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CHAPTER 2POWER AMPLIFIERS
BENE 2163
ELECTRONIC SYSTEMS
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Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Introduction
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Definitions
In small-signal amplifiers the main factors are:
Amplification Linearity Gain
Since large-signal, or power, amplifiers handle relatively largevoltage signals and current levels, the main factors are:
Efficiency Maximum power capability Impedance matching to the output device
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Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Amplifier TypesClass A
The amplifier conducts through the full 360 of the input. The Q-point isset near the middle of the load line.
Class BThe amplifier conducts through 180 of the input. The Q-point is set atthe cutoff point.
Class ABThis is a compromise between the class A and B amplifiers. Theamplifier conducts somewhere between 180 and 360 . The Q-point islocated between the mid-point and cutoff.
Class C
The amplifier conducts less than 180 of the input. The Q-point is locatedbelow the cutoff level.
Class DThis is an amplifier that is biased especially for digital signals .
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Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Amplifier Types
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Power Amplifier
Circuit(Power
transistor)Input Output
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Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Class A Amplifier
The output of a class A amplifierconducts for the full 360 of thecycle.
The Q-point is set at the middle ofthe load line so that the AC signalcan swing a full cycle.
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Remember that the DC load lineindicates the maximum and minimumlimits set by the DC power supply.
Fig. 12.1: Amplifier operating classes
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Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Class B Amplifier
A class B amplifier outputonly conducts for 180 orone-half of the AC inputsignal.
The Q-point is at 0V on theload line, so that the ACsignal can only swing forone-half cycle.
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Fig. 12.1: Amplifier operating classes
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Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Class AB Amplifier
This amplifier is a compromise between theclass A and class B amplifier the Q-pointis above that of the Class B but below the
class A.
The output conducts between 180 and360 of the AC input signal.
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Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Class C
The output of the class Cconducts for less than 180 of the
AC cycle. The Q-point is belowcutoff.
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Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Series-Fed Class A Amplifier
This is similar to thesmall-signal amplifierexcept that it will handlehigher voltages. Thetransistor used is a high-power transistor.
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Fig. 12.2: Series-fed class A large-signalamplifier
DC Bias Operation:
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Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Series-Fed Class A Amplifier
A small input signalcauses the output voltageto swing to a maximum of
Vcc and a minimum of 0V.The current can alsoswing from 0mA to I CSAT (V CC /R C)
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AC Operation:
Fig. 12.4: Amplifier input and output signal variation
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Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Series-Fed Class A Amplifier
The power into the amplifier is from the DC supply. With no inputsignal, the DC current drawn is the collector bias current, I CQ .
CQCCi(dc) IVP
Output Power
C
C(rms)2
o(ac) R
VP
C
p)-CE(p2
o(ac) 8R
VP
Input Power
or
100P
P%
i(dc)
o(ac)
Efficiency:
14
25%100P
P%
i(dc)max
o(ac)max(max)
Maximum Efficiency:
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Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Example 12.1 (Method 1 - calculation)Calculate the input power, output power, and efficiency of theamplifier circuit in Fig. 12.5 for an input voltage that results in a
base current of 10mA peak.
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Fig. 12.5: Operation of a series-fed circuit for Example12.1
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Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Example 12.1 summary
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Results Comparison Method 1(Calculation)
Method 2 (Graphical) Simulation (OrCAD)
IBQ
19.3mA 19.3mA 19.33mA
ICQ 482.5mA 480mA 488.9mA
Input power (P i(dc) ) 9.65W 9.6W 9.45W
IB(p) 10mA(given) 10mA (given) 10mA (given)
IC(p) = I C(max) I CQ@
[I C(max) I C(min) ]/2
250mA 730m 480m = 250mA@
[730m 230m]/2 = 250mA
[717.676m 225.337m]/2= 246.17mA
VCE(p) = V CE(max) V CEQ@[V CE(max) V CE(min) ) ]/2
[16-6]/2 = 5V [15.501 5.6576] /2 =4.9217V
Output power (P o(ac) ) 0.625W 0.625W 0.61W
Efficiency ( %) 6.5% 6.5% 6.4%
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Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Transformer-Coupled Class A Amplifier
This circuit uses atransformer to couple tothe load. This improvesthe efficiency of the ClassA to 50%.
