Datasheet

INA163
SBOS177D
8
www.ti.com
R
5
2.2k
R
6
(2)
5
R
7
(3)
1k
A
1
INA163
+
47
F
R
3
47k
R
2
6.8k
R
1
6.8k
Phantom Power
+48V
+
+
R
4
2.2k
C
1
(1)
47 F
60V
C
2
(1)
47 F 60V
A
2
OPA134
0.1 F
+15V
0.1 F
0.1
F
1M
V
O
15V
Optional DC
Output Control Loop
8
9
10
1
2
3
Female XLR
Connector
NOTES: (1) Use non-polar capacitors if phantom
power is to be turned off. (2) R
6
sets maximum gain.
(3) R
7
sets minimum gain.
1N4148
15V
1N4148
FIGURE 5. Phantom-Powered Microphone Preamplifier.
MICROPHONE AMPLIFIER
Figure 5 shows a typical circuit for a professional
microphone input amplifier. R
1
and R
2
provide a cur-
rent path for conventional 48V phantom power source
for a remotely located microphone. An optional switch
allows phantom power to be disabled. C
1
and C
2
block
the phantom power voltage from the INA163 input
circuitry. Non-polarized capacitors should be used for
C
1
and C
2
if phantom power is to be disabled. For
additional input protection against ESD and hot-plug-
ging, four INA4148 diodes may be connected from the
input to supply lines.
R
4
and R
5
provide a path for input bias current of the
INA163. Input offset current (typically 100nA) creates a
DC differential input voltage that will produce an output
offset voltage. This is generally the dominant source of
output offset voltage in this application. With a maxi-
mum gain of 1000 (60dB), the output offset voltage can
be several volts. This may be entirely acceptable if the
output is AC-coupled into the subsequent stage. An
alternate technique is shown in Figure 5. An inexpen-
sive FET-input op amp in a feedback loop drives the
DC output voltage to 0V. A
2
is not in the audio signal
path and does not affect signal quality.
Gain is set with a variable resistor, R
7
, in series with
R
6
. R
6
determines the maximum gain. The total resis-
tance, R
6
+ R
7
, determines the lowest gain. A special
reverse-log taper potentiometer for R
7
can be used to
create a linear change (in dB) with rotation.