eet 223(1) analog electronics jagjeet

8/17/2019 Eet 223(1) Analog Electronics Jagjeet

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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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63/79

Single-ended push-pull ampliier


64/79

Single ended push pull ampliier.


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66/79

A 7arlingt!n class AB push-pull ampliier.


67/79

a gt! c ass pus pu a p e


68/79

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.


74/79

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.

eet 223(1) analog electronics jagjeet

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