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Fig. 12.6: Transformer-coupled audiopower amplifier
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Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
A transformer improves the efficiency because it is able to transform
the voltage, current, and impedance
Voltage Ratio
Current Ratio
Impedance Ratio
Transformer Action Review
1
2
1
2
N
N
V
V
2
1
1
2
N
N
I
I
22
2
1
2
1
L
L aN
N
R
R
R
R
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Fig. 12.6: Transformer-coupled audiopower amplifier
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Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Example 12.2
Calculate the effective resistance seen looking into the primary of a15:1 transformer connected to an 8 load.
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Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Example 12.3
What transformer turns ratio is required to match a 16 speakerload so that the effective load resistance seen at the primary is10k .
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Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Transformer-Coupled Class A AmplifierDC Load Line
As in all class A amplifiersthe Q-point is establishedclose to the midpoint of theDC load line.
AC Load Line
The saturation point (I Cmax )is at V cc/R L and the cutoffpoint is at V 2 (the secondaryvoltage of the transformer).
This increases the maximumoutput swing because theminimum and maximumvalues of I C and V CE arespread further apart.
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Fig. 12.8: Load lines for class A transformer-coupled amplifier
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Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Transformer-Coupled Class A Amplifier
The voltage swing:
8
)I)(IV(V
P CminCmaxCEminCEmax
o(ac)
Signal Swing and Output AC Power
The AC power:
The current swing:
minCEmaxCE)pp(CE VVV
minCmaxC II
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Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Example 12.4
Calculate the ac powerdelivered to the 8 speakerfor the circuit of Fig. 12.10.The circuit componentvalues result in a dc basecurrent of 6mA, and theinput signal (Vi) results in apeak base current swing of4mA.
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Fig. 12.10: Transformer-coupled class A amplifierfor Example 12.4
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Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
IB=12mA
IB=2mA
IB=4mA
IB=6mA
IB=8mA
IB=10mA
VCE (V)
IC (mA)
42 6 8 14 16 18 2012
150
50
100
200
250
300
350
0 10
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Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Transformer-Coupled Class A AmplifierEfficiency
Power input from the DC source:
CQCCi(dc) IVP
o(ac)i(dc)Q PPP
2
CEminCEmax
CEminCEmax
VVVV
50%
Maximum efficiency :
Power dissipated as heat across the transistor:
Note: The larger the input and output signal, thelower the heat dissipation.
Note: The larger V CEmax and smaller V CEmin , thecloser the efficiency approaches the theoreticalmaximum of 50%.
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Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Example 12.5
For the circuit of Fig. 12.10 and results of Example 12.4, calculatethe dc input power, power dissipated by the transistor, andefficiency of the circuit for the input signal of Example 12.4.
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Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Example 12.6
Calculate the efficiency of a transformer-coupled class A amplifierfor a supply of 12V and outputs of:
a) V(p) = 12Vb) V(p) = 6Vc) V(p) = 2V
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Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Summary
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Example 12.1: = 6.5 % (Series Class A)
Example 12.5: = 35.71% (Transformer-Coupled Class A)
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Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Announcement!
