AN1534
APPLICATION NOTE
TS971 based Electret Condenser Microphone amplifier
by R.CITTADINI & F.POULIN
This application note explains how to implement
the TS971 as a microphone pre-amplifier for an
Electret Condenser Microphone (ECM). This type
of microphone has one of the best price to perfor-
mance ratio on the market.
Microphone pre-amplifiers are very common in to-
day’s appliances, digital appliances have adopted
and kept the typologies used in analog ones. This
block is helping to interface the microphone to the
A/D converter by buffering, filtering and amplifying
the microphone signal.
1 - DEVICE PRESENTATION
Many low noise amplifiers exist in the market. Old-
ies (but still goodies!) are mainly dual ones like
LM833, MC33078. But few are able to reach low
voltages and are not available in today’s smallest
packages.
The TS97x is a family including single, dual and
quad low-noise operational amplifier. It features
excellent audio characteristics: low distortion
(0.003% THD @ F=1kHz) and a 4nV/sqrt(Hz)
equivalent input noise voltage with a 1/f corner @
100Hz. Thanks to those characteristics, it helps
keeping an optimal Signal to Noise ratio, a critical
point at the entry of the audio amplification chain!
These devices also allows a higher fidelity thanks
to a 4V/µs Slew Rate and 12 MHz Gain Bandwidth
Product. This enables the amplifier to cope with
quick variations of the input signal well over the
audio bandwidth.
The family is available in compact packages like
SOT23-5 for TS971 or even the thin and rather
compact package like TSSOP for TS972/4. This
allows them to be used in portable and miniature
digital appliances like PDA or Cellular Phones and
also in thin notebook computers.
Table 1:
Acoustic Units Reference Table
Acoustic
Intensity
I in W/m
1.E+02
2
Acoustic
Pressure
p in Pa
200
Sound
Pressure
Level
SPL in dB
140
130
Activity
Ear Damage
1.E+00
20
120
110
Night Club, Factory Floor
1.E-02
2
100
90
1.E-04
0.2
80
70
Conversation
1.E-06
0.02
60
50
1.E-08
0.002
40
30
Recording Studio
1.E-10
0.0002
20
10
I
REF
=
1.E-12
p
REF
=
0.00002
0
Minimum Level of Audition, Reference Level
SPL
=
10
×
Log (
SPL
=
20
×
Log (
I
I
REF
p
) (dB )
p
REF
) (dB )
2 - MICROPHONE CONSIDERATIONS
Preliminary knowledge of Acoustic Intensity (in
Watt/m
2
), Acoustic Pressure (in Pascal or Pa) and
Sound Pressure Level or SPL (in Decibels or dB)
is important. You can report to
table 1
for more in-
formation.
ECM microphones follow more or less the same
characteristics, however Gain and surrounding
components may vary from one model to another.
June 2002
1/4
AN1534 - APPLICATION NOTE
We selected a popular model from Panasonic: the
WM-60A series. It’s an omni-directional micro-
phone with the following main characteristics:
u
Operating: from 2 to 10V.
u
Sensitivity: -44dB +/- 5dB (0dB=1V/Pa).
u
Impedance: less than 2.2kΩ
u
S/N ratio: more than 58dB
u
Current Consumption: 0.5mA max
u
Recommended Load Resistor: R
L
: 2.2kΩ
The sensitivity of the microphone defines its gain
as per the following formula:
3 - COMPONENTS CALCULATION
Let’s look now on how to implement such an am-
plifier with TS971. You can refer to schematic on
Figure 1
hereafter. We’ve chosen a non-inverter
typology to exploit to the best the low noise char-
acteristics of the device. Indeed, with an inverter
configuration, the input resistor adds significant
noise to the application.
First, let’s look on the behavior in DC mode. The
first goal is to polarize the Electret Condenser Mi-
crophone. By using R
1
and R
2
, we can polarize it
around Vcc/2 as per below formula:.
