NE570 ON Semiconductor, NE570 Datasheet - Page 7

• Manufacturer Part Number: NE570
• Description: Compandor
• Manufacturer: ON Semiconductor

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
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.
VARIABLE GAIN CELL
linearized two−quadrant transconductance multiplier. Q
Q
gain control pair, Q
a current mirror provides the output current.
NOTE:
I
2
V
OUT =
At very high frequencies, the response of the rectifier will
Figure 12 is a diagram of the variable gain cell. This is a
IN
and the op amp provide a predistorted drive signal for the
Figure 11. Rectifier Frequency Response
Figure 12. Simplified DG Cell Schematic
R
20 kW
I
I
G
1
2
0
3
5
140 mA
I
IN =
I
or Q
IN
I
1
I
I
Q
G
1
3
6
1
and Q
10 k
V
vs. Input Level
R
conducting. The rectifier frequency
IN
Q
2
I
280 mA
2
2
FREQUENCY (Hz)
( = 2 I
4
. The gain is controlled by I
1
)
−
+
V+
V−
INPUT = 0 dBm
−40 dBm
−20 dBm
1 MEG
Q
3
Q
I
G
4
http://onsemi.com
G
and
1
,
NE570
7
ground potential (V
input current I
Q
been set at twice the value of I
Q
This drive signal will be linear for small signals, but very
non−linear for large signals, since it is compensating for the
non−linearity of the differential pair, Q
signal conditions.
signal is applied to the gain control pair, Q
two differential pairs of transistors have the same signal
applied, their collector current ratios will be identical
regardless of the magnitude of the currents. This gives us:
plus the relationships I
will yield the multiplier transfer function,
assumes ideal transistors.
non−linearity is generated, which results in second
harmonic distortion. Figure 13 gives an indication of the
magnitude of the distortion caused by a given input level and
offset voltage. The distortion is linearly proportional to the
magnitude of the offset and the input level. Saturation of the
gain cell occurs at a +8.0 dBm level. At a nominal operating
level of 0 dBm, a 1.0 mV offset will yield 0.34% of second
harmonic distortion. Most circuits are somewhat better than
1
1
The op amp maintains the base and collector of Q
The op amp has thus forced a linear current swing between
The key to the circuit is that this same predistorted drive
This equation is linear and temperature−insensitive, but it
If the transistors are not perfectly matched, a parabolic,
Figure 13. DG Cell Distortion vs. Offset Voltage
along with the current I
and Q
0.34
4
3
2
1
2
by providing the proper drive to the base of Q
I 2 * (I 1 ) I IN) + I 1 * I IN + I C2.
IN
(= V
I
−6
OUT
I
I
INPUT LEVEL (dBm)
C1
C2
REF
IN
+
+
) by controlling the base of Q
G
/R
= I
0
I
I
I
I
2
C4
C3
G
1
1
) is thus forced to flow through
, so I
C3
I
+
IN
1
+ I
, the current through Q
+
C1
I
I
1
1
C4
) I
* I
V
= I
R
IN
and I
2
1
IN
IN
+6
1
I
I
and Q
G
1
+ I
V
4 mV
3 mV
2 mv
1 mV
OS
3
OUT
IN
and Q
= 5 mV
. Since I
2
, under large
= I
4
C4
. When
2
. The
− I
2
2
1
has
is:
C3
at
2
.