Philips Semiconductors

Product specification

SA571

Compandor

1997 Aug 14

8

VCA GAIN (0dB)

+20

0

–20

–40

–60

–80

–100

–40

–20

0

MAXIMUM

SIGNAL LEVEL

NOISE IN

20kHz BW

90dB

110dB

SR00690

Figure 16. Dynamic Range

Control signal feedthrough is generated in the gain cell by imperfect

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

shows such a trim network.

R-SELECT FOR

3.6V

470k

TO PIN 3 OR 14

100k

VCC

SR00691

Figure 17. Control Signal Feedthrough

OPERATIONAL AMPLIFIER

The main op amp shown in the chip block diagram is equivalent to a

741 with a 1MHz bandwidth. Figure 18 shows the basic circuit. Split

compensation capacitor of just 10pF may be used. The output

stage, although capable of output currents in excess of 20mA, is

biased for a low quiescent current to conserve power. When driving

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

Q1

Q2

Q4

Q3

I1

I2

Q6

D1

D2

Q2

CC

+IN

–IN

OUT

SR00692

Figure 18. Operational Amplifier

RESISTORS

Inspection of the gain equations in Figures 7 and 8 will show that the

basic compressor and expander circuit gains may be set entirely by

resistor ratios and the internal voltage reference. Thus, any form of

resistors that match well would suffice for these simple hook-ups,

and absolute accuracy and temperature coefficient would be of no

importance. However, as one starts to modify the gain equation with

external resistors, the internal resistor accuracy and tempco become

very significant. Figure 19 shows the effects of temperature on the

diffused resistors which are normally used in integrated circuits, and

the ion-implanted resistors which are used in this circuit. Over the

critical 0

°C to +70°C temperature range, there is a 10-to-1 improve-

ment in drift from a 5% change for the diffused resistors, to a 0.5%

change for the implemented resistors. The implanted resistors have

another advantage in that they can be made the size of the diffused

resistors due to the higher resistivity. This saves a significant

amount of chip area.

1% ERROR

TEMPERATURE

1.15

1.10

1.05

1.00

.95

–40

0

40

80

120

LOW TC

IMPLANTED

RESISTOR

BAND

1k

Ω /

140

Ω /

DIFFUSED

RESISTOR

SR00693

Figure 19. Resistance vs Temperature