Note: Descriptions are shown in the official language in which they were submitted.
CA 02759921 2011-11-30
DIFFERENTIAL MICROPHONE CIRCUIT
Field
The present disclosure is generally directed at microphone circuits and more
specifically at a differential microphone circuits.
Background
Electret microphones have been used for almost half a century since their
introduction in 1962. The microphone itself has a very high output impedance
due to the
capacitance of the electret material. In order to overcome this problem, a
junction gate
field-effect transistor (JFET) or a complementary metal-oxide semiconductor
(CMOS)
buffer transistor is integrated within the microphone capsule to change the
output
impedance. The traditional way to capture the electrical output from these
microphones
has been to measure the voltage across the microphone, amplify the voltage and
then
digitize it inside a codec.
Summary
The current disclosure is generally directed at embodiments of a differential
microphone circuit configuration. In some of the embodiments, the differential
microphone circuit configuration may provide the advantage of a high power
supply
rejection ratio (PSRR) or high attenuation of bias noise. In current
microphone
technology, little attention has been paid to the internal workings of the
junction gate field
effect transistor (JFET) within the microphone capsule. The JFET may operate
as a
current source with high output impedance
In the current disclosure, the electrical output from the microphone may be
measured across the bias resistor supplying current to the JFET. Since the
JFET in the
normal bias point works as a current source, any voltage variations and noise
from the
supply voltage will also happen over the JFET. However, the bias resistor will
see an
almost completely constant current with the result of a very high PSRR and
noise
immunity, typically 17-28 dB being achieved. This is an improvement over
conventional
single ended microphone circuit, and can be accomplished with the same number
of or
fewer external components. Other advantages include, but are not limited to,
improved
performance, lower costs and less board space required. The differential
microphone
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circuit may be implemented in various ways but in each configuration similar
benefits are
achieved. Another advantage of some of the embodiments disclosed within
include that the
supporting circuitry to the microphone may be less costly and more noisy and
still meet
microphone specifications. Furthermore any external interference such as from
battery noise may
be reduced.
In the current disclosure, apparatus for reducing the level of disturbance on
microphone
lines when a headset is connected to a portable electronic device is
disclosed. By having a
portable electronic device which may be able to interact with different
headsets, i.e. with
different ground signal and microphone signal lines, a sensing circuit, such
as a Kelvin sensing
circuit is integrated within the portable electronic device interface to
reduce offset caused by
connection with ground.
In one aspect there is presented an apparatus for a portable electronic device
for receiving
a jack of a headset, the jack including a set of lines, the set of lines
including at least one audio
line, a ground signal line and a microphone signal line, the apparatus
comprising a set of
switches for receiving the ground signal line and the microphone signal line
wherein in use with
a first headset, one of the switches is connected to the ground signal line
and another of the
switches is connected to the microphone signal line, and in another use with a
second headset,
one of the switches is connected to the microphone signal line and another of
the switches is
connected to the ground signal line and a sensing circuit for reducing induced
noise from the
headset; wherein the sensing circuit is connected between the set of switches
and the microphone
signal line and ground signal line and the microphone signal line and the
ground signal line are
connected to a microphone.
Brief Description of the Drawings
Embodiments of the present invention will now be described, by way of example
only,
with reference to the attached Figures, wherein:
Figure 1 is a schematic diagram of a microphone circuit;
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Figure 2 is a schematic diagram of a microphone circuit in accordance with one
embodiment of the disclosure;
Figure 3 is a schematic diagram of another embodiment of a microphone circuit;
Figure 4 is a schematic diagram of another embodiment of a microphone circuit;
Figure 5 is a schematic diagram of another embodiment of a microphone circuit;
Figure 6 is a schematic diagram of another embodiment of a microphone circuit;
Figure 7 is a schematic diagram of another embodiment of a microphone circuit;
Figure 8 is a schematic diagram of another embodiment of a microphone circuit;
Figure 9 is a schematic diagram of another embodiment of a microphone circuit;
Figure 10 is a schematic diagram of another embodiment of a microphone
circuit;
Figure 11 is a schematic diagram of another embodiment of a microphone
circuit;
Figure 12 is a schematic diagram of another embodiment of a microphone
circuit;
Figure 13 is a schematic diagram of another embodiment of a microphone
circuit;
Figure 14 is a schematic diagram of a headset connected with a portable
electronic device
in a first mode;
Figure 15 is a schematic diagram of another headset connected with a portable
electronic
device in a second mode; and
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Figure 16 is a schematic diagram of another embodiment of an apparatus for
connecting a portable electronic device with a headset.
