SA575
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7
Automatic Level Control
The typical Automatic Level Control circuit
configuration is shown in Figure 4. It can be seen that it is
quite similar to the compressor schematic except that the
input to the rectifier cell is from the input path and not from
the feedback path. The input is AC coupled through C
12
and C
13
and the output is AC coupled through C
8
. Once
again, as in the previous cases, if the system input and
output signals are already AC coupled, then C
12
, C
13
and
C
8
could be eliminated. Concerning the compressor,
removing R
6
, R
7
and C
7
will cause motor-boating in
absence of signals. C
COMP
is necessary to stabilize the
summing amplifier at higher input levels. This circuit
provides an input dynamic range greater than 60 dB with
the output within "0.5 dB typical. The necessary design
expressions are given by Equation 5 and Equation 6,
respectively.
(eq. 5)
4V
IN
(avg)
3.8 k x 100 A
ALC gain =
(eq. 6)
R
=
A
= 10 k x C
RECT
= 10 k x C
9
2.2F
10F
10F
V
REF
G
1F
4.7F
ALC IN
ALC OUT
8
12
13
16
15
11
14
C COMP
22pF
C7
Figure 4. Typical ALC Configuration
10k
3.8k
10k
30k 30k
C
12
C
13
R
6
R
7
C
8
C
9
