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Power Amplifiers
25 FEB 2013.
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Basics ….
• In mathematical terms, if the inpt si!nal is
"enote" as S, the otpt of a perfect
amplifier is X*S, where # is a constant $a
fi%e" nm&er'. (he )*) s+m&ol means
mltiplie" &+).
• -o amplifier "oes e%actl+ the i"eal .
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ont"..
• Bt man+ "o a /er+ !oo" o& if the+ areoperate" within their a"/ertise" power ratin!s .
• tpt si!nal of all amplifiers contain a""itional$nwante"' components that are not present in
the inpt si!nal these a""itional characteristics
ma+ &e lmpe" to!ether an" are !enerall+nown as distortion.
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ont"….
• Power amplifiers !et the necessar+ ener!+
for amplification of inpt si!nals from the
A wall otlet to which the+ are pl!!e"
into.
• If +o ha" a perfect amplifier , all of the
ener!+ the amplifier too from the A
otlet wol" &e con/erte" to sefl otpt
$to the speaers'
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ont" ….
• Power is not reall+ somethin! that can &e4amplifie". Voltage an" current can &e
amplifie".
• (he term 4power amplifier altho!h technicall+
incorrect has &ecome n"erstoo" to mean an
amplifier that is inten"e" to "ri/e a loa" $schas a speaer, a motor, etc'.
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Fnctional &locs of an amplifier
• All power amplifiers ha/e
1.A Power supply
2.An input stage
3.An output stage
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1.Power Supply
• (he primar+ prpose of a power sppl+ in a
power amplifier is to tae the 120 6 A power
from the otlet an" con/ert it to a 7 /olta!e.
• (he /er+ &est of amplifiers ha/e two totall+
in"epen"ent power spplies, one for each
channel $the+ "o share a common A power
cor" tho!h'.
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2. Input Stage
• (he !eneral prpose of the inpt sta!e of apower amplifier $sometimes calle" the )front
en")' is to recei/e an" prepare the inpt
si!nals for )amplification) &+ the otpt sta!e.
• (wo t+pes
1.Balanced Input2.Single Ended Input
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2. Input Stage
• Balanced inputs are mch preferre" o/ersingle ended inputs when interconnection
ca&les are lon! an"8or s&ect to nois+
electrical en/ironments &ecase the+ pro/i"e/er+ !oo" noise rejection.
• (he inpt sta!e also contains thin!s lie inpt
le/el controls.
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3.Output Stage
• (he portion which actall+ con/erts the weainpt si!nal into a mch more powerfl )replica)
which is capa&le of "ri/in! hi!h power to a
speaer.• (his portion of the amplifier t+picall+ ses a
nm&er of )power transistors) $or 9:FE(s'
an" is also responsi&le for !eneratin! the mostheat in the nit.
• (he otpt sta!e of an amplifier interfaces to
the speaers.
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Amplifier lasses
• (he Class of an amplifier refers to the "esi!nof the circitr+ within the amp.
• For a"io amplifiers, the Class of amp refersto the output stage of the amp.
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Classes
Collector current waveforms for
transistors operating in (a) class A, (b)
class B, (c) class AB, and (d) class C
amplifier stages.
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• Class-A: tpt "e/ice$s' con"ct thro!h 3;0 "e!rees of inpt c+cle $ne/er switch off' < A sin!le otpt "e/ice is possi&le. (he "e/ice con"ctsfor the entire wa/eform in Fi!re 1
• Class-B: tpt "e/ices con"ct for 1=0 "e!rees $182 of inpt c+cle' <for a"io, two otpt "e/ices in )psh
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• Class-C: tpt "e/ice$s' con"ct for less than 1=0 "e!rees $100 to150 "e!rees t+pical' < @a"io Fre?encies onl+ < cannot &e se" for a"io(his is the son" hear" when one of the otpt "e/ices !oes open circit in
an a"io amp :ee Fi!re 1, showin! the time the otpt "e/ice con"cts
Figure 1 - The Sinewave Cycle
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lass A tpt :ta!e
• lass A otpt sta!e is a simple linear
crrent amplifier.
• It is also /er+ inefficient, t+pical ma%imm
efficienc+ &etween 10 an" 20 .
• nl+ sita&le for low power applications.
• >i!h power re?ires mch &etterefficienc+.
