NE571 Philipss, NE571 Datasheet - Page 6

• Manufacturer Part Number: NE571
• Description: Compandor
• Manufacturer: Philipss

Philips Semiconductors
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, V
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 R
time constant, and then mirrored with a gain of 2 to become I
gain control current.
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 Q
grounded, is actually tied to the internal 1.8V V
input is tied to the op amp output, (the emitters of Q
the input summing resistor R
of Q
the output current of the op amp, we simply use the collector
currents of the output devices Q
input swings positive and Q
negative. The collector currents will be in error by the a of Q
on negative or positive signal swings, respectively. ICs such as this
1997 Aug 14
V
Compandor
IN
C
5
IN
and Q
R
R
3
6
4
assures that only one device is on at a time. To detect
V
IN
V
REF
R
R
1
Figure 8. Basic Compressor
DC
Figure 9. Rectifier Concept
*
G
I = V
5
C
C
1
DC
NOTES:
RECT
conducts when the input swings
IN
GAIN
I
External components
. The single diode between the bases
B
*
R
/ R
C
R
= 140 A
5
DC
R
R
2
*
and Q
1
1
*
5
, CR, which set the averaging
2R 3 V INavg
R 1 R 2 I B
6
V
V+
. Q
OUT
C
10k
R
F
6
*
1
S
), which is shown
will conduct when the
REF
1
2
I
. The inverting
G
5
and Q
IN
R
6
SR00684
1
SR00682
5
), and
, is
G
or Q
, the
6
6
PNP Q
At very high frequencies, the response of the rectifier will fall off. The
0dBm, -20dBm, and -40dBm is shown in Figure 12. The response at
have typical NPN s of 200 and PNP s of 40. The a ’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
0.13dB gain error.
At very low input signal levels the bias current of Q
50nA), will become significant as it must be supplied by Q
low level error can be caused by DC coupling into the rectifier. If an
offset voltage exists between the V
an error current of V
will cause an input current of 100nA which will produce twice the
error of the input bias current. For highest accuracy, the rectifier
should be coupled into capacitively. At high input levels the
the circuit saturates. Saturation can be avoided by limiting the
current into the rectifier input to 250 A. If necessary, an external
resistor may be placed in series with R
value. Figure 11 shows the rectifier accuracy vs input level at a
frequency of 1kHz.
roll-off will be more pronounced at lower input levels due to the
increasing amount of gain required to switch between Q
conducting. The rectifier frequency response for input levels of
all three levels is flat to well above the audio range.
NOTE:
6
I G
Q
will begin to suffer, and there will be an increasing error until
1
Q
Figure 10. Simplified Rectifier Schematic
2
Q
I
2
1
3
V IN avg
+1
–1
0
R 1
Figure 11. Rectifier Accuracy
OS
Q
V+
4
V–
D
/R
I
2
1
–40
1
RECTIFIER INPUT dBm
will be generated. A mere 1mV of offset
Q
–20
IN
6
Q
input pin and the base of Q
5
1
to limit the current to this
Q
7
C
0
R
10k
R
10k
1
R
Product specification
S
2
, (typically
Q
SA571
8
5
V
or Q
5
IN
. Another
SR00685
SR00683
of the
6
Q
2
9
,