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DESCRIPTION JP2015211355

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DESCRIPTION JP2015211355
The present invention provides a microphone bias circuit with improved sound quality. A bias
circuit supplies a bias voltage to a microphone. The variable gain amplifier 104 amplifies the
reference voltage V. The low pass filter 106 removes high frequency components of the variable
gain amplifier 104. The voltage follower amplifier 108 receives the output voltage of the low
pass filter 106 and supplies it to the microphone 2. [Selected figure] Figure 2
Microphone bias circuit, audio interface circuit, electronic equipment
[0001]
The present invention relates to a bias circuit for supplying a bias voltage to a microphone.
[0002]
Microphones, bias circuits that bias the microphones, and amplifiers that amplify electric signals
from the microphones are mounted on electronic devices having a recording function and a call
function, such as camcorders, digital cameras, mobile phone terminals, and tablet terminals.
[0003]
FIG. 1 is a circuit diagram of a bias circuit for a microphone examined by the inventor.
The bias circuit 30 includes a voltage source 32 and a variable gain amplifier 34 and generates a
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bias voltage V BIAS to be supplied to the microphone 2.
The output terminal of the bias circuit 30 is connected to a capacitor C1 for phase compensation,
and the bias voltage V BIAS is supplied to the microphone 2 via the resistor R1.
[0004]
The voltage source 32 divides the stabilized voltage V REG to generate a reference voltage V REF.
The variable gain amplifier 34 is a non-inverting amplifier, which multiplies the reference voltage
V REF by a gain, and outputs a bias voltage V BIAS. The appropriate level of the bias voltage V
BIAS varies depending on the type of microphone 2. Therefore, in order to supply the optimum
bias voltage V BIAS for each microphone 2, the gain of the variable gain amplifier 34 is set.
[0005]
The microphone 2 converts the input acoustic signal into an electrical signal. Among the
electrical signals, an AC component other than the DC bias component is input to the microphone
amplifier 40 via the capacitor C2 for DC block. The microphone amplifier 40 amplifies a weak
electrical signal. The A / D converter 42 converts the output of the microphone amplifier 40 into
a digital signal. The bias circuit 30 of FIG. 1 should not be recognized as a known technique. The
microphone amplifier 40 and the A / D converter 42 are integrated with the bias circuit 30 in the
audio interface circuit 4r.
[0006]
JP, 2000-236383, A
[0007]
As a result of examining the bias circuit 30 of FIG. 1, the inventor has come to recognize the
following problem.
A digital circuit (not shown) is mounted at the subsequent stage of the A / D converter 42. High
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frequency noise caused by the clock signal generated in the digital circuit is mixed in the bias
circuit 30 via the ground line. Specifically, in FIG. 1, the noise is mixed into the bias voltage V
BIAS through the mixing paths (i) to (iii) from the ground, and the noise of the bias voltage V
BIAS is microphoned through the capacitor C2. It is input to the amplifier 40 to reduce the sound
quality.
[0008]
In particular, noise mixed in the paths (i) and (ii) is to be amplified by the variable gain amplifier
34, which causes the sound quality to be degraded.
[0009]
The present invention has been made in view of such problems, and one of the exemplary objects
of an aspect thereof is to provide a bias circuit for a microphone with improved sound quality.
[0010]
In order to solve the above problems, a bias circuit according to an aspect of the present
invention is a bias circuit for supplying a bias voltage to a microphone, which removes a highfrequency component of a variable gain amplifier that amplifies a reference voltage and a
variable gain amplifier. A low pass filter and a voltage follower amplifier that receives an output
voltage of the low pass filter and supplies the output voltage to a microphone.
[0011]
According to this aspect, by providing a voltage follower amplifier at the subsequent stage of the
variable gain amplifier and inserting a filter between them, noise components included in the bias
voltage are reduced while maintaining sufficient drive capability for the microphone. Because it
can, it can improve the sound quality.
[0012]
The cutoff frequency of the low pass filter may be lower than 50 Hz.
Thereby, the noise component of the audio band that affects the sound quality can be suitably
removed.
