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SSM2135

Dual Single-Supply Audio Operational Amplifier

Analog Devices 

Low Noise Microphone Preamplifier

The 5.2 nV/√Hz input noise in conjunction with low distortion

make the SSM2315 an ideal device for amplifying low level signals

such as those produced by microphones. Figure 34 illustrates a

stereo microphone input circuit feeding a multimedia sound

codec. The gain is set at 100 (40 dB), although it can be set to

other gains depending on the microphone output levels. Figure 33

shows the harmonic distortion performance of the preamplifier

with 1 V rms output, while operating from a single 5 V supply.

The SSM2135 is biased to 2.25 V by the VREF pin of the AD1845

codec. The same voltage is buffered by the 2N4124 transistor to

provide phantom power to the microphone. A typical electrets

condenser microphone with an impedance range of 100 Ω to

1 kΩ works well with the circuit. This power booster circuit can

be omitted for dynamic microphone elements.

1

VS = 5V

AV = 40dB

VOUT = 1V rms

80kHz LOW-PASS FILTER

0.1

SSM2135

0.01

10

100

1k

FREQUENCY (Hz)

10k 20k

Figure 33. MIC Preamp THD + N Performance

LEFT CHANNEL

MIC IN

10µF

2kΩ 10kΩ

5V

2N4124

10kΩ

100Ω

5V 10µF

28

1

3

1/2

4 SSM2135 5V

0.1µF

RIGHT CHANNEL

10kΩ

MIC IN

2kΩ 10µF

10µF

0.1µF

5

7

6

1/2

SSM2135

29

L_MIC

35/36

VCC

34/37

GNDA

32

VREF

AD1845

28

R_MIC

100Ω

10kΩ

Figure 34. Low Noise Microphone Preamp for Multimedia Sound Codec

Rev. G | Page 11 of 16

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