Philips Semiconductors RF Communications Products
Product specification
Programmable analog compandor
NE/SA572
V
OUT
2
+
@
V
IN
I
1
(I
1
=140µA)
R
3
@
V
IN
(
AVG
)
R
2
@
R
1
(5)
Both the resistors R
1
and R
2
are tied to
internal summing nodes. R
1
is a 6.8k internal
resistor. The maximum input current into the
gain cell can be as large as 140µA. This
corresponds to a voltage level of 140µA
•
6.8k=952mV peak. The input peak current
into the rectifier is limited to 300µA 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
drive 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
ODC
+
V
REF
1
)
R
3
R
4
*
V
B
R
3
R
4
(6)
V
B
can be tied to a regulated power supply
for a dual supply system and be grounded for
a single supply system. CA sets the attack
time constant and CR sets the recovery time
constant. *5COL
R4
+VB
R3
17.3k
–
C
IN1
V
IN
2.2µF
+
A1
C
IN2
(7,9)
R1
6.8k
∆G
(5,11)
A2
(6,10) R6
V
REF
1k
(2,14)
V
OUT
R5
100k
BUFFER
C
IN3
2.2µF
R2
3.3k
(3,13)
(4,12)
C1
2.2µF
CA CR
1µF 10µF
(8)
(16)
+V
CC
Figure 4. Basic Expandor Schematic
October 7, 1987
7
PHILIPS [ NXP SEMICONDUCTORS ]
