SA571
INTRODUCTION
Much interest has been expressed in high performance
electronic gain control circuits. For non−critical
applications,
an
integrated
circuit
operational
transconductance amplifier can be used, but when
high−performance is required, one has to resort to complex
discrete circuitry with many expensive, well−matched
components. This paper describes an inexpensive integrated
circuit, the SA571 Compandor, which offers a pair of high
performance gain control circuits featuring low distortion
(<0.1%), high signal−to−noise ratio (90 dB), and wide
dynamic range (110 dB).
Circuit Background
requires a simple full−wave averaging rectifier with good
accuracy, since the rectifier accuracy determines the (input)
output level tracking accuracy. The gain cell determines the
distortion and noise characteristics, and the phone system
specifications here are very loose. These specs could have
been met with a simple Operational Transconductance
Multiplier, or OTA, but the gain of an OTA is proportional
to temperature and this is very undesirable. Therefore, a
linearized transconductance multiplier was designed which
is insensitive to temperature and offers low noise and low
distortion performance. These features make the circuit
useful in audio and data systems as well as in
telecommunications systems.
Basic Hook−up and Operation
The SA571 Compandor was originally designed to satisfy
the requirements of the telephone system. When several
telephone channels are multiplexed onto a common line, the
resulting signal−to−noise ratio is poor and companding is
used to allow a wider dynamic range to be passed through
the channel. Figure 4 graphically shows what a compandor
can do for the signal−to−noise ratio of a restricted dynamic
range channel. The input level range of +20 to −80 dB is
shown undergoing a 2−to−1 compression where a 2.0 dB
input level change is compressed into a 1.0 dB output level
change by the compressor. The original 100 dB of dynamic
range is thus compressed to a 50 dB range for transmission
through a restricted dynamic range channel. A
complementary expansion on the receiving end restores the
original signal levels and reduces the channel noise by as
much as 45 dB.
The significant circuits in a compressor or expander are
the rectifier and the gain control element. The phone system
COMPRESSION
EXPANSION
Figure 5 shows the block diagram of one half of the chip,
(there are two identical channels on the IC). The full−wave
averaging rectifier provides a gain control current, I
G
, for the
variable gain (DG) cell. The output of the
DG
cell is a current
which is fed to the summing node of the operational
amplifier. Resistors are provided to establish circuit gain and
set the output DC bias.
The circuit is intended for use in single power supply
systems, so the internal summing nodes must be biased at
some voltage above ground. An internal band gap voltage
reference provides a very stable, low noise 1.8 V reference
denoted V
REF
. The non−inverting input of the op amp is tied
to V
REF
, and the summing nodes of the rectifier and
DG
cell
(located at the right of R
1
and R
2
) have the same potential.
The THD trim pin is also at the V
REF
potential.
THD TRIM
OUTPUT
LEVEL
−20
0dB
8,9
R
3
G
IN
3,14
RECT
IN
R
2
20kW
R
1
R
3
INV
IN
6,11
5,12
INPUT
LEVEL
+20
0dB
20kW
DG
IG
R
4
30kW
V
REF
1.8V
V
CC
PIN 13
1,16
C
RECT
GND PIN 4
−
+
OUTPUT
7,10
−40
NOISE
−80
−40
2,15 10kW
−80
Figure 4. Restricted Dynamic Range Channel
Figure 5. Chip Block Diagram (1 of 2 Channels)
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