NE570

http://onsemi.com

4

13

2, 15

4

1, 16

5, 12

8, 9

7, 10

6, 11

10

mF

0.1

mF

200 pF

8.2 k

W

2.2

mF

−

+

30 k

W

20 k

W

DG

10 k

W

20 k

W

2.2

mF

3, 14

2.2

mF

Figure 3. Typical Test 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 NE570 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

The NE570 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 dB 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

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.

INPUT

LEVEL

OUTPUT

LEVEL

NOISE

+20

0 dB

−40

−80

−20

0 dB

−40

−80

Figure 4. Restricted Dynamic Range Channel