CHAPTER 4CHAPTER 4

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POWER AMPLIFIERPOWER AMPLIFIER

Outcome of Chapter 4Outcome of Chapter 4

Ability to perform simple DESIGN and EVALUATE A, B and AB classes of BJT and FET amplifiers, in terms of their frequency response, equivalent circuit, termal management (power dissipation) and gain.

2

OutlineOutline

3

Introduction

Concept of Power Amplifier

Power Amplifier Classification

BJTs/ MOSFETs Power Amplifier

Class A Power Amplifier

IntroductionIntroductionPower amplifiers are used to deliver a relatively high

amount of power, usually to a low resistance load. Typical load values range from 300W (for transmission

antennas) to 8W (for audio speaker). Although these load values do not cover every

possibility, they do illustrate the fact that power amplifiers usually drive low-resistance loads.

Typical output power rating of a power amplifier will be 1W or higher.

Ideal power amplifier will deliver 100% of the power it draws from the supply to load. In practice, this can never occur.

The reason for this is the fact that the components in the amplifier will all dissipate some of the power that is being drawn form the supply.

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Concept of Power AmplifierConcept of Power Amplifier

5

Provide sufficient power to an output load to drive other power device.

To deliver a large current to a small load resistance e.g. audio speaker;

To deliver a large voltage to a large load resistance e.g. switching power supply;

To provide a low output resistance in order to avoid loss of gain and to maintain linearity (to minimize harmonic distortion)

To deliver power to the load efficiently

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Power Amplifier Power Power Amplifier Power DissipationDissipation

P1 = I12R1

P2 = I22R2

ICQRC

RE

R1

R2

VCC

I1

I2

ICC

PC = ICQ2 RC

PT = ITQ2 RT

PE = IEQ2 RE

IEQ

The total amount of power being dissipated by the amplifier, Ptot , is

Ptot = P1 + P2 + PC + PT + PE

The difference between this total value and the total power being drawn from the supply is the power that actually goes to the load – i.e. output power.

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Power Amplifier Efficiency Power Amplifier Efficiency A figure of merit for the power amplifier is its

efficiency, .

Efficiency ( of an amplifier is defined as the ratio of ac output power (power delivered to load) to dc input power .

By formula :

As we will see, certain amplifier configurations have much higher efficiency ratings than others.

This is primary consideration when deciding which type of power amplifier to use for a specific application.

%100)(

)(%100

dcP

acP

powerinputdc

poweroutputac

i

o

Power Amplifiers Classification Power Amplifiers Classification

8

Class A - The transistor conducts during the whole cycle of sinusoidal input signal

Class B - The transistor conducts during one-half cycle of input signal

Class AB - The transistor conducts for slightly more than half a cycle of input signal

Class C - The transistor conducts for less than half a cycle of input signal

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Efficiency RatingsEfficiency Ratings The maximum theoretical efficiency

ratings of class-A, B, and C amplifiers are:

Amplifier Maximum Theoretical Efficiency, max

Class A 25%

Class B 78.5%

Class C 99%

BJT Power AmplifierBJT Power Amplifier

Comparison of the characteristics and maximum ratings of a small-signal and power BJT

ParameterSmall-signal BJT

(2N2222A)Power BJT(2N3055)

Power BJT(2N6078)

VCE (max) (V) 40 60 250

IC (max) (A) 0.8 15 7

PD (max) (W) 1.2 115 45

35 – 100 5 – 20 12 – 70

fT (MHz) 300 0.8 1

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Typical dc beta characteristics Typical dc beta characteristics ((hhFEFE versus versus IIcc) for 2N3055) for 2N3055

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BJTs Power AmplifierBJTs Power Amplifier

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Current gain is smaller in power amplifier BJT. The gain depends on IC and temperature may be

related to the following:

maximum current that connecting wires can handle

at which current gain falls below a stated value current which leads to maximum power

dissipation. maximum voltage limitation associated with

avalanche breakdown in reverse-biased collector-base junction.

second breakdown in BJT operating at high voltage and current.

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VCE(sus) =115 volt at which these curve merge and the minimum voltage necessary to sustain the transistor in breakdown.

