provided by the parallel impedance combination of the impedance
provided by the first transistor 210 between the input node 250 and
the source/drain terminal 216/218 of the first transistor 210 and
the impedance provided by the second transistor 220 between the
input node 250 and the source/drain terminal 226/228 of the second
transistor 220.
[0024] The voltage and/or impedance at input node 250 may be
controlled. FIG. 3 is a schematic block diagram illustrating an
example high impedance biasing circuit with voltage and/or
impedance control according to some embodiments of the disclosure.
In FIG. 3, a capacitor 302 may represent a microphone and be
coupled to circuitry 304 of an audio controller 300 through
impedance biasing circuit 200. A start-up switch 312 may be
included to initialize input node 306 to a desired voltage, such as
input voltage VCM.sub.IN 394. A high impedance may be desired at
input node 306 and provided with high impedance biasing circuit
200.
[0025] Voltage control of biasing circuit 200 illustrated in FIG. 3
may be provided through the use of a feedback loop. A feedback loop
may be included in audio controller 300 and may be configured to
control an intermediate bias node voltage of biasing circuit 200,
such as the voltage at input node 306. The feedback loop may
include a replica bias circuit 380. Replica bias circuit 380 may
include a third transistor 382 coupled to first power supply 212 of
biasing circuit 200. Replica bias circuit 380 may also include a
fourth transistor 384 coupled to second power supply 222 of biasing
circuit 200. The parameters of the third transistor 382 and fourth
transistor 384, such as their widths, lengths, and electrochemical
properties, may be substantially similar to the first transistor
210 and second transistor 220, respectively. To further replicate
the behavior of the first transistor 210 and the second transistor
220, the gate 386 of the third transistor 382 may be coupled to a
gate 388 of the fourth transistor 384 at a replica bias node 390.
An operational amplifier 392 may be coupled to replica bias node
390 and may be configured to control the first power supply 212 to
obtain a desired input voltage 394 at the replica bias node 390. In
some embodiments, the operational amplifier 392 or additional
operational amplifiers (not shown) may be configured to control the
first power supply 212 and the second power supply 222. Control of
both power supplies may be used to obtain control over voltage and
impedance at intermediate bias nodes, as described in further
detail below.
[0026] Such control of replica bias circuit 380 by operational
amplifier 392 to obtain an input voltage 394 at replica bias node
390 may also control biasing circuit 200 to obtain a voltage at
input node 306 that is substantially similar to input voltage 394.
For example, because the replica bias circuit 380 is designed to
substantially replicate biasing circuit 200, and because the power
supplies 212 and 222 couple to both the biasing circuit 200 and
replica bias circuit 380 in the same way, a voltage present at
replica bias node 390 may be substantially equal to a voltage
present on input node 306. Therefore, by controlling replica bias
circuit 380 to obtain an input voltage 394 at replica bias node
390, a voltage substantially equal to input voltage 394 may also be
set at input node 306.
[0027] In operation, in order to control the voltage at input node
306 through control of biasing circuit 200, operational amplifier
392 may control the voltage at replica bias node 390 through
control of replica bias circuit 380. For example, input voltage 394
may be applied to an input terminal of operational amplifier 392 to
set the voltage at replica bias node 390 to input voltage 394. That
is, the operational amplifier 392 may control the first power
supply 212 to obtain input voltage 394 at replica bias node 390.
Because replica bias circuit 380 is a replica of bias circuit 200,
controlling, by the operation amplifier 392, the first power supply
212 to obtain input voltage 394 at replica bias node 390 also
controls the first power supply 212 coupled to bias circuit 200 to
obtain input voltage 394 at input node 306.
[0028] The feedback loop may be configured to track variation in
the audio controller 300 to modify first power supply 212 as needed
to maintain the voltage at input node 306 at a value approximately
equal to input voltage 394. For example, the feedback loop may be
configured to detect a voltage representative of the voltage at the
intermediate bias node of bias circuit 200, such as input node 306.
