LT6015/LT6016/LT6017
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601567ff
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applicaTions inForMaTion
(Over-The-Top operation), Q9 begins to turn on diverting
bias current away from the common emitter differential
input pair to the current mirror consisting of Q11 and Q12.
The current from Q12 will bias the common base differential
input pair consisting of Q3 to Q6. Because the Over-The-Top
input pair is operating in a common base configuration,
the input bias current will increase to about 14μA. Both
input stages have their voltage offsets trimmed tightly and
are specified in the Electrical Characteristics table.
The inputs are protected against temporary excursions to
as much as 25V below V– by internal 1k resistor in series
with each input and a diode from the input to the negative
supply. Adding additional external series resistance will
extend the protection beyond 25V below V–. The input
stage of the LT6015/LT6016/LT6017 incorporates phase
reversal protection to prevent the output from phase
reversing for inputs below V–.
There are no clamping diodes between the inputs. The
inputs may be over-driven differentially to 80V without
damage, or without drawing appreciable input current.
Figure 1 summarizes the kind of faults that may be applied
to the LT6015/LT6016/LT6017 without damage.
Over-The-Top Operation Considerations
When the input common mode of the LT6015/LT6016/
LT6017 is biased near or above the V+ supply, the amplifier
is said to be operating in the Over-The-Top configuration.
The differential input pair which control amplifier operation
is common base pair Q3 to Q6 (refer to the Simplified
Schematic). If the input common mode is biased between
V– and approximately 1.5V below V+, the amplifier is said
to be operating in the normal configuration. The differential
input pair which control amplifier operation is common
emitter pair Q1 and Q2.
A plot of the Over-The-Top Transition region vs Temperature
(the region between normal operation and Over-The-Top
operation) on a 5V single supply is shown in Figure 2.
Some implications should be understood about Over-
The-Top operation. The first, and most obvious is the
input bias currents change from under ±2nA in normal
operation to 14µA in Over-The-Top operation as the input
stage transitions from common emitter to common base.
Even though the Over-The-Top input bias currents run
around 14 µA, they are very well matched and their offset
is typically under ±100nA.
The second and more subtle change to amplifier operation
is the differential input impedance which decreases from
1MΩ in normal operation, to approximately 3.7kΩ in
Over-The-Top operation (specified as RIN in the Electrical
Characteristics table). This resistance appears across the
summing nodes in Over-The-Top operation and is due to
the common base input stage configuration. Its value is
easily derived from the specified input bias current flowing
into the op amp inputs and is equal to 2 • k • T/(q • Ib)
(k-Boltzmann’s constant, T – operating temperature,
Ib-operating input bias current of the amplifier in the
Over-The-Top region). And because the inputs are biased
proportional to absolute temperature, it is relatively
constant with temperature. The user may think this
effective resistance is relatively harmless because it
appears across the summing nodes which are forced
Figure 2. LT6016/LT6017 Over-The-Top Transition Region vs
Temperature
TEMPERATURE (°C)
–50
VCM (V)
5.0
4.5
3.5
2.5
4.0
3.0
2.0
1.5
1.0
0.5
0500 100 125
601567 F02
15025–25 75
VS = 5V
TRANSISTION REGION
TYPICAL COMMON MODE VOLTAGE
FOR ONSET OF OVER-THE-TOP
OPERATION
TYPICAL COMMON MODE VOLTAGE
WHERE OVER-THE-TOP OPERATION
FULLY ON