The breakdown voltage, VCE0 ~130 volt when the base terminal is open circuited, IB=0

Instantaneous power dissipation

The average power over one cycle

The maximum rated power,

CCEBBECCEQ ivivivp

T

CCEQ dtivT

P0

1

14

CCET IVP

MOSFETs Power AmplifierMOSFETs Power AmplifierCharacteristic of two power MOSFETs

ParameterPower

MOSFET2N6757

Power MOSFET2N6792

VDS (max) (V) 150 400

ID (max) (A) (at T = 25C) 8 2

PD (max) (W) 75 20

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Performance Characteristic of MOSFETs Performance Characteristic of MOSFETs Power AmplifierPower Amplifier

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Faster switching times

no second breakdown.

Stable gain and response over wide temperature range.

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Class A AmplifierClass A Amplifier

output waveform same shape input waveform + phase shift.

The collector current is nonzero 100% of the time. inefficient, since even with zero input signal, ICQ is nonzero(i.e. transistor dissipates power in the rest, or quiescent, condition)

vin voutAv

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Basic OperationBasic OperationCommon-emitter (voltage-divider) configuration (RC-coupled amplifier)

RC

+VCC

RE

R1

R2

RL

v in

ICQI1

ICC

Typical Characteristic Curves for Typical Characteristic Curves for Class A OperationClass A Operation

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Configuration : No inductor @ transformer are used

(a) Common-emitter amplifier,

(b) dc load line (the Q point is at centre of the load line)

(c) instantaneous power dissipation versus time in the transistor

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DC Input PowerDC Input Power

RC

+VCC

RE

R1

R2

RL

v in

ICQI1

ICCThe total dc power, Pi(dc) , that an

amplifier draws from the power supply :

CCCCiIVdcP )(

1III

CQCC

CQCCII )(

1II

CQ

CQCCiIVdcP )(

Note that this equation is valid for most amplifier power analyses. We can rewrite for the above equation for the ideal amplifier as

CQCEQiIVdcP 2)(

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AC Output Power AC Output Power

R1//R2

vcevin

vo

ic

RC//RLrC

AC output (or load) power, Po(ac)

Above equations can be used to calculate the maximum possible value of ac load power. HOW??

L

rmso

rmsormsco R

vviacP

2

)(

)()()(

Disadvantage of using class-A amplifiers is the fact that their efficiency ratings are so low, max 25% .

Why?? A majority of the power that is drawn from the supply by a class-A amplifier is used up by the amplifier itself.

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IC(mA)

VCE

VCE(off) = VCC

IC(sat) = VCC/(RC+RE)

DC Load LineIC

VCE

IC(sat) = ICQ + (VCEQ/rC)

VCE(off) = VCEQ + ICQrC

ac load line

IC

VCE

Q - point

ac load line

dc load line

L

PP

CQCEQ

CQCEQ

o R

VIV

IVacP

82

1

22)(

2

%25%10022

1

%100)(

)( CQCEQ

CQCEQ

dci

aco

IV

IV

P

P

23

LimitationLimitation

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ExampleExampleCalculate the input power [Pi(dc)], output power

[Po(ac)], and efficiency [] of the amplifier circuit for

an input voltage that results in a base current of 10mA peak.

RCRB

+VCC = 20V

IC

Vi

25

20k1

Vo

%5.6%100

6.9)48.0)(20(

625.0)20(2

10250

2

250)10(25

20

1100020

20

4.10)20)(48.0(20

48.05.482)3.19(25

3.191

7.020

)(

)(

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232)(

)(

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)(

dci

aco

CQCCdci

CpeakC

aco

C

CCsatc

B

P

P

WAVIVP

WA

RI

P

peakmApeakmAII

VVV

AmAV

R

VI

VAVRIVV

AmAmAII

mAk

VV

R

VVI

peakbpeakC

CCcutoffCE

CCCCCEQ

CQ

B

BECC

BQ

ExampleExample

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The common source circuit parameters are VDD=10V, RD=5kΩ and the transistor parameters are Kn=1mA/V2, VTN=1V and =0.

Assume the output voltage swing is limited to the range between the transition point and vDS=9V to minimize nonlinear distortion.

Calculate the actual efficiency of a class A output stage.

ExerciseExercise

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The Q-point of common source circuit is VDSQ=4V

a) Find IDQ

b) Determine the max peak to peak amplitude of a symmetrical sinusoidal output voltage if the min value of instantaneous drain current must be no less than 0.1IDQ and the min value of instantaneous drain source voltage must be no less than vDS=1.5V.

c) Calculate the power conversion efficiency where the signal power is the power delivered to RL.

