Application Hints

The LF411 series of internally trimmed JFET input op amps

anteed input offset voltage drift. These JFETs have large re-

verse breakdown voltages from gate to source and drain

eliminating the need for clamps across the inputs. Therefore,

large differential input voltages can easily be accommodated

without a large increase in input current. The maximum dif-

ferential input voltage is independent of the supply voltages.

However, neither of the input voltages should be allowed to

exceed the negative supply as this will cause large currents

to flow which can result in a destroyed unit.

Exceeding the negative common-mode limit on either input

will force the output to a high state, potentially causing a re-

versal of phase to the output. Exceeding the negative

common-mode limit on both inputs will force the amplifier

output to a high state. In neither case does a latch occur

since raising the input back within the common-mode range

again puts the input stage and thus the amplifier in a normal

operating mode.

Exceeding the positive common-mode limit on a single input

will not change the phase of the output; however, if both in-

puts exceed the limit, the output of the amplifier may be

forced to a high state.

The amplifier will operate with a common-mode input voltage

equal to the positive supply; however, the gain bandwidth

and slew rate may be decreased in this condition. When the

negative common-mode voltage swings to within 3V of the

negative supply, an increase in input offset voltage may oc-

cur.

The LF411 is biased by a zener reference which allows nor-

ages less than these may result in lower gain bandwidth and

slew rate.

The LF411 will drivea2k

full temperature range. If the amplifier is forced to drive

heavier load currents, however, an increase in input offset

voltage may occur on the negative voltage swing and finally

reach an active current limit on both positive and negative

swings.

Precautions should be taken to ensure that the power supply

for the integrated circuit never becomes reversed in polarity

or that the unit is not inadvertently installed backwards in a

socket as an unlimited current surge through the resulting

forward diode within the IC could cause fusing of the internal

conductors and result in a destroyed unit.

As with most amplifiers, care should be taken with lead

dress, component placement and supply decoupling in order

to ensure stability. For example, resistors from the output to

an input should be placed with the body close to the input to

minimize “pick-up” and maximize the frequency of the feed-

back pole by minimizing the capacitance from the input to

ground.

A feedback pole is created when the feedback around any

amplifier is resistive. The parallel resistance and capacitance

from the input of the device (usually the inverting input) to AC

ground set the frequency of the pole. In many instances the

frequency of this pole is much greater than the expected

3 dB frequency of the closed loop gain and consequently

there is negligible effect on stability margin. However, if the

feedback pole is less than approximately 6 times the ex-

pected 3 dB frequency, a lead capacitor should be placed

Large Signal Inverting

DS005655-41

Large Signal Non-Inverting

DS005655-42

Current Limit (R

DS005655-43

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