Detailed Disclosure
The present disclosure is generally directed at embodiments of a differential
microphone circuit configuration with a high power supply rejection ratio
(PSRR) and
high attenuation of bias noise. Different implementations of the circuitry are
contemplated
such as the microphone circuit being supplied by a negative or a positive bias
or the
positioning of the bias resistor to have a higher or lower potential than the
junction gate
field transistor (JFET) within the microphone.
In microphone technology, it is desirous to achieve high power supply
rejection
ratio (PSRR) and low noise, and this is typically accomplished with filtering
components
and a special low power supply. Also, various circuit configurations have been
proposed
in the art to increase the PSRR and noise immunity, typically with a penalty
of higher
current consumption, higher cost or with the requirement of non-grounded
connections.
Still, noise and PSRR are regular concerns for the audio electronics designer.
Turning to Figure 1, a schematic diagram of circuitry within a traditional
electret
microphone circuit is shown. The circuit 10 includes a bias resistor 12,
electret
microphone portion 14 (including a two-terminal electret capsule 16 and a JFET
18) and a
pair of microphone lines 20 seen as +MIC OUT line 20a and ¨MIC OUT line 20b.
Each
of the microphone lines 20 includes a capacitor 22 which can be used to block
out DC
signals.
In traditional operation of the microphone of Figure 1, voltage is supplied to
the
JFET 18 via the bias resistor 12 and then an output signal taken between the
microphone
lines 20, or the negative and positive terminals across the microphone. The
differential
voltage between the two microphone lines 20 may provide a voltage proportional
to the
acoustic pressure received at the microphone inlet or input.
Turning to Figure 2, a schematic diagram of circuitry for a differential
microphone
circuit in accordance with the disclosure is shown. In this embodiment, the
microphone
circuit provides the advantage of a higher PSRR and a high attenuation of bias
noise.
The microphone circuit 30 comprises a bias resistor 32 which is connected to a
voltage source 34 (providing a positive bias) and to an electret microphone
circuit 36. The
electret microphone circuit 36 is also connected to ground and includes a two-
terminal
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electret capsule 38 and a JFET 40. A pair of microphone lines 42, seen as a
+MIC OUT
line 42a and a ¨MIC OUT line 42b are connected across the bias resistor 32.
Each
microphone line 42 may include a capacitor 44. Selection of higher resistance
values for
the bias resistors may result in an increase of acoustic sensitivity, however,
the selection of
the resistance value for the bias resistor is such that the JFET should not go
out of
saturation during operation of the microphone circuit.
In another embodiment, a very high bias voltage and a bias resistor with a
large
resistance value may be used. In this example, a large output signal would be
sensed over
the microphone lines which may also provide an improved immunity to
electromagnetic
interference (EMI). In this embodiment, there may be no need for a pre-
amplifier circuit.
Operation of the microphone circuit 30 is similar to operation of the
traditional
microphone circuit of Figure 1, however the sensing is performed at a
different location
within the circuit 30. In this embodiment and the ones disclosed below, the
sensing of the
output signal is performed across the bias resistor 32.
Advantages of measuring the differential voltage or output signal, across the
bias
resistor include the benefit that the bias resistor 32 experiences an almost
constant current
which results in the microphone circuit 30 having a very high PSRR and
improved noise
immunity over other circuits. Another advantage is that the resistance value
of the bias
resistor 32 may be increased with respect to bias resistors in traditional
electret
microphone circuits. Another advantage is that by increasing the PSRR or
reducing the
noise or both within the microphone circuit, fewer components are required to
implement
the microphone of the current disclosure and therefore the size and cost of
the microphone
circuit 30 can be reduced with improved performance. Furthermore,
implementation of
the biasing or sensing circuitry over the bias resistor allows the supporting
circuitry of the
microphone to be cheaper and noisier while still meeting microphone
specifications. Also,
any interference from battery noise or any external interference will be
lowered.
Turning to Figure 3, yet another embodiment of a microphone circuit is shown.
The microphone circuit 50 includes a bias resistor 52 which is connected to a
voltage
source 54 (providing a negative bias) and to electret microphone circuit 56.