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Transfer Characteristics
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Basic class A ampliier !perati!n. "utput is sh!wn 1#$% !ut ! phase with the input &inverted'.
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(a)imum class A !utput !ccurs when the -p!int is centered !n the ac l!ad line.
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-p!int cl!ser t! cut!.
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-p!int cl!ser t! saturati!n.
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F+,/
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F+,/ 0-$ Class A p!wer ampliier with c!rrect !utput v!ltage swing.
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Ch+ is class A so inefficient D
• :in!le transistor can onl+ con"ct in one
"irection.
• 7.. &ias crrent is nee"e" to cope with
ne!ati/e !oin! si!nals.
• 5 $or more' of the spplie" power is
"issipate" &+ ".c.
• :oltion eliminate the &ias crrent.
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lass A
• lass A amplifiers ha/e /er+ low "istortion $lowest "istortion occrs when the /olme is
low'
• (he+ are /er+ inefficient an" are rarel+ se" for
hi!h power "esi!ns.
• (he "istortion is low &ecase the transistors in
the amp are &iase" sch that the+ are half )on)
when the amp is i"lin!
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lass A
• As a reslt of &ein! half on at i"le, a lot ofpower is "issipate" in the "e/ices e/en when
the amp has no msic pla+in!
• lass A amps are often se" for )si!nal) le/elcircits $where power re?irements are small'
&ecase the+ maintain low "istortion.
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Class-A Beneits
• (he first is circit simplicit+.
• (he si!nal is s&ecte" to comparati/el+ little
amplification, resltin! in an open loop !ainwhich is !enerall+ fairl+ low.
• (his means that /er+ little o/erall fee"&ac is
se", so sta&ilit+ an" phase shol" &ee%cellent o/er the a"io fre?encies.
• 7o not re?ire an+ fre?enc+ compensation.
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Class-A Beneits
• 2! cr!ss !ver dist!rti!n
• 2! switching dist!rti!n
• 3!wer harm!nic dist!rti!n in the v!ltage ampliier
• 3!wer harm!nic dist!rti!n in the current ampliier
• 2! signal dependent dist!rti!n r!m the p!wer
supply
• C!nstant and l!w !utput impedance• Simpler design
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Circuit Operation
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Basic class B ampliier !perati!n &n!ninverting'.
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C!mm!n-c!llect!r class B ampliier.
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Class B push-pull ac !perati!n.
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Transfer Characteristics
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Crossover Distortion
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+llustrati!n ! cr!ss!ver dist!rti!n in a class B push-pull ampliier. The transist!rs c!nduct !nly during the p!rti!ns !
the input indicated 4y the shaded areas.
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Trans!rmer c!upled push-pull ampliiers. 1 c!nducts during the p!sitive hal-cycle5 6 c!nducts during the
negative hal-cycle. The tw! halves are c!m4ined 4y the !utput trans!rmer.
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Biasing the push-pull ampliier t! eliminate cr!ss!ver dist!rti!n.
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lass B tpt :ta!e
• Q1 an" Q2 form two n&iase"emitter followers Q1 onl+ con"cts when the inpt is
positi/e
Q2 onl+ con"cts when the inpt isne!ati/e
• on"ction an!le is, therefore,1=0G
• Chen the inpt is Hero, neithercon"cts
• i.e. the ?iescent power"issipation is Hero
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lass B rrent Ca/eforms
Iout
IC1
IC2
time
time
time
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lass B Efficienc+
Average power drawn fromthe positive supply:
( ) 1C S ve I V P =+
( ) ( ) L L
C
R
A
R
A I I
π θ θ
π θ θ
π
π π
=== ∫ ∫ 0
2
0
C11 dsin2
1d
2
1
IC1
Phase, θ
A/RL
π 2π
A sin!θ "
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#y symmetry, power drawn from $ve and %vesupplies will &e the same' (otal power,therefore:
( ) ( ) ( ) L
S veveveS
R
AV P P P P
π 22 ==+= +−+
Load power:
( )
L L L
out
L R
A
R
t A
R
v P
2
sin 2222===
ω
)*+ien+y:
S S
L
LS
L
V
A
AV
R
R
A
P
P
422
2 π π η ===
( )%5.784/ NB. =≤⇒≤ π η S
V A
( ) ( ) ( ) L
S
veveveS
R
AV P P P P π 22 ==+= +−+
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Power 7issipation
(o sele+t appropriate output transistors, themaimum power dissipation must &e +al+ulated'
L L
S
LS D
R
A
R
AV P P P
2
2 2−=−=
π
-ust need to .nd the maimum value of PD to
sele+t transistors/heatsins
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P0 is a uadrati+ fun+tion of A,
9a%imm Power 7issipation
0d
d=
A
P D
maimum when:
L L
S
D
R
A
R
AV P
2
2 2−=
π
L
S
L
S
L
S
D
R
V
R
V
R
V P
2
2
2
2
2
2
(max)
224
π π π =−=∴
0
2=−⇒
L L
S
R
A
R
V
π π
S V
A 2=⇒
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Efficienc+ 8 Power 7issipation
• Pea efficienc+ of the class B otpt
sta!e is =.5 , mch hi!her than
class A.