Test 1Date : 3 rd April 2013 (Wednesday)Time : 8.00 9.30 pmVenue : UTeM Hall (Dewan Besar)
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Cl A A lifi S
http://www.google.com.my/imgres?imgurl=http://1.bp.blogspot.com/-TTLFthzYoHg/USY67brcHII/AAAAAAAAGU8/ZhOaUsyI1VI/s1600/surprisedCat.jpg&imgrefurl=http://mycatsaredemocrats.blogspot.com/2013_02_01_archive.html&usg=__N4nTLPTcQTqJVWGQs5g2rNAVgw0=&h=257&w=271&sz=9&hl=en&start=15&zoom=1&tbnid=nKoNY_AuYfkBHM:&tbnh=107&tbnw=113&ei=nq4-UbuhBY6srAfohoCgAg&prev=/search%3Fq%3Dsurprised%2Bcat%26um%3D1%26hl%3Den%26gbv%3D2%26tbm%3Disch&um=1&itbs=1&sa=X&ved=0CEUQrQMwDghttp://www.google.com.my/imgres?imgurl=http://churchwhisperer.files.wordpress.com/2010/02/surprised-little-boy1.jpg&imgrefurl=http://churchwhisperer.com/2011/07/12/71211/&usg=__oCMBJ2qK2mdy-llIi2Y-JpIr4EY=&h=374&w=319&sz=95&hl=en&start=6&zoom=1&tbnid=ak-TVr2qf6A6EM:&tbnh=122&tbnw=104&ei=Xa4-UYWaIYW8rAeVtoCAAw&prev=/search%3Fq%3Dsurprised%26um%3D1%26hl%3Den%26gbv%3D2%26tbm%3Disch&um=1&itbs=1&sa=X&ved=0CDMQrQMwBQhttp://www.google.com.my/imgres?imgurl=http://www.desicomments.com/funnypics/funnyanimal67.jpg&imgrefurl=http://www.desicomments.com/funny/oh-my-god-2/&usg=__W2n-f-oQOkbwaMbdGNFnEEDy5_c=&h=483&w=475&sz=36&hl=en&start=2&zoom=1&tbnid=nKbALvkOusceQM:&tbnh=129&tbnw=127&ei=Da4-Uc63KsePrgfT94DgCQ&prev=/search%3Fq%3Doh%2Bmy%2Bgod%2Bfunny%26hl%3Den%26gbv%3D2%26tbm%3Disch&itbs=1&sa=X&ved=0CCsQrQMwAQhttp://www.google.com.my/imgres?imgurl=http://www.dreamstime.com/the-young-sad-student-with-the-books-isolated.-thumb16755705.jpg&imgrefurl=http://www.dreamstime.com/royalty-free-stock-photo-young-sad-student-books-isolated.-image16755705&usg=__c2cb16gmRBqkPKfrFfHGGp7Le60=&h=365&w=400&sz=36&hl=en&start=15&zoom=1&tbnid=Y8bSPBokc-PZPM:&tbnh=113&tbnw=124&ei=3qw-UYbLJYiKrgfN4YGIBg&prev=/search%3Fq%3Dstudent%2Bsad%26hl%3Den%26gbv%3D2%26tbm%3Disch&itbs=1&sa=X&ved=0CEUQrQMwDghttp://www.google.com.my/imgres?imgurl=http://blogs.studentlife.utoronto.ca/lifeatuoft/files/2012/10/shock1.jpg&imgrefurl=http://blogs.studentlife.utoronto.ca/lifeatuoft/2012/10/31/the-5-stages-of-rejection/&usg=__aADK4t2xg2j0dHppvYQyvMYB4hM=&h=262&w=350&sz=16&hl=en&start=20&zoom=1&tbnid=8AXCTAB47wDcMM:&tbnh=90&tbnw=120&ei=uas-Uav8MMmzrAfu7YGoCA&prev=/search%3Fq%3Dshock%26hl%3Den%26gbv%3D2%26tbm%3Disch&itbs=1&sa=X&ved=0CE8QrQMwEwhttp://www.google.com.my/imgres?imgurl=http://us.123rf.com/400wm/400/400/doglikehorse/doglikehorse1004/doglikehorse100400164/6875240-a-look-of-shock-on-this-young-mans-face.jpg&imgrefurl=http://www.123rf.com/photo_6875240_a-look-of-shock-on-this-young-mans-face.html&usg=__rygzBwFOsMX9Pf8hR2wUy0WTpsw=&h=1200&w=1200&sz=112&hl=en&start=15&zoom=1&tbnid=0UmO-QN_XQ0AwM:&tbnh=150&tbnw=150&ei=uas-Uav8MMmzrAfu7YGoCA&prev=/search%3Fq%3Dshock%26hl%3Den%26gbv%3D2%26tbm%3Disch&itbs=1&sa=X&ved=0CEUQrQMwDg -
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Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Class A Amplifier Summary
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Q-point set at the middle of the load line
Conduction angle - 360
Maximum output swing = Vcc/2
Maximum efficiency for series class A amplifier = 25%
Maximum efficiency for transformer-coupled class A amplifier = 50%
DC loadline DC analysis, I CQ , V CEQ , I c(sat) and V CE(cutoff)
AC loadline AC analysis, slope =
Output (ac) current swing and output (ac) voltage swing
Calculation and graphical method
Pi(dc), Po(ac) , % and P Q
L R '1
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Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Class B Amplifier
A class B amplifier outputonly conducts for 180 orone-half of the AC inputsignal.
