Datasheet NE570 (ON Semiconductor) - 4

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NE570
VCC = 15 V 0.1 mF 10 mF 13 6, 11 20 kW 3, 14 20 kW V DG − 7, 10 1 VO 2.2 + mF VREF 2, 15 10 kW 30 kW V2 2.2 mF 4 1, 16 5, 12 8, 9 2.2 mF 8.2 kW 200 pF
Figure 3. Typical Test Circuit INTRODUCTION
The significant circuits in a compressor or expander are Much interest has been expressed in high performance the rectifier and the gain control element. The phone system electronic gain control circuits. For non−critical applications, requires a simple full−wave averaging rectifier with good an integrated circuit operational transconductance amplifier accuracy, since the rectifier accuracy determines the (input) can be used, but when high−performance is required, one has output level tracking accuracy. The gain cell determines the to resort to complex discrete circuitry with many expensive, distortion and noise characteristics, and the phone system well−matched components. This paper describes an specifications here are very loose. These specs could have inexpensive integrated circuit, the NE570 Compandor, which been met with a simple operational transconductance offers a pair of high performance gain control circuits multiplier, or OTA, but the gain of an OTA is proportional featuring low distortion (<0.1 %), high signal−to−noise ratio to temperature and this is very undesirable. Therefore, a (90 dB), and wide dynamic range (110 dB). linearized transconductance multiplier was designed which is insensitive to temperature and offers low noise and low
CIRCUIT BACKGROUND
distortion performance. These features make the circuit The NE570 Compandor was originally designed to satisfy useful in audio and data systems as well as in the requirements of the telephone system. When several telecommunications systems. 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 ANSION INPUT OUTPUT channel. The input level range of +20 dB to −80 dB is shown LEVEL LEVEL COMPRESSION EXP undergoing a 2−to−1 compression where a 2.0 dB input level +20 −20 change is compressed into a 1.0 dB output level change by the 0 dB 0 dB 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 −40 −40 NOISE expansion on the receiving end restores the original signal levels and reduces the channel noise by as much as 45 dB. −80 −80
Figure 4. Restricted Dynamic Range Channel http://onsemi.com 4