Datasheet

LM13700
SNOSBW2E –NOVEMBER 1999–REVISED MARCH 2013 www.ti.com
Typical Performance Characteristics (continued)
Figure 20. Leakage Current Test Circuit Figure 21. Differential Input Current Test Circuit
Circuit Description
The differential transistor pair Q
4
and Q
5
form a transconductance stage in that the ratio of their collector currents
is defined by the differential input voltage according to the transfer function:
(1)
where V
IN
is the differential input voltage, kT/q is approximately 26 mV at 25°C and I
5
and I
4
are the collector
currents of transistors Q
5
and Q
4
respectively. With the exception of Q
12
and Q
13
, all transistors and diodes are
identical in size. Transistors Q
1
and Q
2
with Diode D
1
form a current mirror which forces the sum of currents I
4
and I
5
to equal I
ABC
:
I
4
+ I
5
= I
ABC
(2)
where I
ABC
is the amplifier bias current applied to the gain pin.
For small differential input voltages the ratio of I
4
and I
5
approaches unity and the Taylor series of the In function
can be approximated as:
(3)
(4)
Collector currents I
4
and I
5
are not very useful by themselves and it is necessary to subtract one current from the
other. The remaining transistors and diodes form three current mirrors that produce an output current equal to I
5
minus I
4
thus:
(5)
The term in brackets is then the transconductance of the amplifier and is proportional to I
ABC
.
Linearizing Diodes
For differential voltages greater than a few millivolts, Equation 3 becomes less valid and the transconductance
becomes increasingly nonlinear. Figure 22 demonstrates how the internal diodes can linearize the transfer
function of the amplifier. For convenience assume the diodes are biased with current sources and the input
signal is in the form of current I
S
. Since the sum of I
4
and I
5
is I
ABC
and the difference is I
OUT
, currents I
4
and I
5
can be written as follows:
(6)
Since the diodes and the input transistors have identical geometries and are subject to similar voltages and
temperatures, the following is true:
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