The electret
microphone circuit 56 is also connected to ground and includes a two-terminal
electret
capsule 58 and a JFET 60. A pair of microphone lines 62, seen as a +MIC OUT
line 62a
and a ¨MIC OUT line 62b are connected across the bias resistor 52. Each
microphone line
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62 may include a capacitor 64. The output signal is then sensed over the
microphone lines
62.
Turning to Figure 4, yet another embodiment of a microphone circuit in
accordance with the disclosure is shown. The microphone circuit 70 includes a
bias
resistor 72 which is connected to ground and to electret microphone circuit
76. The
electret microphone 76 is also connected to a voltage source 74 (providing a
negative bias)
and includes a two-terminal electret capsule 78 and a JFET 80. A pair of
microphone lines
82, seen as a +MIC OUT line 82a and a ¨MIC OUT line 82b are connected across
the bias
resistor 72. In the current embodiment, the +MIC OUT line 82a includes a
capacitor 84.
Turning to Figure 5, yet another embodiment of a microphone circuit is shown.
The microphone circuit 90 includes a bias resistor 92 which is connected to
ground and to
electret microphone circuit 96. The electret microphone 96 is also connected
to a voltage
source 94 (providing a positive bias) and includes a two-terminal electret
capsule 98 and a
JFET 100. A pair of microphone lines 102, seen as a +MIC OUT line 102a and a
¨MIC
OUT line 102b are connected across the bias resistor 92. In the current
embodiment, both
of the microphone lines 102 may include a capacitor 104.
Turning to Figure 6, yet another embodiment of a microphone circuit is shown.
The microphone circuit 110 includes a bias resistor 112 which is connected to
ground and
to electret microphone circuit 116. The electret microphone 116 is also
connected to a
voltage source 114 (providing a negative bias) and includes a two-terminal
electret capsule
118 and a JFET 120. A pair of microphone lines 122, seen as a +MIC OUT line
122a and
a ¨MIC OUT line 122b are connected across the bias resistor 112. In the
current
embodiment, both of the microphone lines 122 may include a capacitor 124.
Turning to Figure 7, yet another embodiment of a microphone circuit is shown.
The microphone circuit 130 includes a bias resistor 132 which is connected to
ground and
to electret microphone circuit 136. The electret microphone 136 is also
connected to a
voltage source 134 (providing a positive bias) and includes a two-terminal
electret capsule
138 and a JFET 140. A pair of microphone lines 142, seen as a +MIC OUT line
142a and
a ¨MIC OUT line 142b are connected across the bias resistor 132. In the
current
embodiment, a capacitor 144 is located on the +MIC OUT line 142a.
Turning to Figure 8, yet another embodiment of a microphone circuit is shown.
The microphone circuit 150 includes a bias resistor 152 which is connected to
a voltage
source 154 (providing a positive bias) and to electret microphone circuit 156.
The electret
CA 02759921 2011-11-30
microphone 156 is also connected to ground and includes a two-terminal
electret capsule
158 and a JFET 160. A pair of microphone lines 162, seen as a +MIC OUT line
162a and
a -MIC OUT line 162b are connected across the bias resistor 152. In the
current
embodiment, each microphone line 162 includes a capacitor 164 and the -MIC OUT
line
162b also includes a resistor, or resistive element 166 although the
capacitors 164 and
resistive elements 166 are not mandatory components.
Turning to Figure 9, yet another embodiment of a microphone circuit is shown.
The microphone circuit 170 includes a bias resistor 172 which is connected to
a voltage
source 174 (providing a negative bias) and to electret microphone circuit 176.
The electret
microphone 176 is also connected to ground and includes a two-terminal
electret capsule
178 and a JFET 180. A pair of microphone lines 182, seen as a +MIC OUT line
182a and
a ¨MIC OUT line 182b are connected across the bias resistor 172. In the
current
embodiment, each microphone line 182 includes a capacitor 184 and the -MIC OUT
line
182b also includes a resistor, or resistive element 186.
Turning to Figure 10, yet another embodiment of a microphone circuit is shown.
The microphone circuit 190 includes a bias resistor 192 which is connected to
ground and
to electret microphone circuit 196. The electret microphone 196 is also
connected to a
voltage source 194 (providing a negative bias) and includes a two-terminal
electret capsule
198 and a JFET 200. A pair of microphone lines 202, seen as a +MIC OUT line
202a and
a ¨MIC OUT line 202b are connected across the bias resistor 192. In the
current
embodiment, each microphone line 202 includes a capacitor 204 and the -MIC OUT
line
202b also includes a resistor, or resistive element 206.