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[0013]
ローパスフィルタは、RCフィルタであってもよい。
[0014]
The variable gain amplifier may be a non-inverting amplifier.
[0015]
The variable gain amplifier includes an operational amplifier receiving a reference voltage at its
non-inverting input terminal, and a variable voltage dividing circuit that divides the output
voltage of the operational amplifier with a variable voltage division ratio and inputs it to the
inverting input terminal of the operational amplifier. May be.
[0016]
Another aspect of the present invention relates to an audio interface circuit.
The audio interface circuit performs the signal processing on the bias circuit according to any
one of the above aspects, a microphone amplifier for amplifying the output signal of the
microphone, an A / D converter for converting the output of the microphone amplifier into a
digital signal, And a digital signal processing unit.
[0017]
The audio interface circuit may be integrated on one semiconductor substrate.
"Integrated integration" includes the case where all of the circuit components are formed on a
semiconductor substrate, and the case where the main components of the circuit are integrally
integrated. A resistor, a capacitor or the like may be provided outside the semiconductor
substrate.
By integrating the circuit as one IC, the circuit area can be reduced and the characteristics of the
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circuit elements can be kept uniform.
[0018]
Another aspect of the present invention relates to an electronic device.
The electronic device comprises a microphone and the above-mentioned audio interface circuit.
[0019]
It is to be noted that any combination of the above-described constituent elements, or one in
which the constituent elements and expressions of the present invention are mutually replaced
among methods, apparatuses, systems, etc. is also effective as an aspect of the present invention.
[0020]
According to the bias circuit of the present invention, voice can be improved.
[0021]
It is a circuit diagram of the bias circuit for microphones which this inventor examined.
It is a circuit diagram of an audio interface circuit provided with a bias circuit concerning an
embodiment.
3 (a) is a spectrum of the audio interface circuit of FIG. 2, and FIG. 3 (b) is a spectrum of the
audio interface circuit of FIG.
It is a block diagram of an electronic device provided with an audio interface circuit.
5 (a) and 5 (b) are diagrams showing an electronic device. It is a circuit diagram of a bias circuit
concerning the 2nd modification.
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[0022]
Hereinafter, the present invention will be described based on preferred embodiments with
reference to the drawings. The same or equivalent components, members, and processes shown
in the drawings are denoted by the same reference numerals, and duplicating descriptions will be
omitted as appropriate. In addition, the embodiments do not limit the invention and are merely
examples, and all the features and combinations thereof described in the embodiments are not
necessarily essential to the invention.
[0023]
In the present specification, “a state in which the member A and the member B are connected”
refers to the case where the member A and the member B are physically directly connected, or
the member A and the member B are electrically It also includes the case of being indirectly
connected via other members that do not substantially affect the connection state or do not
impair the function or effect exerted by the connection. Similarly, "a state where the member C is
provided between the member A and the member B" means that the member A and the member
C, or the member B and the member C are directly connected, and It also includes the case of
indirect connection via other members that do not substantially affect the connection state of the
connection or do not impair the function or effect provided by the connection.
[0024]
FIG. 2 is a circuit diagram of an audio interface circuit (also referred to as a codec IC) 4 including
the bias circuit 10 according to the embodiment. The audio interface circuit 4 is a functional IC
(Integrated Circuit) that includes a microphone amplifier 40 and an A / D converter 42 in
addition to the bias circuit 10, and is integrated on one semiconductor substrate.
[0025]
The power supply voltage (V DD) is supplied to the power supply (VDD) terminal of the audio
interface circuit 4. Each element in audio interface circuit 4 operates by receiving power supply
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voltage V DD. A bias resistance R1 is provided between the microphone bias (MICBIAS) terminal
of the audio interface circuit 4 and the microphone 2. A capacitor C1 for phase compensation is
connected to the MICBIAS terminal. Further, the microphone 2 is connected to the microphone
input (MICIN) terminal of the audio interface circuit 4 through the capacitor C2 for DC block.
[0026]
The microphone amplifier 40 amplifies the audio electrical signal input to the MICIN terminal.
