LF155 - LF156 - LF157
APPLICATION HINTS
in a socket as an unilimited current surge throughthe
resultingforward diode within the IC couldcausefusin-
goftheinternalconductorsandresultina destroyedunit.
Because these amplifiers are JFET rather than MOS-
FET input op amps they do not require special han-
dling.
AllofthebiascurrentsintheseamplifiersaresetbyFET
current sources. The drain currents for the amplifiers
are therefore essentially independent of supply volt-
ages.
Aswith most amplifiers, care should betakenwith lead
dress, componentsplacement and supply decoupling
in orderto ensure stability. Forexample, resistorsfrom
the output to an input should be placedwith the body
close to theinputto minimiz ”pickup”andmaximize the
frequencyof the feedbackpole by minimizing the ca-
pacitancefromthe input to ground.
A feedbackpole is createdwhen the feedbackaround
any amplifier is resistive. The parallel resistance and
capacitancefromtheinput of thedevice(usuallythe in-
vertinginput)toacgroundsetthefrequencyofthepole.In
many instances the frequency of this pole is much
greaterthanthe expected3 dBfrequencyof the closed
loopgainand consequentlythereisnegligible effect on
stability margin. However, if the feedback pole is less
than approximately six time the expected 3 dB fre-
quencyaleadcapacitorshould be placed from the out-
putto the inputof the op amp. The value of that added
capacitorshould be such that the RC time constant of
thiscapacitorand theresistance it parallels is greater
than or equal to the original feedbackpole time con-
stant.
The LF155, LF156, LF157 series areop amps with J-
FETinput transistors. TheseJFETs havelarge reverse
breakdown voltagesfromgatetosource or drain elimi-
natingtheneed of clampsacrossthe inputs.Therefore
large differential input voltagescan easily be accom-
modatedwithoutalarge increaseof inputcurrents.The
maximum differential input voltage is independent of
the supplyvoltage.However, neitherof thenegativein-
put voltagesshouldbe allowed to exceedthenegative
supply as this will cause large currents to flow which
can result in a destroyedunit. Exceeding the negative
common-modelimit oneitherinputwillcauseareversal
of thephasetotheoutputandforce the amplifier output
to the correspondinghigh or lowstate. Exceedingthe
negativecommon-mode limit on bothinputs will force
theamplifieroutputtoa highstate.Inneithercasedoes
a latch occur since raising the input back within the
common-mode range again puts the input stage and
thustheamplifierin a normaloperatingmode.Exceed-
ingthepositivecommon-modelimit onasingleinputwill
not changethephase of the output however, if bothin-
putsexceedthe limit, theoutput of theamplifier will be
forcedtoahighstate.Theseamplifierswill operatewith
the common-mode input voltageequal to the positive
supply. In fact, the common-modevoltagecanex-
ceedthepositivesupplybyapproximately100mV inde-
pendentof supply volt-age and over thefull operat-
ingtemperaturerange.The positive suplly can there-
forebeusedasa referenceonaninputas, forexample,
in asupplycurrent monitorand/orlimiter. Precautions-
shouldbe takentoensurethat thepowersupplyforthe
integratedcircuit neverbecomesre-versedin polarity
orthatthe unit isnot inadvertentlyin-stalledbackwards
5/14
STMICROELECTRONICS [ ST ]
