Data Sheet

SSM2220

Rev. C | Page 9 of 12

AD8671

2

3

1

8

4

10µF

+

+

0.01µF

0.01µF

10µF

+15V

–15V

LED

Q1

SSM2220

Q2

2N29007A

18

36

27

R1

250Ω

R2

27kΩ

R3

5kΩ

R4

5kΩ

R6

100Ω

R5

100Ω

C1

50pF

THD < 0.005% 20Hz TO 20kHz

0.5nV/ Hz

1/f CORNER < 1Hz

Figure 18. Low Noise Microphone Preamplifier

LOW NOISE MICROPHONE PREAMPLIFIER

Figure 18 shows a microphone preamplifier that consists of an

SSM2220 and a low noise op amp. The input stage operates at a

relatively high quiescent current of 2 mA per side, which reduces

the SSM2220 transistor voltage noise. The 1/f corner is less than

1 Hz. Total harmonic distortion is under 0.005% for a 10 V p-p

signal from 20 Hz to 20 kHz. The preamp gain is 100, but can be

input stage emitter current of 4 mA is provided by Q2. The con-

stant current in Q2 is set by using the forward voltage of a GaAsP

LED as a reference. The difference between this voltage and the

percent) over a wide temperature range. The voltage difference,

approximately 1 V, is dropped across the 250 Ω resistor, which

produces a temperature stabilized emitter current.