ch2 poweramp student

8/12/2019 CH2 Poweramp Student

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CHAPTER 2POWER AMPLIFIERS

BENE 2163

ELECTRONIC SYSTEMS


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Copyright 2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 All rights reserved.

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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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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Amplifier Types

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Power Amplifier

Circuit(Power

transistor)Input Output


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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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Announcement!

Test 1Date : 3 rd April 2013 (Wednesday)Time : 8.00 9.30 pmVenue : UTeM Hall (Dewan Besar)

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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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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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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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Class B Amplifier

Input (DC) Power;

P dcccdci I Vcc I V P

2)(

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Output (AC) Power; L

p L

L

p p L

L

rms Laco R

V

R

V

R

V P

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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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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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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

[email protected] )(

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Maximum power dissipated by output transistors;


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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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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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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)

41

Figure aFigure b


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Think-pair-share (floyd)

42

Figure a

Figure b


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Figure a

Figure b


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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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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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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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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.

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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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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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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.

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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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Heat Sink and Thermal Considerations

The difference in temperature from the junction temperature T J to the ambient

TA;

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)(* SACS JC D A J P T T

Derating Factor;

JC

DF

1*
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Example (Ali Aminian)

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PBL ASSIGNMENT1. CHAN CHUN YEW2. MUHAMMAD KHAIRULANWAR BIN

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ch2 poweramp student

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