SA571
Figure 6 shows how the circuit is hooked up to realize an
expandor. The input signal, V
IN
, is applied to the inputs of
both the rectifier and the
DG
cell. When the input signal
drops by 6.0 dB, the gain control current will drop by a factor
of 2, and so the gain will drop 6.0 dB. The output level at
V
OUT
will thus drop 12 dB, giving us the desired 2−to−1
expansion.
R
3
*C
IN1
DG
R
2
R
1
R
DC
*
C
IN
* R
3
−
C
RECT
*
R
DC
*
C
DC
*
C
F
*
V
OUT
V
IN
R
4
V
REF
+
R
2
DG
R
4
−
+
V
IN
*C
IN2
R
1
V
OUT
V
REF
NOTE: GAIN
+
I
B
= 140mA
R
1
R
2
I
B
2R
3
V
INavg
1
2
*EXTERNAL COMPONENTS
Figure 7. Basic Compressor
*C
RECT
Circuit Details − Rectifier
NOTE:
GAIN
+
I
B
= 140mA
2 R
3
V
IN
(avg) 2
R
1
R
2
I
B
*EXTERNAL COMPONENTS
Figure 6. Basic Expander
Figure 7 shows the hook−up for a compressor. This is
essentially an expandor placed in the feedback loop of the op
amp. The
DG
cell is setup to provide AC feedback only, so
a separate DC feedback loop is provided by the two R
DC
and
C
DC
. The values of R
DC
will determine the DC bias at the
output of the op amp. The output will bias to:
V
OUT
DC
+
1
)
R
DC1
)
R
DC2
V
REF
R
4
R
DCTOT
30kW
1.8V
Figure 8 shows the concept behind the full−wave
averaging rectifier. The input current to the summing node
of the op amp, V
IN
/R
1
, is supplied by the output of the op
amp. If we can mirror the op amp output current into a
unipolar current, we will have an ideal rectifier. The output
current is averaged by R
5
, CR, which set the averaging time
constant, and then mirrored with a gain of 2 to become I
G
,
the gain control current.
I = V
IN
/ R
1
V+
R
1
V
IN
−
+
R
5
10kW
C
R
I
G
V
OUT
DC
+
1
)
The output of the expander will bias up to:
V
OUT
DC
+
1
)
R
3
V
REF
R
4
Figure 8. Rectifier Concept
V
OUT
DC
+
1
)
20kW 1.8V
+
3.0V
30kW
The output will bias to 3.0 V when the internal resistors are
used. External resistors may be placed in series with R
3
,
(which will affect the gain), or in parallel with R
4
to raise the
DC bias to any desired value.
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