UTC571 参数 Datasheet PDF下载

UTC571

INTRODUCTION

LI

NEAR I

NTEGRATED CI

RCUI

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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.

COMPRESSION

INPUT

LEVEL

+20

0dB

EXPANSION

OUTPUT

LEVEL

+20

0d

B

-40

-40

NOISE

-80

-80

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.

YOUW ANG ELECTRONI CO.

CS

LTD

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