SSM2135

Rev. G | Page 11 of 16

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.

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

0.01

0.1

10

20k

10k

1k

100

FREQUENCY (Hz)

80kHz LOW-PASS FILTER

Figure 33. MIC Preamp THD + N Performance

LEFT CHANNEL

MIC IN

RIGHT CHANNEL

MIC IN

2kΩ

5V

10µF

10kΩ

2N4124

L_MIC

GNDA

AD1845

R_MIC

2kΩ

10µF

10kΩ

10µF

0.1µF

100Ω

10kΩ

100Ω

10kΩ

0.1µF

10µF

5V

2

3

4

1

8

5

6

7

5V

29

35/36

34/37

32

28

1/2

SSM2135

1/2

SSM2135

Figure 34. Low Noise Microphone Preamp for Multimedia Sound Codec