SA572
C
1
+
R
1
3.3k
R
X
ATTACK
CAP
C
A
+
3, 13
2.2
mF
4, 12
1
mF
5, 11
BUFFER
2, 14
C
R
+
RECOVERY
CAP
10
mF
D
G
6.8k
R
2
7, 9
C
2
+
2.2
mF
R
DC2
R
4
100k
R
DC1
9.1k
+
9.1k
C
DC
10
mF
C
3
V
IN
+
R
3
17.3k
R
5
1k
C
5
+
V+
2
3
22
mF
−
2.2
mF
TO THD
TRIM PIN
OF 572
PINS 6, 10
5532
+
1 V
OUT
DC
+
2.2
mF
V−
V
OUT
Figure 9. Automatic Level Control
Automatic Level Control (ALC)
In the ALC configuration, the variable gain cell is placed
in the feedback loop of the operational amplifier and the
rectifier is connected to the input. As the input amplitude
increases above the crossover point, the overall system
gain decreases proportionally, holding the output
amplitude constant. As the input amplitude decreases
below the crossover point, the overall system gain
increases proportionally, holding the output amplitude at
the same constant level.
Gain
+
R1 R2 I1
2 R3 VIN(avg)
The output level is calculated using the following
equation:
VOUT_LEVEL
+
R1 R2 I1
2 R3
VIN
VIN (avg)
where:
VIN
+
p
+
1.11 (for sine waves)
VIN (avg)
2 2
R
1
= 6.8 kW (Internal)
R
2
= 3.3 kW
R
3
= 17.3 kW
I
1
= 140
mA
where:
R
1
= 6.8 kW (Internal)
R
2
= 3.3 kW
R
3
= 17.3 kW
I
1
= 140
mA
Note that for very low input levels, ALC may not be
desired and to limit the maximum gain, resistor R
X
has
been added.
Gain max.
+
R1
)R
x
VREF
· R2 · IB
26 kW)
*
10 kW
2 R3
The output DC level can be set using the following
equation:
V
OUT
DC
+
1
)
R
DC1
)
R
DC2
V
REF
R
4
Rx
^
((desired max gain)
where:
R
4
= 100 kW
R
DC1
= R
DC2
= 9.1 kW
V
REF
= 2.5 V
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