The Q-point is at 0V on the
load line, so that the ACsignal can only swing forone-half cycle.
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Fig. 12.1: Amplifier operating classes
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Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Class B Amplifier
In class B, the transistor isbiased just off. The AC signalturns the transistor on.
The transistor only conductswhen it is turned on by one-half of the AC cycle.
In order to get a full AC cycleout of a class B amplifier, youneed two transistors:
An npn transistor that provides thepositive half of the AC cycle
A pnp transistor that provides thenegative half.
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Fig. 12.12: Block representation ofpush-pull operation
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Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Class B Amplifier
Input (DC) Power;
P dcccdci I Vcc I V P
2)(
35
Output (AC) Power; L
p L
L
p p L
L
rms Laco R
V
R
V
R
V P
28
2)(
2)(
2)(
)(
100P
)P%
)dc(i
ac(oEfficiency;
Power dissipated by output transistors; )()(2 acodciQ P P P
Power dissipated by each transistor;
22Q
Q
P P
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Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Class B Amplifier: Maximum Power Considerations
For maximum power, V L(p) =V CC
L
CC2
L
CCCCdcCCi(dc) R
2V
R
2VV)I(maximumVPmaximum
The maximum efficiency of a class B is 78.5%..
100%max)(
max)(max
dci
aco
P
P
L
2CC
o(ac) 2R V
Pmaximum
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Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Class B Amplifier: Maximum Power Considerations
L2
CC2
2Q R 2V
Pmaximum
For class B operation, the maximum power dissipated by the outputtransistors does not occurs at the maximum power input or output condition.
The maximum power dissipated by the two output transistors occurs when;
CC CC P L V V
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Maximum power dissipated by output transistors;
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Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Example 12.7
For a class B amplifier providing a 20V peak signal to a 16 load(speaker) and a power supply of Vcc = 30V, determine the inputpower, output power, and circuit efficiency.
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Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Example 12.8
For a class B amplifier using a supply of Vcc = 30V and driving aload of 16 , determine the maximum input power, output power,and transistor dissipation.
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Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Example 12.9
Calculate the efficiency of a class B amplifier for a supply voltage ofVcc = 24V with peak output voltages of:
a) VL(p) = 22Vb) VL(p) = 6V
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Think-pair-share (floyd)
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Figure aFigure b
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Think-pair-share (floyd)
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Figure a
Figure b
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Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
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Figure a
Figure b
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Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Transformer-Coupled Push-PullClass B Amplifier
The center-tappedtransformer on the inputproduces oppositepolarity signals to thetwo transistor inputs.
The center-tappedtransformer on theoutput combines the twohalves of the ACwaveform together.
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Fig. 12.15: Push-pull circuit
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Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Class B Amplifier Push-Pull Operation
During the positivehalf-cycle of the ACinput, transistor Q 1 (npn ) is conductingand Q 2 ( pnp ) is off.
During the negativehalf-cycle of the ACinput, transistor Q 2 ( pnp ) is conductingand Q 1 (npn ) is off.
Each transistor produces one-half of an AC cycle. The transformer combines thetwo outputs to form a full AC cycle.
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Fig. 12.15: Push-pull circuit
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Crossover Distortion
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Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Example 12.10
For the circuit of Fig. 12.19,
calculate the input power , outputpower , and power handled by eachoutput transistor and the circuitefficiency for an input of 12Vrms.Assume the amplifier has unityvoltage gain.