The A / D converter 42 converts the output of the microphone amplifier 40 into a digital audio
signal.
[0027]
The bias circuit 10 generates a bias voltage V BIAS and supplies it to the microphone 2 through
the MICBIAS terminal and the resistor R1. The bias circuit 10 includes a voltage source 102, a
variable gain amplifier 104, a low pass filter 106, and a voltage follower amplifier 108.
[0028]
The voltage source 102 generates a reference voltage V REF. For example, the voltage source
102 includes an LDO (Low Drop Output) 110 and resistors R11 to R13. LDO 110 receives power
supply voltage V DD and generates internal power supply voltage V REG stabilized at a
predetermined level. Internal power supply voltage V REG is supplied not only to voltage source
102 but also to a digital circuit (not shown) or the like. たとえばV DD =3.3V、V REG
=1.8Vである。
[0029]
The resistors R11 and R12 divide the internal power supply voltage V REG to generate a
reference voltage V REF. The reference voltage V REF is supplied to the subsequent variable gain
amplifier 104 via the resistor R13. When the input impedance of the first operational amplifier
112 is sufficiently high, the resistor R13 may be omitted.
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[0030]
The variable gain amplifier 104 amplifies the reference voltage V REF. The variable gain
amplifier 104 is a non-inverting amplifier and includes a first operational amplifier 112 and a
voltage dividing circuit 114. The reference voltage V REF is input to the non-inverting input
terminal (+) of the first operational amplifier 112. The voltage dividing circuit 114 divides the
output voltage V O1 of the first operational amplifier 112 with a variable voltage division ratio,
and inputs it to the inverting input terminal (−) of the first operational amplifier 112. The
voltage dividing circuit 114 includes resistors R21 and R22, and the voltage dividing ratio K is
given by the equation (1). K = R22 / (R21 + R22) (1) At least one of the resistors R21 and R22 is
a variable resistor. The gain g of the variable gain amplifier 104 is expressed by Equation (2).
g=1/K=(R21+R22)/R22 …(2)
[0031]
The low pass filter 106 removes high frequency components of the variable gain amplifier 104.
The low-pass filter 106 only needs to pass 0 Hz DC components, so the cutoff frequency fc is
preferably as low as possible, for example, 50 Hz or less. More preferably, the cutoff frequency fc
is lower than the lower limit of the frequency band of the microphone 2. The low pass filter 106
is an RC filter and includes a resistor R31 and an external capacitor C31.
[0032]
The voltage follower amplifier 108 receives the output voltage V O2 of the low pass filter 106.
The output voltage of the voltage follower amplifier 108 is supplied to the microphone 2 as a
bias voltage V BIAS. The voltage follower amplifier 108 includes a second operational amplifier
116 whose output terminal and inverting input terminal are shorted.
[0033]
The above is the configuration of the bias circuit 10. Subsequently, the operation will be
described.
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[0034]
In FIG. 2, noise mixing paths (i) to (iii) from the ground line 44 in the bias circuit 10 are shown.
The noise introduced from these paths is removed or attenuated by the low pass filter 106.
[0035]
Further, a voltage follower amplifier 108 is provided at a stage subsequent to the low pass filter
106. In the voltage follower amplifier 108, only the lower power supply terminal 118 of the
second operational amplifier 116 is connected to the ground line 44, and other noises are mixed
There is no route.
[0036]
According to this bias circuit 10, compared to the bias circuit 30 shown in FIG. 1, noise coming
from the ground line 44 to the MICIN terminal via the bias circuit 10 can be reduced, so the S / N
ratio of the input signal of the microphone amplifier 40 can be reduced. The sound quality can be
improved.
[0037]
In order to verify the effect of the bias circuit 10 of FIG. 2, the spectrum of the signal input to the
MICIN terminal was measured for each of the audio interface circuits of FIG. 2 and FIG.
3 (a) is a spectrum of the audio interface circuit of FIG. 2, and FIG. 3 (b) is a spectrum of the
audio interface circuit of FIG.
