Datasheet

SummingaDifferentialInputSignalanda
DecouplingCapacitor(C
S
)
Gain 1 +
V
O
V
I1
+ *
R
F
R
I1
ǒ
V
V
Ǔ
(6)
Gain 2 +
V
O
V
I2
+ *
R
F
R
I2
ǒ
V
V
Ǔ
(7)
USINGLOW-ESRCAPACITORS
SUMMINGINPUTSIGNALSWITHTHE
DIFFERENTIALOUTPUTVERSUS
SummingTwoDifferentialInputSignals
Gain 1 +
V
O
V
I1
+ *
R
F
R
I1
ǒ
V
V
Ǔ
(4)
Gain 2 +
V
O
V
I2
+ *
R
F
R
I2
ǒ
V
V
Ǔ
(5)
TPA6205A1
SLOS490BJULY2006REVISEDJUNE2008..............................................................................................................................................................
www.ti.com
(R
I
,C
I
)andthefeedbackresistor(R
F
)totheload.
Single-EndedInputSignal
Thisleakagecurrentcreatesadcoffsetvoltageatthe
inputtotheamplifierthatreducesusefulheadroom,
Figure31showshowtosumadifferentialinputsignal
especiallyinhighgainapplications.Forthisreason,a
andasingle-endedinputsignal.Groundnoisecan
ceramiccapacitoristhebestchoice.Whenpolarized
coupleinthroughIN+withthismethod.Itisbetterto
capacitorsareused,thepositivesideofthecapacitor
usedifferentialinputs.Toassurethateachinputis
shouldfacetheamplifierinputinmostapplications,
balanced,thesingle-endedinputmustbedrivenbya
asthedclevelthereisheldatV
DD
/2,whichislikely
low-impedancesourceeveniftheinputisnotinuse.
higherthanthesourcedclevel.Itisimportantto
Bothinputnodesmustseethesameimpedancefor
confirmthecapacitorpolarityintheapplication.
optimumperformance,thustheuseofR
P
andC
P
.
TheTPA6205A1isahigh-performanceCMOSaudio
amplifierthatrequiresadequatepowersupply
decouplingtoensuretheoutputtotalharmonic
distortion(THD)isaslowaspossible.Powersupply
decouplingalsopreventsoscillationsforlonglead
Where
lengthsbetweentheamplifierandthespeaker.For
C
P
=C
I1
//C
I2
higherfrequencytransients,spikes,ordigitalhashon
R
P
=R
I1
//R
I2
theline,agoodlowequivalent-series-resistance
(ESR)ceramiccapacitor,typically0.1µFto1µF,
placedascloseaspossibletothedeviceV
DD
lead
worksbest.Forfilteringlowerfrequencynoise
Low-ESRcapacitorsarerecommendedthroughout
signals,a10-µForgreatercapacitorplacednearthe
thisapplicationssection.Areal(asopposedtoideal)
audiopoweramplifieralsohelps,butisnotrequired
capacitorcanbemodeledsimplyasaresistorin
inmostapplicationsbecauseofthehighPSRRofthis
serieswithanidealcapacitor.Thevoltagedrop
device.
acrossthisresistorminimizesthebeneficialeffectsof
thecapacitorinthecircuit.Thelowertheequivalent
valueofthisresistancethemoretherealcapacitor
TPA6205A1
behaveslikeanidealcapacitor.
MostwirelessphonesorPDAsneedtosumsignals
attheaudiopoweramplifierorjusthavetwosignal
sourcesthatneedseparategain.TheTPA6205A1
SINGLE-ENDEDOUTPUT
makesiteasytosumsignalsoruseseparatesignal
Figure32showsaClass-ABaudiopoweramplifier
sourceswithdifferentgains.Manyphonesnowuse
(APA)inafullydifferentialconfiguration.The
thesamespeakerfortheearpieceandringer,where
TPA6205A1amplifierhasdifferentialoutputsdriving
thewirelessphonewouldrequireamuchlowergain
bothendsoftheload.Thereareseveralpotential
forthephoneearpiecethanfortheringer.PDAsand
benefitstothisdifferentialdriveconfiguration,but
phonesthathavestereoheadphonesrequire
initiallyconsiderpowertotheload.Thedifferential
summingoftherightandleftchannelstooutputthe
drivetothespeakermeansthatasonesideis
stereosignaltothemonospeaker.
slewingup,theothersideisslewingdown,andvice
versa.Thisineffectdoublesthevoltageswingonthe
loadascomparedtoagroundreferencedload.
Twoextraresistorsareneededforsumming
Plugging2×V
O(PP)
intothepowerequation,where
differentialsignals(atotalof10components).The
voltageissquared,yieldstheoutputpowerfrom
gainforeachinputsourcecanbesetindependently
thesamesupplyrailandloadimpedance(see
(seeEquation4andEquation5,andFigure30).
Equation8).
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