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Fig. 12.19: Class B power amplifier forExamples 12.10 to 12.12
L
p Laco R
V P
2*
)(2
)(
dcccdci I V P
)(*
)(
2*
p Ldc I I
22* 2 oiQQ
P P P P
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Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Example 12.11
For the circuit of Fig. 12.19, calculate the maximum input power,maximum output power , efficiency for maximum power operation ,and power dissipated by the output transistors at this voltage.
49
L
cc
dci RV
P
2
max)(
2*
L
ccaco R
V P
2*
2
max)(
)()(2* acodciQ P P P
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Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Example 12.12
For the circuit of Fig. 12.19, determine the maximum powerdissipated by the output transistors and the output voltage at whichthis occurs.
50
L
ccQ RV
P 22
(max)22
*
CC L V V 636.0*
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Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Heat Sink and Thermal ConsiderationsThe power transistor in power amplifier is subject to dissipating alarge amount of power which is converted to heat and can causethe transistors junction temperature to rise.
51
D
A J JA
P
T T *
Thermal resistance (heat transfer from the transistor junction to the ambient);
T J = Junction temperature ( C)TA = Ambient temperature ( C)P Q@P D = Power transistor dissipation
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Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Heat Sink and Thermal Considerations
The difference in temperature from the junction temperature T J to the ambient
TA;
52
)(* SACS JC D A J P T T
Derating Factor;
JC
DF
1*
http://localhost/var/www/apps/works/data%20hardisk%20KJ/fail%20jabatan/fkekk/kerja/bene2163/bene2163_sem%202_2012_2013/slide%20for%20bene%202163/LECTURESLIDE/New%20Folder%20(sistem%20e%20yus)/TIP29C.pdfhttp://localhost/var/www/apps/works/data%20hardisk%20KJ/fail%20jabatan/fkekk/kerja/bene2163/bene2163_sem%202_2012_2013/slide%20for%20bene%202163/LECTURESLIDE/New%20Folder%20(sistem%20e%20yus)/TIP29C.pdfhttp://localhost/var/www/apps/works/data%20hardisk%20KJ/fail%20jabatan/fkekk/kerja/bene2163/bene2163_sem%202_2012_2013/slide%20for%20bene%202163/LECTURESLIDE/New%20Folder%20(sistem%20e%20yus)/TIP29C.pdfhttp://localhost/var/www/apps/works/data%20hardisk%20KJ/fail%20jabatan/fkekk/kerja/bene2163/bene2163_sem%202_2012_2013/slide%20for%20bene%202163/LECTURESLIDE/New%20Folder%20(sistem%20e%20yus)/TIP29C.pdf -
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Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Example (Ali Aminian)
53
http://localhost/var/www/apps/works/data%20hardisk%20KJ/fail%20jabatan/fkekk/kerja/bene2163/bene2163_sem%202_2012_2013/slide%20for%20bene%202163/LECTURESLIDE/New%20Folder%20(sistem%20e%20yus)/TIP29C.pdfhttp://localhost/var/www/apps/works/data%20hardisk%20KJ/fail%20jabatan/fkekk/kerja/bene2163/bene2163_sem%202_2012_2013/slide%20for%20bene%202163/LECTURESLIDE/New%20Folder%20(sistem%20e%20yus)/TIP29C.pdfhttp://localhost/var/www/apps/works/data%20hardisk%20KJ/fail%20jabatan/fkekk/kerja/bene2163/bene2163_sem%202_2012_2013/slide%20for%20bene%202163/LECTURESLIDE/New%20Folder%20(sistem%20e%20yus)/TIP29C.pdfhttp://localhost/var/www/apps/works/data%20hardisk%20KJ/fail%20jabatan/fkekk/kerja/bene2163/bene2163_sem%202_2012_2013/slide%20for%20bene%202163/LECTURESLIDE/New%20Folder%20(sistem%20e%20yus)/TIP29C.pdf -
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