Because the operational amplifier 392 is coupled to replica bias
node 390, the detection of the voltage representative of the
voltage at input node 306 may be performed by the operational
amplifier 392 through its detection of the voltage at replica bias
node 390 because the voltage at replica bias node 390 may be a
voltage representative of the voltage at input node 306. Upon
detecting the voltage at replica bias node 390, the feedback loop,
through operational amplifier 394, may control the first power
supply 212 to set an intermediate bias node 306 of bias circuit 200
to the input voltage 394 based on the detected voltage at replica
bias node 390.
[0029] FIG. 4 is a plot illustrating the relationship between
resistance and voltage in an example high impedance biasing circuit
according to some embodiments of the disclosure. FIG. 4 illustrates
that the DC impedance provided by biasing circuit 200 may also be
controlled. For example, in FIG. 4 the vertical axis represents
increasing resistance and the horizontal axis represents increasing
voltage. Resistance plot 410 may correspond to a resistance
provided by a PMOS transistor, such as first transistor 210
illustrated in FIGS. 2-3. Resistance plot 420 may correspond to a
resistance provided by a NMOS transistor, such as second transistor
220 illustrated in FIGS. 2-3. Input impedance plot 430 may
correspond to the parallel combination of resistances 410 and 420.
Therefore, in the embodiments illustrated in FIGS. 2-3, the
impedance at input nodes 250 and 306 may be represented in FIG. 4
as input impedance 430. The voltage corresponding to the horizontal
axis may correspond to VCMin input voltage 394.
[0030] The impedance presented at input node 306 may be controlled
based on the value of input voltage 394 according to the plots of
FIG. 4. The maximum impedance is achieved when input voltage 394
has a value approximately equal to 0.25 V. In other embodiments,
the voltage where the maximum resistance is achieved may be a
different voltage and may depend on the design parameters of the
transistors in bias circuit 200. As illustrated in the plot 430 of
FIG. 4, as input voltage 394 decreases or increases from the
voltage where the maximum resistance is achieved, the resistance
present at input node 306 decreases. By changing the value of input
voltage 394, the value of impedance present at input node 306 may
also change accordingly such that the impedance present at input
node 306 may be controlled by input voltage 394.
[0031] Returning to FIG. 3, impedance control of biasing circuit
200 illustrated in FIG. 3 may be provided through the use of a
feedback loop in some embodiments. The feedback loop described with
respect to FIG. 3 for voltage control may be configured to also or
alternatively control the DC impedance provided at input node 306.
In particular, control of replica bias circuit 380 by operational
amplifier 392 to obtain an input voltage 394 at replica bias node
390 may also control biasing circuit 200 to obtain a desired DC
impedance at input node 306. In order to control the DC impedance
provided at input node 306 through control of biasing circuit 200,
operational amplifier 392 may control the voltage at replica bias
node 390 through control of replica bias circuit 380. For example,
as illustrated in FIG. 3, input voltage 394 may be applied to an
input terminal of operational amplifier 392 to set the voltage at
replica bias node 390 to input voltage 394. That is, the
operational amplifier 392 may control the first power supply 212 to
obtain input voltage 394 at replica bias node 390. Because replica
bias circuit 380 is a replica of bias circuit 200, controlling the
first power supply 212 to obtain input voltage 394 at replica bias
node 390 also controls the first power supply 212 coupled to bias
circuit 200 to obtain input voltage 394 at input node 306. The DC
impedance present at input node 306 may be a function of the
voltage present at input node 306. Thus, the first power supply 212
may be controlled to obtain input voltage 394 at input node 306 and
controlled to obtain a selected DC impedance at input node 306.
