NE570D(2003) Просмотр технического описания (PDF) - Philips Electronics
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NE570D Datasheet PDF : 12 Pages
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Philips Semiconductors
Compandor
Product data
NE570
Figure 8 shows the hook-up for a compressor. This is essentially an
expander placed in the feedback loop of the op amp. The ∆G cell is
set-up to provide AC feedback only, so a separate DC feedback
loop is provided by the two RDC and CDC. The values of RDC will
determine the DC bias at the output of the op amp. The output will
bias to:
VOUT
DC
+
1)
RDC1 ) RDC2
R4
ǒ Ǔ VREF +
1
)
RDC
30
TOT
k
1.8 V
The output of the expander will bias up to:
VOUT DC
+
1
)
R3
R4
VREF
ǒ Ǔ VREF +
1
)
20
30
k
k
1.8 V + 3.0 V
The output will bias to 3.0 V when the internal resistors are used.
External resistors may be placed in series with R3, (which will affect
the gain), or in parallel with R4 to raise the DC bias to any desired
value.
R2
∆G
R1
*RDC
*CIN R3
VIN
–
R4 VREF +
NOTES:
( ) GAIN =
R1 R2 IB
2 R3 VIN (avg.)
IB = 140 µA
* EXTERNAL COMPONENTS
*CRECT
*CF
*RDC
*CDC
VOUT
SR02511
Figure 8. Basic compressor
CIRCUIT DETAILS—RECTIFIER
Figure 9 shows the concept behind the full-wave averaging rectifier.
The input current to the summing node of the op amp, VINR1, is
supplied by the output of the op amp. If we can mirror the op amp
output current into a unipolar current, we will have an ideal rectifier.
The output current is averaged by R5, CR, which set the averaging
time constant, and then mirrored with a gain of 2 to become IG, the
gain control current.
V+
I = VIN/R1
R1
VIN
–
+
R5
10 kΩ
CR
IG
Figure 9. Rectifier concept
SR02512
Figure 10 shows the rectifier circuit in more detail. The op amp is a
one-stage op amp, biased so that only one output device is on at a
time. The non-inverting input, (the base of Q1), which is shown
grounded, is actually tied to the internal 1.8 V VREF. The inverting
input is tied to the op amp output, (the emitters of Q5 and Q6), and
the input summing resistor R1. The single diode between the bases
of Q5 and Q6 assures that only one device is on at a time. To detect
the output current of the op amp, we simply use the collector currents
of the output devices Q5 and Q6. Q6 will conduct when the input
swings positive and Q5 conducts when the input swings negative.
The collector currents will be in error by the α of Q5 or Q6 on negative
or positive signal swings, respectively. ICs such as this have typical
NPN βs of 200 and PNP βs of 40. The α’s of 0.995 and 0.975 will
produce errors of 0.5% on negative swings and 2.5% on positive
swings. The 1.5% average of these errors yields a mere 0.13 dB
gain error.
V+
Q3
Q7
Q4
Q1 Q2
I1
Q5
R1
D1
10 kΩ
VIN
R5
10 kΩ
Q6
I2
Q8
Q9
CR
NOTE:
V–
VIN avg
IG = 2 R1
SR02513
Figure 10. Simplified rectifier schematic
2003 Apr 03
6
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