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APPLICATIONINFORMATION
gainsettingresistors,r
F
,r
i,
andR
增益
R
F
R
I
或者 增益 (db)
20 log
R
F
R
I
(1)
有效的 阻抗
R
F
R
I
R
F
R
I
(2)
f
c
1
2
R
F
C
F
(3)
inputcapacitor,c
I
f
c
1
2
R
I
C
I
(4)
C
I
1
2
R
I
f
c
(5)
TPA6100A2D
SLOS269B–JUNE2000–REVISEDSEPTEMBER2004
ThevoltagegainfortheTPA6100A2DissetbyresistorsR
F
andR
I
accordingtoequation1.
giventhatthetpa6100a2disanmosamplifier,theinputimpedanceishigh.consequently,inputleakage
currentsarenotgenerallyaconcern,althoughnoiseinthecircuitincreasesasthevalueofr
F
增加.在
增加,acertainrangeofr
F
valuesisrequiredforproperstart-upoperationoftheamplifier.takentogether,它
isrecommendedthattheeffectiveimpedanceseenbytheinvertingnodeoftheamplifierbesetbetween5k
Ω
and20k
Ω
.theeffectiveimpedanceiscalculatedinequation2.
asanexample,consideraninputresistanceof20k
Ω
andafeedbackresistorof20k
Ω
.thegainoftheamplifier
wouldbe–1andtheeffectiveimpedanceattheinvertingterminalwouldbe10k
Ω
,whichiswithinthe
recommendedrange.
forhigh-performanceapplications,metalfilmresistorsarerecommendedbecausetheytendtohavelowernoise
levelsthancarbonresistors.forvaluesofr
F
above50k
Ω
,theamplifiertendstobecomeunstableduetoapole
formedfromR
F
andtheinherentinputcapacitanceofthemosinputstructure.forthisreason,asmall
compensationcapacitorofapproximately5pFshouldbeplacedinparallelwithR
F
.ineffect,thiscreatesa
低-passfilternetworkwiththecutofffrequencydefinedinequation3.
forexample,ifr
F
is100k
Ω
andC
F
is5pf,thenf
c
is318khz,whichiswelloutsidetheaudiorange.
formaximumsignalswingandoutputpoweratlowsupplyvoltageslike1.6vto3.3v,bypassisbiasedto
V
DD
/4.不管怎样,toallowtheoutputtobebiasedatv
DD
/2,aresistor,r,equaltor
F
mustbeplacedfromthe
negativeinputtoground.
inthetypicalapplication,aninputcapacitor,c
I
,isrequiredtoallowtheamplifiertobiastheinputsignaltothe
properdclevelforoptimumoperation.inthiscase,c
I
andR
I
formahigh-passfilterwiththecornerfrequency
determinedinequation4.
ThevalueofC
I
isimportanttoconsider,asitdirectlyaffectsthebass(低-频率)performanceofthecircuit.
ConsidertheexamplewhereR
I
is20k
Ω
andthespecificationcallsforaflatbassresponsedownto20hz.
equation4isreconfiguredasequation5.
inthisexample,c
I
is0.4µf,soonewouldlikelychooseavalueintherangeof0.47µfto1µf.afurther
considerationforthiscapacitoristheleakagepathfromtheinputsourcethroughtheinputnetwork(r
I
,c
I
)和
thefeedbackresistor(r
F
)totheload.thisleakagecurrentcreatesadcoffsetvoltageattheinputtotheamplifier
thatreducesusefulheadroom,especiallyinhigh-gainapplications(>10).forthisreasonalow-leakagetantalum
orceramiccapacitoristhebestchoice.whenpolarizedcapacitorsareused,thepositivesideofthecapacitor
shouldfacetheamplifierinputinmostapplications,asthedclevelthereisheldatv
DD
/4,whichislikelyhigher
thanthesourcedclevel.itisimportanttoconfirmthecapacitorpolarityintheapplication.
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