[0032] The feedback loop may be configured to track variation in
the audio controller 300 to modify first power supply 212 as needed
to maintain the voltage at input node 306 at a value substantially
equal to input voltage 394 so as to control the impedance present
at input node 306. For example, the feedback loop may be configured
to detect a voltage at replica bias node 390 that may be
representative of the voltage at the intermediate bias node of bias
circuit 200, such as input node 306. Upon detecting the voltage at
replica bias node 390 that is representative of the voltage at
input node 306, the feedback loop, through operational amplifier
394, may control the first power supply 212 to set an intermediate
bias node of bias circuit 200, such as input node 306, to the input
voltage 394 based on the detected voltage at replica bias node 390,
thereby also setting the impedance present at input node 306 to a
selected DC impedance value.
[0033] The impedance provided at intermediate bias node 306 may be
used to high-pass filter a signal from a terminal at which an
alternating current (AC) signal is applied. For example, as
illustrated in FIG. 3, capacitor 302 may represent an acoustic
transducer of a MEMS microphone. The MEMS microphone 302 may
generate an AC signal representative of audio received from the
environment around the microphone 302. When the impedance provided
by biasing circuit 200 is combined with capacitor 302, a high-pass
filter may be created and the high-pass filter may have a corner
frequency at a selected frequency for filtering the input audio
voltage signal. In other words, the selected DC impedance provided
by bias circuit 200 may have an impedance value that may be
combined with a capacitance of an attached device, such as
capacitor 302, to create a high-pass filter with a corner at a
selected frequency, such as an audible frequency, for filtering the
input audio voltage signal.
[0034] The impedance provided at intermediate bias node 306 may
alternatively be used to low-pass filter noise from a terminal at
which a DC signal is applied. For example, the DC signal may
include the signals applied at first power supply 212 and/or second
power supply 222. When the impedance provided by biasing circuit
200 is combined with capacitance 302, a low-pass filter may be
created and the low-pass filter may filter noise present at first
power supply 212 and/or second power supply 222.
[0035] FIG. 5 is a schematic block diagram illustrating another
example high impedance biasing circuit with voltage and/or
impedance control according to some embodiments of the disclosure.
In particular, FIG. 5 illustrates an embodiment in which a replica
bias circuit is not used to provide voltage and/or impedance
control of biasing circuit 200. Voltage and/or impedance control
may be provided with control block 520. Control block includes a
first variable resistor 522 and second variable resistor 524.
Variable resistors 522 and 524 may be controlled to set node 530 to
a specific voltage. For example, variable resistors 522 and 524 may
create a voltage divider that can be adjusted to set node 530 to a
voltage between power supply 510 and ground. By setting node 530 to
a specific voltage, control block 520 may control the first power
supply of bias circuit 200 to set an intermediate bias node 550 of
bias circuit 200 to a specific voltage between the voltage at node
530 and ground. Therefore, the voltage at intermediate bias node
550 may be controlled and set based on the voltage at node 530,
which in turn may be controlled and set based on the values of
variable resistors 522 and 524 in control block 520. Similarly,
because the DC impedance present at node 550 may be a function of
the voltage present at node 550, as is illustrated in FIG. 4,
controlling the first power supply 530 coupled to bias circuit 200
to set intermediate bias node 550 of bias circuit 200 to a specific
voltage may also control bias circuit 200 to set a selected DC
impedance at node 550. Therefore, the impedance at intermediate
bias node 550 may also be controlled and set based on the voltage
at node 530, which in turn may be controlled and set based on the
values of variable resistors 522 and 524 in control block 520.
[0036] FIG. 6 is a schematic block diagram illustrating an example
high impedance biasing circuit with voltage control and impedance
control according to some embodiments of the disclosure. The
biasing circuit illustrated in FIG. 6 may be configured similar to,
and may operate similar to, the biasing circuit illustrated in FIG.
