OP176_15 Datasheet, PDF (12/21 Pages) Analog Devices – Bipolar/JFET, Audio Operational Amplifier

OP176
A Low Noise Microphone Preamplifier with a Phantom
Power Option
Figure 39 is an example of a circuit that combines the strengths
of the SSM2017 and the OP176 into a variable gain micro-
phone preamplifier with an optional phantom power feature.
The SSM2017’s strengths lie in its low noise and distortion, and
gain flexibility/simplicity. However, rated only for 2 kΩ or
higher loads, this makes driving 600 Ω loads somewhat limited
with the SSM2017 alone. A pair of OP176s are used in the
circuit as a high current output buffer (U2) and a DC servo
stage (U3). The OP176’s high output current drive capability
provides a high level drive into 600 Ω loads when operating
from ± 18 V supplies. For a complete treatment of the circuit
design details, the interested reader should consult application
note AN-242, available from Analog Devices.
This amplifier’s performance is quite good over programmed
gain ranges of 2 to 2000. For a typical audio load of 600 Ω,
THD + N at various gains and an output level of 10 V rms is
illustrated in Figure 40. For all but the very highest gain, the
THD + N is consistent and well below 0.01%, while the gain of
2000 becomes more limited by noise. The noise performance of
the circuit is exceptional with a referred-to-input noise voltage
spectral density of 1 nV/√Hz at a circuit gain of 1000.
1.0
±V_S = ±18V
80kHz LPF
0.1
0.010
G = 2000
0.001
20
100
G = 200
G = 20
G=4
1k
10k 20k
Figure 40. Low Noise Microphone Preamplifier THD + N
Performance at Various Gains (V_OUT = 10 V rms and
R_L = 600 Ω)
C8
+
47µF/
63V
R9
6.81kΩ
+48V
ΩR10
100Ω
R8
6.81kΩ
PHANTOM POWER SUPPLY CONNECTIONS,
INTERLOCKED WITH +/–V_S (SEE NOTE 5).
Z1
Z2
+V_S
–V_S
C5
33pF
C6
0.1µF
+18V
+ _C3
100µF/25V
C7
0.1µF
+ _C4
100µF/25V
–18V
–IN
TO MICROPHONE
COMMON
+IN
+
C_IN1 ΩR_P1
47µF/ 49.9Ω
63V
+
C_IN2 ΩR_P2
47µF/ 49.9Ω
63V
1)
R_B1
10kΩ
C_N
4.7nF/
FILM
R_B2
10kΩ
2200µF/ 10V
+
C_G1
R_G ³⁾
C_G2
+
2200µF/ 10V
Z3
Z4
C_RF2+V^S
100pF
U1
SSM2017P
37
81
6
R1
10kΩ
2 45
R7
C_RF1
1kΩ
100pF
–V_S
R6
10kΩ
NOTES:
1) Z1–Z4 1N752 (SEE TEXT).
2) C_INX, C_GX LOW LEAKAGE ELECTROLYTIC TYPES (SEE TEXT).
3) GAIN = G = 2 x ((10k/R_G ) +1) (SEE TEXT).
4) ALL RESISTORS 1% METAL FILM.
5) DOTTED PHANTOM POWER RELATED COMPONENTS OPTIONAL (SEE TEXT).
R2
20kΩ
–V_S
U2
OP176
24
6
3
7
+V_S
C1
1µF FILM
U3
OP176 _+VS
72
6
3
4
–V_S
ΩR3
49.9Ω
R4
221kΩ
D1
1N458
D2
1N458
R5
221kΩ
C2
1µF FILM
OUTPUT
OUT COMMON
Figure 39. A Low Noise Microphone Preamplifier
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