UTC571
LINEAR INTEGRATED CIRCUIT
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 UTC571 Compandor, which offers a pair of high
performance gain control circuits featuring low distortion (<0.1%), high signal-to-noise ratio (90dB), and wide
dynamic range (110dB).
CIRCUIT BACKGROUND
The UTC571 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 1 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 -80dB is shown undergoing a 2-to-1 compression where a 2dB input level change is compressed into a
1dB output level change by the compressor. The original 100dB of dynamic range is thus compressed to a 50dB
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 45dB.
INPUT
LEVEL
+20
OUTPUT
LEVEL
+20
0d
B
0dB
-40
-80
-40
-80
NOISE
Figure 1. Restricted Dynamic Range Channel
The significant circuits in a compressor or expander are the rectifier and the gain control element. The phone
system 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 Tran conductance 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.
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YOUW ANG ELECTRONICS CO.LTD
UTC [ Unisonic Technologies ]