3. In FIG. 6, the second power supply 222 of biasing circuit 200 is
coupled to the source and drain of fourth transistor 684 of replica
bias circuit 680. In addition, another biasing branch 670 that
includes a resistor 672 and a variable current source 674 is
coupled to first power supply 212 and to second power supply 222 to
provide additional voltage and impedance control of biasing circuit
200. Specifically, the resistor 672 may have one of its terminals
coupled to first power supply 212 and may have its other terminal
coupled to second power supply 222. With the addition of biasing
branch 670, the voltages on both the first power supply 212 and the
second power supply 222 and the impedance between the first power
supply 212 and the second power supply 222 may be controlled to
control the voltage and/or impedance present at intermediate bias
node 650. The operational amplifier 692 may be configured to
control the second power supply 222 to obtain an input voltage 694
at the replica bias node 690 as well as to obtain the input voltage
694 and a specific resistance at intermediate bias node 650.
[0037] FIG. 7 is a schematic block diagram illustrating another
example high impedance biasing circuit with voltage control and
impedance control according to some embodiments of the disclosure.
Voltage and/or impedance control may be provided with low-pass
filter (LPF) 770. LPF 770 may receive as a first input an output
voltage signal 772 from an additional circuitry block 704. LPF 770
may also receive as a second input another voltage signal, such as
VCMout voltage signal 774. LPF 770 may process output signal 772
and voltage signal 774 to control first power supply 212 of bias
circuit 200. LPF 770 may be configured to set first power supply
212 to a specific voltage based on output signal 772 and voltage
signal 774. By setting first power supply 212 to a specific
voltage, LPF 770 may control the first power supply 212 of bias
circuit 200 to set an intermediate bias node 750 of bias circuit
200 to a specific voltage between the voltage at first power supply
212 and ground. Therefore, the voltage at intermediate bias node
750 may be controlled and set based on the voltage at first power
supply 212, which in turn may be controlled and set based on the
processing performed by LPF 770 on output signal 772 and voltage
signal 774. Similarly, because the DC impedance present at node 750
may be a function of the voltage present at node 750, as is
illustrated in FIG. 4, controlling the first power supply 212
coupled to bias circuit 200 to set intermediate bias node 750 of
bias circuit 200 to a specific voltage may also control bias
circuit 200 to set a selected DC impedance at node 750. Therefore,
the impedance at intermediate bias node 750 may also be controlled
and set based on the voltage at first power supply 212, which in
turn may be controlled and set based on the processing performed by
LPF 770 on output signal 772 and voltage signal 774.
[0038] Another biasing branch 760 may also be included along with
LPF 770 to provide additional voltage and impedance control of bias
circuit 200. Biasing branch 760 may include a resistor 762 and a
variable current source 764. The resistor 762 may have one terminal
coupled to first power supply 212 and may have another terminal
coupled to second power supply 222. The current source 764 may be
coupled to second power supply 222 and to ground. In this
configuration, the second power supply 222 is not coupled to
ground, and is instead coupled to terminal 763 of biasing branch
760. With the addition of biasing branch 760, the voltages on both
the first power supply 212 and the second power supply 222 and the
impedance between the first power supply 212 and the second power
supply 222 may be controlled to control the voltage and/or
impedance present at intermediate bias node 750.
[0039] FIG. 8 is a flow chart illustrating an example method for
providing an ultra-high input impedance with an
ultra-high-input-impedance biasing circuit according to one
embodiment of the disclosure. Method 800 may be implemented with
the systems described with respect to FIGS. 2-7 or other systems.
Method 800 begins, at block 802, with providing an impedance at an
intermediate bias node. In some embodiments, the impedance may be
provided with a bias circuit. The bias circuit may include a first
transistor coupled to a first power supply and a second transistor
coupled to a second power supply. A gate of the first transistor
may be coupled to a gate of the second transistor at the
intermediate bias node. The first transistor and the second
transistor may provide a selected DC impedance at the intermediate
bias node between the first power supply and the second power
supply.
