LF155 - LF156 - LF157
APPLICATION HINTS
The LF155, LF156, LF157 series are op amps with J-
FETinput transistors. TheseJFETs havelarge reverse
breakdown voltagesfromgatetosource or drain elimi-
natingtheneed of clamps across the inputs.Therefore
large differential input voltages can easily be accom-
modatedwithoutalarge increaseof inputcurrents. The
maximum differential input voltage is independent of
the supply voltage. However, neitherof thenegativein-
put voltagesshouldbe allowed to exceedthe negative
supply as this will cause large currents to flow which
can result in a destroyed unit. Exceeding the negative
common-modelimit on either inputwill causeareversal
of thephasetotheoutputandforce the amplifier output
to the correspondinghigh or lowstate. Exceedingthe
negativecommon-mode limit on bothinputs will force
the amplifier outputto a highstate.In neithercasedoes
a latch occur since raising the input back within the
common-mode range again puts the input stage and
thustheamplifierin a normal operatingmode. Exceed-
ingthepositive common-modelimit on asingle input will
not changethephase of the output however, if bothin-
putsexceedthe limit, theoutput of theamplifier will be
forcedto ahighstate.Theseamplifiers will operatewith
the common-mode input voltage equal to the positive
supply. In fact, the common-modevoltagecanex-
ceedthepositivesupplyby approximately 100mV inde-
pendentof supply volt-age and over thefull operat-
ingtemperaturerange.The positive suplly can there-
forebe used as a referenceonaninput as, forexample,
in a supply current monitor and/orlimiter. Precautions-
shouldbe taken to ensurethat thepowersupply forthe
integrated circuit never becomes re-versed in polarity
or that the unit is not inadvertentlyin-stalledbackwards
in a socket as an unilimited current surge throughthe
resulting forward diode within the IC couldcausefusin-
goftheinternalconductorsandresultin a destroyedunit.
Because these amplifiers are JFET rather than MOS-
FET input op amps they do not require special han-
dling.
Allof thebiascurrentsintheseamplifiersareset by FET
current sources. The drain currents for the amplifiers
are therefore essentially independent of supply volt-
ages.
As with most amplifiers, care should betakenwith lead
dress, components 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 theinput to minimiz ”pickup”and maximize the
frequency of the feedback pole by minimizing the ca-
pacitancefromthe input to ground.
A feedback pole is createdwhen the feedbackaround
any amplifier is resistive. The parallel resistance and
capacitancefromthe input of thedevice(usually the in-
vertinginput)toacgroundsetthefrequencyofthepole.In
many instances the frequency of this pole is much
greaterthanthe expected3 dBfrequencyof the closed
loopgain and consequentlythereisnegligible effect on
stability margin. However, if the feedback pole is less
than approximately six time the expected 3 dB fre-
quencyaleadcapacitor should be placed from the out-
put to the input of the op amp. The value of that added
capacitor should be such that the RC time constant of
this capacitor and the resistance it parallels is greater
than or equal to the original feedback pole time con-
stant.
5/14
STMICROELECTRONICS [ STMICROELECTRONICS ]