Turning to Figure 11, yet another embodiment of a microphone circuit is shown.
The microphone circuit 210 includes a bias resistor 212 which is connected to
ground and
to electret microphone circuit 216. The electret microphone 216 is also
connected to a
voltage source 214 (providing a positive bias) and includes a two-terminal
electret capsule
218 and a JFET 220. A pair of microphone lines 222, seen as a +MIC OUT line
222a and
a ¨MIC OUT line 222b are connected across the bias resistor 212. In the
current
embodiment, both of the microphone lines 222 may include a capacitor 224 while
the ¨
MIC OUT line 222b also includes a resistive element, seen as resistor 226.
Turning to Figure 12, yet another embodiment of a microphone circuit is shown.
The microphone circuit 230 includes a bias resistor 232 which is connected to
ground and
to electret microphone circuit 236. The electret microphone 236 is also
connected to a
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voltage source 234 (providing a negative bias) and includes a two-terminal
electret capsule
238 and a JFET 240. A pair of microphone lines 242, seen as a +MIC OUT line
242a and
a ¨MIC OUT line 242b are connected across the bias resistor 232. In the
current
embodiment, both of the microphone lines 242 may include a capacitor 244 and
the -MIC
OUT line 242b includes a resistive element 246.
Turning to Figure 13, yet another embodiment of a microphone circuit is shown.
The microphone circuit 250 includes a bias resistor 252 which is connected to
ground and
to electret microphone circuit 256. The electret microphone 256 is also
connected to a
voltage source 254 (providing a positive bias) and includes a two-terminal
electret capsule
258 and a JFET 260. A pair of microphone lines 262, seen as a +MIC OUT line
262a and
a ¨MIC OUT line 262b are connected across the bias resistor 252. In the
current
embodiment, a capacitor 264 is located on the +MIC OUT 262a along with a
resistive
element 266.
Another benefit of the embodiments of Figures 2 to 13 is that when the JFET
within the preamplifier is biased in a particular setup, the JFET functions as
a current
source with a high output impedance. This allows for a bias resistor with a
higher resistive
value to be implemented within the microphone circuit, thereby increasing the
acoustic
sensitivity of the microphone. In the preferred embodiment, the resistive
value for the bias
resistor is selected so that after the voltage drop over the bias resistor
there is enough
voltage supplied to the JFET so that it does not go out of saturation.
Furthermore, by having a high value resistive value for the bias resistor
along with
a high bias voltage, a high output signal would be experienced over the
microphone lines
and therefore, reduce the needed gain for any following stages
In a further embodiment of the disclosure, in order to provide further noise
reduction within the circuit when this circuit is combined with ground
switching, such as
via ground noise, a extra set of switches can be implemented within the
microphone circuit
as will be discussed below.
As schematically shown in Figure 14, further circuitry for use with a headset
is
shown. The headset 300 includes a pair of speakers 302, seen as a right
headset speaker
302a and a left headset speaker 302b, and a microphone 304. Alternatively, the
headset
may include only one headphone. The headset 300 further includes a jack
(represented by
wires 306) which may be inserted into a portable electronic device, such as
via a port, in
order to connect the headset with the device. As schematically shown, the jack
includes
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four separate wires which are a left speaker audio line 306a, a right speaker
audio line 306b, a
ground signal line 306c and a microphone signal line 306d. In this embodiment,
the left speaker
302b is connected to the left speaker audio line 306a and to the ground line
306c. The right
speaker 302a is connected to the right speaker audio line 306b and the ground
signal line 306c
while the microphone is connected to the microphone signal line 306d and the
ground line 306c.
Within the device, the left speaker audio line 306a is connected to a left
headphone
output signal (HPL) signal line 310 while the right speaker audio line 306b is
connected to a
right headphone output signal (HPR) signal line 312. In one embodiment the
lines are
communicatively connected via the ports.
