ncy9100 ON Semiconductor, ncy9100 Datasheet - Page 8

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

Variable Gain Cell
linearized two−quadrant transconductance multiplier. Q
Q
gain control pair, Q
a current mirror provides the output current.
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
yield the multiplier transfer function,
V
2
1
1
Figure 12 is a diagram of the variable gain cell. This is a
IN
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
and the op amp provide a predistorted drive signal for the
along with the current I
and Q
20k
R
2
140mA
Figure 12. Simplified DG Cell Schematic
I
IN
2
I
1
by providing the proper drive to the base of Q
IN
Q
NOTE:
I
1
2
(= V
I
− (I
OUT
I
I
3
C1
C2
REF
and Q
I
2
280mA
1
Q
IN
G
+
(= 2I
I
+
+ I
OUT
2
) by controlling the base of Q
/R
= I
IN
I
I
4
I
I
2
C4
C3
1
C3
G
1
. The gain is controlled by I
1
) = I
+
)
) is thus forced to flow through
, so I
I
+ I
IN
+
I
1
I
G
1
+
1
−
, the current through Q
C4
+
− I
I
C1
I
I
V+
V−
IN
1
1
and I
IN
) I
* I
V
= I
R
+
= I
IN
2
1
IN
IN
1
I
I
I
OUT
and Q
I
C2
G
G
1
+ I
2
V
R
.
3
IN
IN
2
= I
and Q
Q
. Since I
2
, under large
3
C4
− I
I
4
G
Q
. When
http://onsemi.com
C3
4
2
G
. The
2
2
1
will
and
has
is:
at
1
2
,
.
8
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 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
this, which means our overall offsets are typically about mV.
The distortion is not affected by the magnitude of the gain
control current, and it does not increase as the gain is
changed. This second harmonic distortion could be
eliminated by making perfect transistors, but since that
would be difficult, we have had to resort to other methods.
A trim pin has been provided to allow trimming of the
internal offsets to zero, which effectively eliminated second
harmonic distortion. Figure 14 shows the simple trim
network required.
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
.34
4
3
2
1
Figure 14. THD Trim Network
To THD Trim
INPUT LEVEL (dBm)
−6
≈200pF
6.2kW
0
V
CC
20kW
3.6V
+6
R
4mV
3mV
2mV
1mV