ELECTRONICS
A Passive Current Source
for Two-Wire
Microphones
Application Note 16
R1
1 0V
VBat
4 53 k
R2
1 3V
15 4 k
20 µA To Microphone
Cc
C
To A/D
Figure 1. Recommended passive network for supplying two-wire microphone with 20 µA current.
Many Knowles microphones require three terminal connections, one each for supply voltage
and ground, and a third for the output signal
from the microphone. A Knowles two-wire microphone has only two terminal connections,
where the supply and signal are on the same
pin. The microphone is powered by a current
source, and the output signal is separated from
the current source through a coupling capacitor
(Cc).
satisfactory performance for many applications
can be obtained with passive components using
the circuit in Figure 1.
The Knowles two-wire microphone requires
20 µA (nominal), and the recommended minimum compliance of the current source driving
it is 0.9 V to insure maximum peak-to-peak
output voltage. Power Supply Rejection (PSR)
is a factor since the passive current source is
connected directly to the hearing instrument
battery. Figure 2 shows the PSR for various
values of filter capacitor (C). The maximum
capacitance values for a given form factor and
For optimum performance, an active current
source (i.e., composed of matched bipolar or
MOS transistors) should be used. However,
90
C
C
C
C
80
PSR (dB)
70
60
= 1µF
= 10µF
= 22µF
= 47µF
50
40
30
20
10
0
2
3
4
5
6
7
2
8 910
2
3
4
5
6
7
3
8 910
2
3
4
5
6
7
8 910
4
Frequency (Hz)
Figure 2. Power Supply Rejection of the current source with battery voltage of 1 volt and
various values of the filter capacitor (C).
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Issue 060630
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AN16 Two-Wire Circuit
dielectric type are in Table 1. As an example,
for a PSR of 40 dB at 200 Hz, choose a 22 µF
capacitor; this value is available in a 0805 for
ceramic or a 0603 for tantalum.
The graphs in Figure 2 were generated from
simulations. It is recommended that prototype
circuits be evaluated prior to incorporation into
a hearing instrument. Knowles Electronics can
provide assistance if desired in the design and
component selection for a particular application.
Capacitor
Size
0402
0603
0805
1206
Ceramic
Tantalum
1 µF
10 µF
22 µF
22 µF
not available
22 µF
not available
47 µF
Table 1. Maximum capacitor values for different
package sizes and dielectric types.
Knowles Electronics, LLC
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USA
+1 630 250 5100 voice
+1 630 250 0575 fax
www.knowleselectronics.com
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England
+44 1444 235432 voice
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Suzhou, JiangSu 215021
China
+86 512 6258 8258 voice
+86 512 6258 9258 Fax
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Bhd.
Plot 104
Lebuhraya Kampung Jawa
Georgetown, Penang 11900
Maylasia
+60 4 6437 466 voice
+60 4 6437 446 Fax
Knowles Electronics, Japan
5/F YK Building 2-16
Sangenjaya 2-Chome
Setagaya-Ky, Tokyo 154-0024
Japan
+81 3 3439 1151 voice
+81 3 3439 8822 fax
Micromax Pty Ltd
5 Orangegrove Avenue
Unanderra NSW 2526
Australia
+61 2 4271 1300 voice
+61 2 4271 8091 fax
www.micromax.com.au
Note: All data and performance specifications in this document are for reference only.
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Knowles Electronics