Philips Semiconductors
Product specification
Compandor
SA571
+20
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.
1.15
NORMALIZED RESISTANCE
140Ω /
DIFFUSED
RESISTOR
1.10
1kΩ /
1.05
LOW TC
IMPLANTED
RESISTOR
0
MAXIMUM
SIGNAL LEVEL
90dB
–40
110dB
OUTPUT (dBm)
–20
–60
–80
NOISE IN
20kHz BW
–100
–40
–20
VCA GAIN (0dB)
0
SR00690
Figure 16. Dynamic Range
Control signal feedthrough is generated in the gain cell by imperfect
device matching and mismatches in the current sources, I
1
and I
2
.
When no input signal is present, changing I
G
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
∆G
input pin. This effectively trims I
1
. Figure 17
shows such a trim network.
V
CC
1.00
.95
Figure 19. Resistance vs Temperature
R-SELECT FOR
3.6V
470k
100k
TO PIN 3 OR 14
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
collectors are used in the input pair to reduce g
M
, so that a small
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.
I
1
I
2
Q
6
–IN
Q
1
Q
2
+IN
C
C
Q
2
Q
3
Q
4
D
1
D
2
OUT
SR00692
Figure 18. Operational Amplifier
1997 Aug 14
8
ÇÇÇÇÇÇ
ÇÇÇÇÇÇ
–40
0
40
80
120
TEMPERATURE
1% ERROR
BAND
SR00693
PHILIPS [ NXP SEMICONDUCTORS ]