When focusing on, for example, around 420 Hz in FIGS. 3A and 3B, the noise level in FIG. 3B is
−70 dBFS (Full Scale), whereas the noise level in FIG. 3A is − It is 110 dBFS, and an
improvement of 40 dB is confirmed. As shown in FIG. 3A, it is confirmed that the noise floor level
is suppressed to -110 dBFS or less in the bias circuit 10 of FIG. 2 even at other frequencies, and
the S / N ratio is improved. It was done.
[0038]
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(Application) FIG. 4 is a block diagram of an electronic device 500 provided with the audio
interface circuit 4. The electronic device 500 is a device having a recording function and a call
function, such as a camcorder, a digital camera, a mobile phone terminal, and a tablet terminal.
The electronic device 500 includes the microphone 2, the audio interface circuit 4 and the
memory 6.
[0039]
The audio interface circuit 4 further includes a DSP (Digital Signal Processor) 46 provided
downstream of the A / D converter 42. The DSP 46 performs various signal processing on the
audio signal converted into a digital signal by the A / D converter 42. Examples of signal
processing by the DSP 46 include audio encoding, equalizing, volume control, and the like. The
audio signal encoded by the DSP 46 is recorded in the memory 6.
[0040]
5A and 5B illustrate the electronic device 500. FIG. The electronic device 500a in FIG. 5A is a
camcorder, and includes an imaging element 502, an image processing circuit 504, a lens 506,
and the like in addition to the microphone 2, the audio interface circuit 4, and the memory 6. The
audio interface circuit 4 and the image processing circuit 504 are mounted on the substrate 510.
[0041]
The light that has passed through the lens 506 is converted into an electrical signal by the
imaging device 502, subjected to image processing by the image processing circuit 504,
encoded, and then stored in the memory 6. At the same time, the microphone 2 and the audio
interface circuit 4 record voice.
[0042]
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The electronic device 500b of FIG. 5B is a smart phone. In the electronic device 500b, the
microphone 2 and the audio interface circuit 4 are used for voice communication. The digital
audio signal generated by the audio interface circuit 4 is input to the baseband circuit 512. The
baseband circuit 512 encodes a digital audio signal. An RF (Radio Frequency) circuit 514
converts the signal from the baseband circuit 512 into a high frequency signal, and transmits the
signal from an antenna (not shown).
[0043]
The present invention has been described above based on the embodiments. It is understood by
those skilled in the art that this embodiment is an exemplification, and that various modifications
can be made to the combination of each component and each processing process, and such a
modification is also within the scope of the present invention. is there. Hereinafter, such
modifications will be described.
[0044]
First Modification The low pass filter 106 of FIG. 2 may be an LC filter.
[0045]
Second Modification The low pass filter 106 and the voltage follower amplifier 108 may be
integrally configured as an active filter.
FIG. 6 is a circuit diagram of a bias circuit 10a according to a second modification. The low pass
filter 106 a and the voltage follower amplifier 108 constitute a second-order active filter. The
order of the low pass filter is not particularly limited. The capacitor of the low pass filter 106 a is
externally attached to the audio interface circuit 4. The same effect as FIG. 2 can be acquired also
by this modification.
[0046]
Third Modification The variable gain amplifier 104 may be an inverting amplifier. The voltage
source 102 may use a configuration other than the voltage dividing circuit, for example, a
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constant voltage circuit using a zener diode.
[0047]
Although the present invention has been described using specific terms based on the
embodiments, the embodiments only show the principles and applications of the present
invention, and the embodiments are defined in the claims. Many variations and modifications of
the arrangement can be made without departing from the concept of the present invention.
[0048]
Reference Signs List 2 microphone, 4 audio interface circuit, 6 memory 10 bias circuit 102
voltage source 104 variable gain amplifier 106 low pass filter 108 voltage follower amplifier 110
LDO 112 first operation Amplifier 114: voltage divider circuit 116: second operational amplifier
30: bias circuit 32: voltage source 34: variable gain amplifier 40: microphone amplifier 42: A / D
converter 44: ground line 46: DSP, 500: electronic device, 502: imaging device, 504: image
processing circuit, 506: lens, 510: substrate, 512: baseband circuit, 514: RF circuit.
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