5193Ch11 Power Amplifier 1

7/25/2019 5193Ch11 Power Amplifier 1

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Power Amplifers

Power Amplifers are used in thetransmitter

There are dierent classes or the poweramplifer

Class A, Class B, Class AB, Class C, ClassD

and Class E

Those amplifer diers in their angle o

conduction, eciency, linearity and the inthe amount o distortion they introduce tothe system


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Power Amplifers

Class AClass A

The amplifer conducts through theull 360

o the input. The Q-point

is set near the middle o the loadline.

Class BClass BThe amplifer conducts through

!0

o the input. The Q-point is setat the cuto" point.


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Power Amplifers

Class ABClass AB

This is a compromise #etween theclass A and B amplifers. The amplifer

conducts somewhere #etween !0and 360 . The Q-point is located#etween the mid-point and cuto".

Class CClass C

The amplifer conducts less than !0o the input. The Q-point is located#elow the cuto" le$el.


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Power Amplifers

Class %Class %

This is an amplifer that is #iasedespeciall& or digital signals


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Class C power amplifers Class C amplifer is o!tained i the output

current conduction angle is less than "#$

Class C ha%e a greater eciency than !othclass A and class B

Class C also ha%e larger distortion comparedwith the A and B

Class C amplifer is used when there is no%ariation in signal amplitude and the outputcircuit contains a tuned circuit to flter out theharmonics

Class C amplifer is used or applications similarto &' 'odulation


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Class C power amplifers

The output o the class C conducts orless than "#$ o the AC cycle

The ()point is !elow cuto as shown!elow


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Power Amplifers Class C


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Class C power amplifer

The circuitshown to thelet will !econsidered in

analysis o theclass Camplifer


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The amplifer eciency can !e computedrom the ollowing e*uation

The output power is the AC power +owingin the load resistance

The input power is the supply power which

can !e determined rom the multiplicationo the supply %oltage and the a%eragecollector current

The main step in the analysis is tocom ute the a%era e collector current

DC

AC

i

o

P

P

P

P==


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According to thefgure shown tothe let, thecollector current

is gi%en !y

-here ID isgi%en !y


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The direct collector current is determined!y its a%erage %alue which is gi%en !y

.t is desired to fnd the current as aunction o the angles 1and 2rather than

t1and t2

This can !e achie%ed !y using theollowing mapping and

=2

1

)sin(1

t

t

DpCav dtItIT

I

11 =t

22 =t


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The a%erage collector current can !erewritten as

By e%aluating this e*uation we may ha%ethe ollowing epression

To simpliy the notation, the conductionangle will !e defned as

=

2

1

)sin(1

dtItIT

I DpCav

ICav


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By su!stituting the conduction angleepression into the a%erage collectorcurrent e*uation we get

/ow the supplied input power can !ewritten as

. the output 01C circuit is a narrow !andflter tuned to the undamental re*uency

o the current pulses, then the outputower will !e

)cos(sin

== pCC

CavCCi IV

IVP

)cos(sin

= pCavI

I


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-here I1 is the amplitude o theundamental current components which isdetermined !y the trigonometric &ourierseries as shown !elow

By sol%ing the pre%ious e*uation we may

ha%e

The output power now can !e written as

)2sin2(

4

1212

121

=== pCC

CCO

IVIVRLIP


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The eciency o the amplifer can !ewritten as

A plot o the eciency as a a unction othe conduction angle is shown !elow

)cos(sin4

2sin2121

===

CavCC

CC

i

O

IV

IV

P

P


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Class C power ampliferdesign

There are our important designparameters are o great importance orPA design in general

These parameters are

The output power Transistor power dissipation

'aimum collector to emitter %oltage VCEmax The maimum transistor output currentIp


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The maimum collector current is gi%en !y

2ince

The collector current can !e rewritten as

The maimum current in terms o the outputcurrent can !e written as

/ote that the %alue o the collector %oltage VCCcan !e written as


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/ow the maimum collector current can!e rewritten as

A normali3ed pea4 collector current is

defned as


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A plot o the normali3ed pea4 current%ersus the conduction angle is shown!elow


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The power dissipated in the transistor is gi%en!y

/ote the %alue o Ipcan !e epressed as

&rom we can conclude that

. the %alue o Ipis su!stituted in the PTe*uation

then

2)cos( 1

IVSin

IVPPP CCPCCOiT ==

cos1=

Mp

II

2sin2

2 1

=I

Ip


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The power dissipated in the transistor isgi%en !y

5r PTcan !e rewritten as

2)

2sin2

cossin(2 11

IVIVP CCCCT

=


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A normali3ed plot o PT/PO%ersus theta isshown !elow


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Class C power ampliferdesign e'ample

Example:Design a class C amplifer that willdeli%er 6)- a%erage power to a 6$ 7 loadat a re*uency o " '83 using a transistorwith a sae power dissipation rating o $6

-Solution:

The a%erage output power is gi%en !y

5rVPRVCC OL 4.2255022 ===


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Class C power ampliferdesign e'ample

Solution:2ince the allowa!le power dissipation is

The maimum conduction angle can !eound rom the graph shown in slide "9or !y sol%ing the PT/POe*uation

The %alue o the normali3ed currentcorresponds to this angle isreer to the fgure in slide ":

The pea4 collector current is gi%en !y

5.57=


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An alternate design procedure or classC amplifers is

2elect the power supply

2elect the transistor

Determine the maimum output powerwithout eceeding the transistor ratings

The transistor then can !e dri%en to itsmaimum allowed %alue o output current

Determine the %alue o the load resistancethat twill result in the maimum currentaccording to


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/ow the transistor power e*uation can!e modifed as

The normali3ed transistor powerdissipation is gi%en !y

-here

cos1

)2sin2()cos(sin4)(

=f


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Class C power ampliferdesign e'ample (

Example 2: Determine the maimumoutput power and the conduction angleo a class C amplifer using a transistorwith maimum power dissipation rating

o ; - and a maimum output current o"6 A The supply %oltage is ;#

5193Ch11 Power Amplifier 1

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