Datasheet

Data Sheet SSM2220
Rev. C | Page 9 of 12
AD8671
2
3
1
8
4
10µF
+
+
0.01µF
0.01µF
10µF
+15V
–15V
V
OUT
LED
Q1
SSM2220
Q2
2N29007A
18
36
27
R1
250Ω
V
IN
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
03096-018
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
modified by varying R5 or R6 (V
OUT
/V
IN
= R5/R6 + 1). A total
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
V
BE
of a silicon transistor is predictable and constant (to a few
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.