[0040] At block 804, method 800 includes low-pass filtering, with
the impedance provided at the intermediate bias node, noise from a
terminal at which a direct current (DC) signal is applied. At block
806, method 800 includes high-pass filtering, with the impedance
provided at the intermediate bias node, a signal from a terminal at
which an alternating current (AC) signal is applied.
[0041] In some embodiments, the provided DC impedance may be
controlled with a feedback loop, for example as illustrated in
FIGS. 3-7. According to some embodiments, the step of controlling
the provided DC impedance may include replicating the process of
providing the selected DC impedance with a third transistor and a
fourth transistor coupled to a replica bias node, and controlling
the first power supply to control the selected DC impedance based,
at least in part, on a voltage at the replica bias node, for
example as illustrated in FIGS. 3 and 6. In other embodiments, an
input audio voltage signal may be received from a MEMS microphone,
and the step of providing the impedance may create a high-pass
filter with a corner at an audible frequency for filtering the
input audio signal from the MEMS microphone.
[0042] The schematic flow chart diagram of FIG. 8 is generally set
forth as a logical flow chart diagram. As such, the depicted order
and labeled steps are indicative of aspects of the disclosed
method. Other steps and methods may be conceived that are
equivalent in function, logic, or effect to one or more steps, or
portions thereof, of the illustrated method. Additionally, the
format and symbols employed are provided to explain the logical
steps of the method and are understood not to limit the scope of
the method. Although various arrow types and line types may be
employed in the flow chart diagrams, they are understood not to
limit the scope of the corresponding methods. Indeed, some arrows
or other connectors may be used to indicate only the logical flow
of the methods. For instance, an arrow may indicate a waiting or
monitoring period of unspecified duration between enumerated steps
of the depicted methods. Additionally, the order in which a
particular method occurs may or may not strictly adhere to the
order of the corresponding steps shown.
[0043] If implemented in firmware and/or software, functions
described above may be stored as one or more instructions or code
on a computer-readable medium. Examples include non-transitory
computer-readable media encoded with a data structure and
computer-readable media encoded with a computer program.
Computer-readable media includes physical computer storage media. A
storage medium may be any available medium that can be accessed by
a computer. By way of example, and not limitation, such
computer-readable media can comprise random access memory (RAM),
read-only memory (ROM), electrically-erasable programmable
read-only memory (EEPROM), compact disc read-only memory (CD-ROM)
or other optical disk storage, magnetic disk storage or other
magnetic storage devices, or any other medium that can be used to
store desired program code in the form of instructions or data
structures and that can be accessed by a computer. Disk and disc
includes compact discs (CD), laser discs, optical discs, digital
versatile discs (DVD), floppy disks and Blu-ray discs. Generally,
disks reproduce data magnetically, and discs reproduce data
optically. Combinations of the above should also be included within
the scope of computer-readable media.
[0044] In addition to storage on computer readable medium,
instructions and/or data may be provided as signals on transmission
media included in a communication apparatus. For example, a
communication apparatus may include a transceiver having signals
indicative of instructions and data. The instructions and data are
configured to cause one or more processors to implement the
functions outlined in the claims.
[0045] Although the present disclosure and certain representative
advantages have been described in detail, it should be understood
that various changes, substitutions and alterations can be made
herein without departing from the spirit and scope of the
disclosure as defined by the appended claims. Moreover, the scope
of the present application is not intended to be limited to the
particular embodiments of the process, machine, manufacture,
composition of matter, means, methods and steps described in the
specification. For example, although impedance-providing bias
circuits are described in embodiments above, aspects of the
disclosed invention may also be applied to other bias circuits. As
one of ordinary skill in the art will readily appreciate from the
present disclosure, processes, machines, manufacture, compositions
of matter, means, methods, or steps, presently existing or later to
be developed that perform substantially the same function or
achieve substantially the same result as the corresponding
embodiments described herein may be utilized. Accordingly, the
appended claims are intended to include within their scope such
processes, machines, manufacture, compositions of matter, means,
methods, or steps.
* * * * *