ncy9100 ON Semiconductor, ncy9100 Datasheet - Page 7

• Manufacturer Part Number: ncy9100
• Description: Compandor
• Manufacturer: ON Semiconductor

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
1.8 V, V
(the emitters of Q
R
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 Q
input swings positive and Q
swings negative. The collector currents will be in error by
the a of Q
respectively. ICs such as this have typical NPN b’s of 200
and PNP b’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 0.13
dB gain error.
(typically 50 nA), will become significant as it must be
supplied by Q
coupling into the rectifier. If an offset voltage exists between
the V
V
cause an input current of 100 nA which will produce twice
1
1
OS
Figure 9 shows the rectifier circuit in more detail. The op
At very low input signal levels the bias current of Q
. The single diode between the bases of Q
), which is shown grounded, is actually tied to the internal
/R
Q
IN
1
1
REF
Figure 9. Simplified Rectifier Schematic
Q
will be generated. A mere 1.0 mV of offset will
input pin and the base of Q
2
Q
I
1
5
. The inverting input is tied to the op amp output,
3
or Q
5
. Another low level error can be caused by DC
NOTE:
Q
V+
5
V−
6
D
4
I
and Q
2
1
on negative or positive signal swings,
I
5
G
and Q
6
+ 2
), and the input summing resistor
Q
6
5
Q
6
V
. Q
conducts when the input
5
IN
R 1
Q
avg
6
7
will conduct when the
C
10kW
2
R
, an error current of
R
10kW
1
R
S
5
and Q
Q
8
V
IN
6
assures
http://onsemi.com
Q
9
2
,
7
the error of the input bias current. For highest accuracy, the
rectifier should be coupled capacitively. At high input levels
the b of the PNP Q
increasing error until the circuit saturates. Saturation can be
avoided by limiting the current into the rectifier input to
250 mA. If necessary, an external resistor may be placed in
series with R
shows the rectifier accuracy vs. input level at a frequency of
1.0 kHz.
fall off. The roll−off will be more pronounced at lower input
levels due to the increasing amount of gain required to
switch between Q
frequency response for input levels of 0 dBm, −20 dBm, and
−40 dBm is shown in Figure 11. The response at all three
levels is flat to well above the audio range.
At very high frequencies, the response of the rectifier will
Figure 11. Rectifier Frequency Response vs.
0
3
+1
−1
1
Figure 10. Rectifier Accuracy
0
to limit the current to this value. Figure 10
6
will begin to suffer, and there will be an
RECTIFIER INPUT dBm
10k
5
−40dBm
FREQUENCY (Hz)
−40
or Q
Input Level
6
−20
conducting. The rectifier
INPUT = 0dBm
1MEG
0
−20dBm