A MIC+ line 314, such as the +MIC OUT lines of Figures 2 to 13, is connected
via a
switch 316 to the microphone signal line 306d. Similarly, a MIC- line 318,
such as the ¨MIC
OUT line of Figures 2 to 13, is connected via a switch 320 to the ground
signal line 306c. As
some headsets have different ground connections, the switches 316 and 320
enable the portable
electronic device to support headsets that have ground and microphone signal
reversed, as in
Figure 15. A ground signal 322 is also connected via a switch 324 to the
ground signal line 306c
in Figure 14. A MIC Bias voltage signal 326 is connected to the microphone
signal line 306d via
a switch 327 after passing a resistor 328.
In the current embodiment, such as for use with a first headset, the switches
316 and 327
are set such that the MIC+ line 314 and the MIC Bias lines are connected to
the microphone
signal line 306c. The switches 320 and 324 are set such that the MIC- line 318
and the ground
reference voltage 322 are connected to the ground signal line 306c.
In the embodiment of Figure 15, such as for use with a second headset with the
ground
signal line and microphone signal line reversed (from the viewpoint of the
device), the switches
316 and 327 are set such that the MIC+ line 314 and MIC Bias are connected to
the ground
signal line 306c and switches 320 and 324 connect the MIC- line and the ground
reference
voltage 322 to the microphone line 306d. The advantage of using separate
switches for the
microphone signals and for the ground current switch is that the voltage that
will be generated
over the ground switch will not be sensed by the microphone input terminals,
since these
switches are placed after the ground switch. Fhis will be described in more
detail with respect to
Figure 16.
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Each of the pair of speakers 302 is connected to respective audio lines 306a
and 306b
which provide the audio signals to the user via the speakers 302. The audio
signals are generated
by the portable electronic device and transmitted to the headset via the jack
which is connected
to the device, typically via a port.
Turning to Figure 16, a more detailed schematic of the connections between a
portable
electronic device and a headset is shown. In the embodiment shown in Figure
16, the headset is
connected to a chip within the device. The chip may be a switch matrix having
ports for
receiving the individual lines within the jack of the headset.
A video buffer or path (represented by amplifier 500) may also be connected to
the
microphone line 306d of the headset via a switch 502. The video buffer or path
is not a
necessary part but may be included in various embodiments. The MIC+ line 314
and the MIC-
line 318 are connected to a low noise microphone pre-amplifier 504.
In the embodiment of Figure 16, the MIC+ line 314 and the MIC- line 318 are
connected
to the microphone signal line 306d and the ground signal line 306c via a
sensing circuit, such as
a Kelvin sensing circuit 506. By including a sensing circuit between the
switches and ground
322 and the headset, the effect of any changes to the ground potential 322
will be reduced. A
delta-sigma connector 508 may also be located within the device for digitizing
analog signals.
Kelvin sensing may be used on the microphone lines (MIC+ and MIC-) to reduce
the
affect on the microphone input by changes in or the ground signal 322, or
ground potential itself.
The switch 324 for the ground line will still be modulated by signals from the
headset, but the
microphone shall use the signal before this switch 324 to reduce the effect of
the modulation.
Thus, the microphone pre-amplifier 504 shall sense the differential signal at
the jack, before the
ground switch 324. Furthermore the switched microphone ground signal may be
used in another
configuration for reducing or eliminating any ground potential offset observed
by the headset or
the device (ground loop elimination)
In one embodiment, for economic and space reasons, this is most economically
achieved
via low-resistance switches for the ground switching, while somewhat larger
resistance switches
may be used for the separate set of switches used to carry the microphone
signals. As an
example, a resistance of 0.5 Q may be used for switching the ground line,
while a resistance of
S2 may be used to switch the microphone lines. In this manner, a larger
resistance may be
used for the differential microphone input since the
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input impedance is high and the output from the microphone itself is also
relatively high as
compared to the headphone impedances.
In the preceding description, for purposes of explanation, numerous details
are set
forth in order to provide a thorough understanding of the embodiments of the
disclosure.
However, it will be apparent to one skilled in the art that these specific
details are not
required in order to practice the disclosure. In other instances, well-known
electrical
structures and circuits are shown in block diagram form in order not to
obscure the
disclosure. For example, specific details are not provided as to whether the
embodiments
of the disclosure described herein are implemented as a software routine,
hardware circuit,
firmware, or a combination thereof.
The above-described embodiments of the disclosure are intended to be examples
only. Alterations, modifications and variations can be effected to the
particular
embodiments by those of skill in the art without departing from the scope of
the
disclosure, which is defined solely by the claims appended hereto.