G
mike
=
10
(
Sensitivit y
)
20
( V / Pa)
So with a -44dB sensitivity, we can conclude that
the gain of the microphone is 0.0063V/Pa or
6.3mV/Pa. With this value, we can get a good idea
of the output voltage of the microphone. It would
be around 12.6µV for a quiet room (2mPa or
40dB) and would reach approximately 6.3mV for
the climax of a symphonic orchestra (1Pa or
110dB). This sound data is with a source at 1
meter from the microphone. This reference is
mandatory, the distance between the microphone
and the audio signal is illustrated by the Acoustic
Intensity (in Watt/m
2
).
Let’s take the example of a conversation. It’s
equivalent to roughly 20mPa or 60dB SPL at 1
meter. So an Acoustic Intensity of 1µW and 126µV
at the output of the microphone. The intensity de-
creases with the squared value of the distance be-
tween the source and the microphone. So for a
distance of 5cm, you would get a value of 400µW.
As per the
Table 1
formulas, we can calculate the
"equivalent SPL value at 1m": 86dB, then we get
the Acoustic Pressure: 0.4Pa which gives us the
output of the microphone:
0.4 x 0.0063 = 2.52mV (distance divided by 20
and output voltage increased by the same ratio).
We can summarize these considerations into the
following checklist:
u
What type of signal do you want to amplify?
u
How powerful?At what distance?
u
What are the minimum and maximum of
each above parameters?
With these values, you will be able the calculate
the microphone’s output voltage range and be
able to choose the right gain of the amplifier here-
after.
Also, if you want to implement a noise canceling
function, you can also choose another type of mi-
crophone called bi-directional microphone or
noise canceling microphone.
2/4
I
pol
−
mike
≈
Vcc
(A )
2
×
(R
1
+
R
2
)
The only criteria is that this current must remain
below 0.5mA over the supply range (otherwise,
you can increase R1 value).
R
1
is also acting together with C
1
as a filter for the
power supply line of the microphone. Then in AC
mode, C
1
is fixing the gain of the microphone by
allowing only R
2
to act (and not R
2
+R
1
as C
1
is
equivalent to a short circuit to the ground). And R
2
must equal R
L
=2.2kΩ for the microphone we’ve
chosen. In AC mode, this type of microphone can
be simplified and compared to a current source in
parallel with R
2
, hence a voltage source.
Then to avoid extra offset drift due to bias current
mismatching, following resistor values need to
comply with the following rule:
R
8
≈
R
4
+
R
3
+
R
5
×
R
6
( Ohms )
R
5
+
R
6
The second step is, still in DC mode, to polarize
the reference pin of the TS971. It’s the inverting
pin here that will be set at Vcc/2 by the R5 and R6
bridge. C4 adds here additionnal filtering of this
reference voltage. This configuration allows the bi-
asing or the "centring" of the signal at mid-supply
voltage. Hence it allows to maximize the swing
within the supply voltage range. This bias voltage
just needs to be kept within V
ICM
range. This
means V
ICM
or Common Mode Input Voltage must
be at least 1.15V inside the supply voltage rails,
i.e. from Vdd+1.15 to Vcc-1.15V.
AN1534 - APPLICATION NOTE
Figure 1:
ECM Microphone Pre-amp. with TS971
Vcc = 2.7V min.
R1
1k
Optionnal
EMI filter
C1
10u
C2*
10u
R2
2.2k C5
R4
18k
R8
R
EMI
100R
C
EMI
47p
100n
Vcc
82k
Electret
Condenser
Microphone
R5
100k
R3
C8*
180p
R7
820R
C3
4.7u
This bridge can also be supplied by an ASIC’s
"Vref out" pin or by another operator of the op-amp
connected as a buffer (using the TS972, a dual
op-amp, this can be implemented easily).
An important note on the impedance of the ampli-
fier is that, in AC mode, R
4
is equivalent to the in-
put impedance of the amplifier stage. It must not
be too small to avoid the collapsing of the micro-
phone signal!
The coupling capacitor (C
6
) makes this application
universal, however, you could omit it when attack-
ing an A/D converter. In this case, you only have
to adapt the bridge set by R
5
& R
6
to match the in-
put voltage range of the converter. Thanks to its
Rail-to-Rail output, the TS971 simplifies the pro-
cess.
