SA572
Basic Expandor
Figure 6 shows an application of the circuit as a simple
expandor. The gain expression of the system is given by:
V
OUT
+
V
IN
2
2
@
R
3
@
V
IN(AVG)
I
1
R
2
@
R
1
(I
1
= 140
mA)
(eq. 4)
Both the resistors R
1
and R
2
are tied to internal summing
nodes. R
1
is a 6.8 kW internal resistor. The maximum input
current into the gain cell can be as large as 140
mA.
This
corresponds to a voltage level of 140
mA•6.8
kW = 952 mV
peak. The input peak current into the rectifier is limited to
300
mA
by the internal bias system. Note that the value of
R
1
can be increased to accommodate higher input level. R
2
and R
3
are external resistors. It is easy to adjust the ratio of
R
3
/R
2
for desirable system voltage and current levels. A
small R
2
results in higher gain control current and smaller
static and dynamic tracking error. However, an impedance
buffer A
1
may be necessary if the input is voltage driven
with large source impedance.
The gain cell output current feeds the summing node of
the external OPA A
2
. R
3
and A
2
convert the gain cell output
current to the output voltage. In high-performance
applications, A
2
has to be low-noise, high-speed and wide
band so that the high-performance output of the gain cell
will not be degraded. The non-inverting input of A
2
can be
biased at the low noise internal reference Pin 6 or 10.
Resistor R
4
is used to bias up the output DC level of A
2
for
maximum swing. The output DC level of A
2
is given by:
V
OUT
DC
+
V
REF
1
)
R
3
R
4
*
V
B
R
3
R
4
(eq. 5)
V
B
can be tied to a regulated power supply for a dual
supply system and be grounded for a single supply system.
C
A
sets the attack time constant and C
R
sets the recovery
time constant.
R
4
+VB
R
3
17.3kW
−
C
IN1
+
2.2mF
R
5
100kW
A1
C
IN2
(7,9)
2.2mF
R
1
6.8kW
DG
V
REF
(5,11)
(6,10) R
6
1kW
(2,14)
(4,12)
C
1
2.2mF
A2
V
OUT
V
IN
C
IN3
2.2mF
R
2
3.3kW
(3,13)
BUFFER
C
A
C
R
1mF 10mF
(8)
(16)
+V
CC
Figure 6. Basic Expandor Schematic
Basic Compressor
Figure 7 shows the hook-up of the circuit as a
compressor. The IC is put in the feedback loop of the OPA
A
1
. The system gain expression is as follows:
V
OUT
+
V
IN
I
1
R
2
@
R
1
@
2
R
3
@
V
IN(AVG)
(I
1
= 140
mA)
1
2
R
DC1
, R
DC2
, and CDC form a DC feedback for A
1
. The
output DC level of A
1
is given by:
V
OUT
DC
+
V
REF
1
)
R
DC1
)
R
DC2
R
4
(eq. 6)
R
DC1
)
R
DC2
*
V
B
@
R
4
(eq. 7)
The zener diodes D
1
and D
2
are used for channel
overload protection.
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