• nlie class A, power "issipation /aries
with otpt amplit"e.
• @emem&er, there are two otpt
"e/ices so the power "issipation isshare" &etween them.
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ross
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Actal Inpt
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Effect of ross
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lass B
• A class B otpt sta!e can &e far more
efficient than a class A sta!e $=.5
ma%imm efficienc+ compare" with 25
'.
• It also re?ires twice as man+ otpt
transistors…
• …an" it isnMt /er+ linear cross
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lass B
• lass B amplifiers are se" in low cost "esi!ns or "esi!ns where son" ?alit+ is not that
important.
• lass B amplifiers are si!nificantl+ more
efficient than class A amps.
• (he+ sffer from &a" "istortion when the si!nal
le/el is low $the "istortion in this re!ion of
operation is calle" )crosso/er "istortion)'.
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lass B
• lass B is se" most often where econom+ of"esi!n is nee"e".
• Before the a"/ent of I amplifiers, class Bamplifiers were common in cloc ra"io circits,
pocet transistor ra"ios, or other applications
where ?alit+ of son" is not that critical.
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lass AB
• lass AB is pro&a&l+ the most commonamplifier class crrentl+ se" in home stereo
an" similar amplifiers.
• lass AB amps com&ine the !oo" points of
class A an" B amps.
• (he+ ha/e the impro/e" efficienc+ of class B
amps an" "istortion performance that is a lot
closer to that of a class A amp.
/liminating cr!ss!ver dist!rti!n in a trans!rmer-c!upled push-pull ampliier. The di!de c!mpensates !r the 4ase-
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g
emitter dr!p ! the transist!rs and pr!duces class AB !perati!n.
3!ad lines !r a c!mplementary symmetry push pull ampliier "nly the l!ad lines !r the npn transist!r are sh!wn
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3!ad lines !r a c!mplementary symmetry push-pull ampliier. "nly the l!ad lines !r the npn transist!r are sh!wn.
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Single-ended push-pull ampliier
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Single ended push pull ampliier.
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A 7arlingt!n class AB push-pull ampliier.
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a gt! c ass pus pu a p e
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F+,/ 0-6 A Class AB push-pull ampliier with c!rrect !utput v!ltage.
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p p p p g
F+,/ 0- +nc!rrect !utput wave!rms !r the ampliier in Figure 0-6.
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p p g
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lass AB
• Cith sch amplifiers, "istortion is worst when
the si!nal is low, an" !enerall+ lowest when the
si!nal is st reachin! the point of clippin!.
• lass AB amps se pairs of transistors, &oth of
them &ein! &iase" sli!htl+ - so that the
crosso/er "istortion $associate" with lass Bamps' is lar!el+ eliminate".
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lass •
lass amps are ne/er se" for a"io circits.
• (he+ are commonl+ se" in @F circits.
• lass amplifiers operate the otpt transistor
in a state that reslts in tremen"os "istortion
$it wol" &e totall+ nsita&le for a"iorepro"ction'.
F+,/ 0-66 Basic class C ampliier !perati!n &n!n inverting'.
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F+,/ 0-6 Basic class C !perati!n.
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F+,/ 0-68 Class C wave!rms.
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F+,/ 0-69 Tuned class C ampliier.
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F+,/ 0-6# Tuned class C ampliier with clamper 4ias.
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lass
• >owe/er, the @F circits where lass
amps are se", emplo+ filterin! so that the
final si!nal is completel+ accepta&le.
• lass amps are uite eicient.