Ans:

60mA,

5V,

31.25mW, 5.2%

Design of Class A C-E Design of Class A C-E AmplifierAmplifierTo find R1, R2, RE, RC use the DC analysis and

design formula;

To find Zi, Zo, Av, Ai use AC analysis (without loading effect)

To find Zi, Zo, Avs, Ai use AC analysis (with loading effect if have Ri and RL

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LC

CQC

ECCCQ

CCEQ

II

II

RR

VV

VV

10; Rhave If

sheet.data in given10

;2

;10

L

2

Example & ExerciseExample & Exercise

Will be given in our class.

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Class B Power AmplifierClass B Power Amplifier Consists of complementary pair electronic devices

One conducts for one half cycle of the input signal and the other conducts for another half of the input signal

When the input is zero, both devices are off, the bias currents are zero and the output is zero.

Ideal voltage gain is unity

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For input larger than zero, A turn ON and supplies current to the load.

For input less than zero, B turn ON and sinks current from the load

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Complimentary Push-Pull CircuitComplimentary Push-Pull Circuit

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DEAD BAND

CROSSOVER DISTORTION

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The Ideal Class BThe Ideal Class B

tVv po sin

tR

Vi

L

pCn sin

i Cn

i Cp

tVv Po sin

sin tR

Vi

L

pCn

33

• Maximum possible value of Vp is VCC

• The instantaneous power in Qn is;

tVVv pCCCEn sin

CnCEnQn ivp

t

R

VtVVp

L

ppCCQn sinsin

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The average power in Qn is

Differentiating for maximum PQn with respect to Vp equal to zero gives us

Maximum average power dissipation;

L

p

L

pCCQn R

V

R

VVP

4

2

L

CCQn R

VP

2

2

max

CCP

L

p

L

CC

p

Qn

VVthen

R

V

R

V

dV

dP

2

04

2

35

The average power delivered to the load is

Power source supplies half sinewave of current,

the average value is;

The total power supplied by the two sources is

L

pS R

VI

L

pL R

VP

2

2

1

L

pCCSCCsS R

VVIVPP

L

pCCSCCS R

VVIVP

22

36

The efficiency is

%5.78785.04

when efficiency maximum

42

2

21

CCP

CC

p

RV

CC

RV

S

L

VV

V

V

VP

P

L

P

L

P

37

Class AB Power AmplifierClass AB Power Amplifier

Small quiescent bias on each output transistor to eliminate crossover distortion

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Class C Power AmplifierClass C Power Amplifier

39

Class AB Voltage Transfer CurveClass AB Voltage Transfer Curve

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Collector Currents & Output CurrentCollector Currents & Output Current

CpLCn iii

41

ExampleExample

The parameters are VDD=10V, RL=20Ω. The transistor are matched and K=0.2A/V2, VT=1V, IDQ=0.05 when vo=5V.

Determine the required biasing in a MOSFET class AB output stage.

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Inductively Coupled AmplifierInductively Coupled Amplifier

43

The maximum possible average signal power delivered to the load

The possible average signal power supply by VCC

The maximum possible power conversion efficiency

L

CCLCQL R

VRIP

22

2

1

2

1(max)

L

CCCQCCS R

VIVP

2

%505.02

1(max)(max) 2

2

21

L

CC

L

CC

RV

RV

S

L

P

P

44

Transformer Coupled AmplifierTransformer Coupled Amplifier

45

• The theoretical maximum efficiency of a basic RC-coupled class-A amplifier is limited to 25%.

• In practical circuit, the efficiency is less than 25%.

• Used for output power of about 1 W only.

• Transformer coupling can increase the maximum efficiency to 50%

• Disadvantage of transformer coupling – expensive & bulky.

46

Neglecting transformer resistance and assuming RE is small;

CCCEQ VV

47

a

vvaii CL

12 and

For ideal transformer;

2

1ratioturn N

Na

CLL ai

av

i

vR

/12

21 1

ai

vR

CL

i C

CL i

vR 1' LL RaR 2'

48

Turn ratio is designed for maximum symmetrical swing, hence;

The maximum average power delivered to load equals maximum average power delivered to the primary of the transformer

LCQ

CC

CQ

CCL Ra

I

V

I

VR 2

2

2'

CQCCL IVP2

1max

(VCC and ICQ are maximum possible amplitudes of signal)

49

The average power supplied by the VCC source is;

CQCCS IVP

The maximum possible efficiency is;

%505.0

max

S

L

P

P