SA575
Compressor
The typical compressor configuration is shown in
Figure 3. In this mode, the rectifier cell and variable gain
cell are in the feedback path. R
6
and R
7
provide the DC
feedback to the summing amplifier. The input is AC
coupled through C
12
and output is AC coupled through C
8
.
In a system with inputs and outputs AC coupled, C
8
and C
12
could be eliminated and only R
6
, R
7
, C
7
, and C
13
would be
required. If the external components R
6
, R
7
and C
7
are
eliminated, then the output of the summing amplifier will
motor-boat in absence of signals or at extremely low
signals. This is because there is no DC feedback path from
R
6
30kW
C7
V
REF
8
12
the output to input. In the presence of an AC signal this
phenomenon is not observed and the circuit will appear to
function properly.
The compressor gain expression and the attack and
release time constant is given by Equation 3 and
Equation 4, respectively.
Compressor gain =
3.8 kW x 100
mA
4V
IN
(avg)
where V
IN
(avg) = 0.95V
IN(RMS)
t
R
=
t
A
= 10 kW x C
RECT
= 10 kW x C
4
R
7
30kW
1mF
1/2
(eq. 3)
(eq. 4)
C
12
13
COMP IN
10mF
10kW
S
C
8
14
10mF
COMP OUT
D
G
10kW
11
16
3.8kW
15
C
9
2.2mF
C
13
4.7mF
Figure 3. Typical Compressor Configuration
http://onsemi.com
6
ONSEMI [ ON SEMICONDUCTOR ]
