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INPUTRESISTANCE
C
在
R
i
R
f
输入 信号
ƒ
3 dB
1
2
CR
i
(1)
inputcapacitor,c
i
f
c(高通)
1
2
R
i
C
i
−3 db
f
c
(2)
C
i
1
2
R
i
f
c
(3)
TPA6011A4
SLOS392A–FEBRUARY2002–REVISEDJULY2004
eachgainsettingisachievedbyvaryingtheinputresistanceoftheamplifier,whichcanrangefromitssmallest
valuetooversixtimesthatvalue.asaresult,ifasinglecapacitorisusedintheinputhigh-passfilter,这-3db
orcutofffrequencyalsochangesbyoversixtimes.
figure35.resistoroninputforcut-offfrequency
theinputresistanceateachgainsettingisgiveninfigure26.
这-3-dbfrequencycanbecalculatedusingequation1.
inthetypicalapplicationaninputcapacitor(c
i
)isrequiredtoallowtheamplifiertobiastheinputsignaltothe
properdclevelforoptimumoperation.inthiscase,c
i
andtheinputimpedanceoftheamplifier(r
i
)forma
高-passfilterwiththecornerfrequencydeterminedinequation2.
ThevalueofC
i
isimportanttoconsiderasitdirectlyaffectsthebass(lowfrequency)performanceofthecircuit.
ConsidertheexamplewhereR
i
is70k
Ω
andthespecificationcallsforaflat-bassresponsedownto40hz.
equation2isreconfiguredasequation3.
inthisexample,c
i
is56.8nf,soonewouldlikelychooseavalueintherangeof56nfto1µf.afurther
considerationforthiscapacitoristheleakagepathfromtheinputsourcethroughtheinputnetwork(c
i
)andthe
feedbacknetworktotheload.thisleakagecurrentcreatesadcoffsetvoltageattheinputtotheamplifierthat
reducesusefulheadroom,especiallyinhighgainapplications.forthisreason,alow-leakagetantalumor
ceramiccapacitoristhebestchoice.whenpolarizedcapacitorsareused,thepositivesideofthecapacitor
shouldfacetheamplifierinputinmostapplicationsasthedclevelthereisheldatV
DD
/2,whichislikelyhigher
thanthesourcedclevel.notethatitisimportanttoconfirmthecapacitorpolarityintheapplication.
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