NE570 Просмотр технического описания (PDF) - ON Semiconductor

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NE570

Compandor

ON Semiconductor 

NE570

this, which means our overall offsets are typically about mV.

The distortion is not affected by the magnitude of the gain

control current, and it does not increase as the gain is

changed. This second harmonic distortion could be

eliminated by making perfect transistors, but since that

would be difficult, we have had to resort to other methods.

A trim pin has been provided to allow trimming of the

internal offsets to zero, which effectively eliminated second

harmonic distortion. Figure 14 shows the simple trim

network required.

VCC

Control signal feedthrough is generated in the gain cell by

imperfect device matching and mismatches in the current

sources, I1 and I2. When no input signal is present, changing

IG will cause a small output signal. The distortion trim is

effective in nulling out any control signal feedthrough, but

in general, the null for minimum feedthrough will be

different than the null in distortion. The control signal

feedthrough can be trimmed independently of distortion by

tying a current source to the DG input pin. This effectively

trims I1. Figure 16 shows such a trim network.

VCC

6.2 kW

To THD Trim

≈200 pF

R

3.6 V

20 kW

R−SELECT FOR

100 kW

3.6 V

470 kW

TO PIN 3 OR 14

Figure 14. THD Trim Network

Figure 15 shows the noise performance of the DG cell. The

maximum output level before clipping occurs in the gain cell

is plotted along with the output noise in a 20 kHz bandwidth.

Note that the noise drops as the gain is reduced for the first

20 dB of gain reduction. At high gains, the signal to noise

ratio is 90 dB, and the total dynamic range from maximum

signal to minimum noise is 110 dB.

+20

0

MAXIMUM

−20

SIGNAL LEVEL

−40

110 dB

90 dB

−60

−80

−100

−40

NOISE IN

20 kHz BW

−20

0

VCA GAIN (dB)

Figure 15. Dynamic Range

Figure 16. Control Signal Feedthrough

OPERATIONAL AMPLIFIER

The main op amp shown in the chip block diagram is

equivalent to a 741 with a 1.0 MHz bandwidth. Figure 17

shows the basic circuit. Split collectors are used in the input

pair to reduce gM, so that a small compensation capacitor of

just 10 pF may be used. The output stage, although capable

of output currents in excess of 20 mA, is biased for a low

quiescent current to conserve power. When driving heavy

loads, this leads to a small amount of crossover distortion.

I1

I2

D1

−IN

Q1 Q2

+IN

D2

CC

Q5

Q4

Q3

Q6

OUT

Figure 17. Operational Amplifier

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