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LT6015/LT6016/LT6017

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.

ThecurrentfromQ12willbiasthecommonbasedifferential

inputpairconsistingofQ3toQ6.BecausetheOver-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/

LT6017isbiasednearorabovetheV+supply,theamplifier

is said to be operating in the Over-The-Top configuration.

Thedifferentialinputpairwhichcontrolamplifieroperation

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

tobeoperatinginthenormalconfiguration. Thedifferential

input pair which control amplifier operation is common

emitter pair Q1 and Q2.

AplotoftheOver-The-TopTransitionregionvsTemperature

(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

5.0

4.5

3.5

2.5

4.0

3.0

2.0

1.5

1.0

0.5

100

125

601567 F02

150

–25

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

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