We’re coming now to the filter definition, when
looking at
figure 1,
we can see three filters: two
high pass and a low pass. Each has a 6dB/octave
attenuation factor.
The high pass filter is built by C
5
, R
4
and also R
2
.
The theoretical formula to calculate F
CL
or the
Lower Cutoff Frequency (here approximately 79
Hertz) is the following
F
CL
Another high pass filter is made by C
7
and R
7
. The
cutoff frequency is better set at a lower value than
F
CL
to have a stronger reduction (i.e. -12dB/oc-
tave) of low frequencies (here 59Hz).
F
CL
Then for the low pass filter (optional), to calculate
F
CH
or the Higher Cutoff Frequency (here approx-
imately 10.7kHz), we have the following formula::
F
CH
The next step is to configure G or the gain of the
amplifier (here 90 or +39dB):
G
≈
(1
+
+
+
15k
R6
C4
100k
2.2u
+
TS461/971
C6*
+
Output
1u
* Optional : refer to text
C7
3.3u
+
+
1
≈
1
2
× π ×
(
R
2
+
R
4
)
×
C
5
(Hz )
2
≈
1
2
× π ×
R
7
×
C
( Hz )
7
=
1
2
× π ×
R
8
×
C
( Hz )
8
R
4
R
8
)
×
Gain of the microphone
)
×
(
R
4
+
R
2
R
7
3/4
AN1534 - APPLICATION NOTE
This represents the gain of the amplifier in the
"non-filtered" bandwidth area. R
2
and R
4
have to
be taken into consideration as they establish a
voltage divider. Changing R
7
value helps to modi-
fy the gain without being forced to change the val-
ues of the other components (apart from the high
pass filter with C
7
).
What we refer to as the "gain of the microphone" is
the gain we talk about in the "Microphone consid-
erations" chapter. You need to consider the signal
fed to the microphone to avoid saturation or insuf-
ficient gain.
To be completely exhaustive, if you use C
6
, you
have to consider that it creates together with the
impedance of the load Z
L
a high pass filter with
cutoff frequency defined like above (replacing R
4
with Z
L
and C
5
with C
6
.
4- SPECIAL NOTE ON EMI
As you can notice on Figure 1, we have suggested
an optional Electro Magnetic Interference filter
(cutting frequencies above 33.8MHz). With cellu-
lar phones being a constant company to our daily
life, high power RF interferences need to be prop-
erly managed into sensitive amplification devices.
This filter has to be implemented close enough to
the microphone and we also have to pay attention
on not having too long connecting wires (anten-
nas!). Ideally this application should be wired on a
double sided PCB with one side acting as ground
plane.
5 - COMPONENTS CALCULATION
So, in this example, we have a gain value of 90 or
+39dB and we have globally a band pass filter
with F
CL
~60-80Hz and F
CH
=10.7kHz. Concerning
the overall gain, we also have also to consider the
stage after the pre-amplifier. If it’s a CODEC or A/
D converter, it will most probably have its own am-
plification (usually +20dB gain). All together, this
gives the below chain for the chapter 1 example:
Stage \ V
OUT
Microphone Output
at 1 meter
0.13mV
(126µV)
TS971 Output
(+39dB)
Codec Output
(+20dB)
0.11mV
(11.34mV)
0.11V
2.27V
0.23V
at 5 cm
2.5mV
This configuration is well adapted to battery pow-
ered equipment as the overall maximum con-
sumption of this application is 3.3mA (0.5mA for
the microphone and 2.8mA max for TS971).
Information furnished is believed to be accurate and reliable. However, STMicroelectronics assumes no responsibility for the
consequences of use of such information nor for any infringement of patents or other rights of third parties which may result from
its use. No license is granted by implication or otherwise under any patent or patent rights of STMicroelectronics. Specifications
mentioned in this publication are subject to change without notice. This publication supersedes and replaces all information
previously supplied. STMicroelectronics products are not authorized for use as critical components in life support devices or
systems without express written approval of STMicroelectronics.
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© 2002 STMicroelectronics - Printed in Italy - All Rights Reserved
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