BENE 3143_poweramp_classAamplifier (1)

8/3/2019 BENE 3143_poweramp_classAamplifier (1)

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BENE 3143: SYSTEM

ELECTRONICS 2

Power Amplifier

Outline

• Introduction

• The class A amplifier

The class B and class AB Push-Pull amplifier • The class C amplifier

Sadhiqin 2011 2

Introduction

• A power amplifier is designed to deliver a

large amount of power to the load.

• To perform this function, a power amplifier

must itself be capable of dissipating large

amount of power.

Sadhiqin 2011 3

AMPLIFIER

Outline

• It must so designed that the heat generated

when it is operated at high current and

voltage levels is released into the

surroundings at a rate fast enough to prevent

destructive temperature buildup.

Sadhiqin 2011 4

HEAT RELEASE

• Consequently, power amplifiers typically

contain bulky component s having large

surface areas to enhance heat transfer to

environment.

• A power amplifier is often the last, or output,

stage of an amplifier system.

• The preceding stages may be designed to

provide amplification, to provide buffering to

a high impedance signal source, or to modify signal characteristics in some predictable way.

Sadhiqin 2011 5

• Power amplifier arewidely used in audiocomponent such asradio and televisionreceivers, phonographsand tape players, stereoand high fidelity system,recording-studioequipment, and publicaddress system.

Sadhiqin 2011 6

• The load of these

applications is most often a

loudspeaker (“speaker”)

which requires considerable

power to convert electrical

signal to sound waves.

Sadhiqin 2011 7

Sound Test Room!

Sadhiqin 2011 8

• It also used inelectromechanicalcontrol system to drive

electric motors.Example includecomputer disk andtape drives, roboticmanipulators,autopilots, antennarotator

Sadhiqin 2011 9

• One important aspect in the

design power amplifiers is that itdelivers a specified power to the

load efficiently.

Sadhiqin 2011 10

• Power amplifiers

are classifiedaccording to the

percent time the

outputtransistors are

conducting, or

‘turn on’.

Sadhiqin 2011 11

Outline

Class A Amplifier

• Class A operation

• Heat dissipation and Centered Q-Point

• 2 load lines

• Power Gain

• DC Quiescent Power

• Output Power

• Efficiency

• Examples

Sadhiqin 2011 12

Outline

Class A operation

• A power amplifier is a large signal amplifier

that produces a replica of the input signal on

its output. In the case shown here, the output

is an inverted replica of the input.

Sadhiqin 2011 13

V in V out A

• Output is shown 180o out of phase with the

input (inverted)

Sadhiqin 2011 14

• The output of a class A amplifier conducts

for the full 360 of the cycle.

Sadhiqin 2011 15

Class A Amplifier

• 2 load lines

• Power Gain

• Output Power

• Efficiency

• Examples

Sadhiqin 2011 16

Outline

Notice that a class A amplifierdissipates dc power even with no

signal. The dc power dissipated is

the product of ICQ and V CEQ .

Heat Dissipation and Centered Q-point

• A class A power amplifier is a large signal amplifier that

operates in the linear region. Ideally, a class A amplifier is

designed to operate in the center of the ac load line.

Sadhiqin 2011 170

I c(sat)

AC load line

V CEQ V ce(cutoff) V CE

DC load line

The Q-point is set at the middle

of the load line so that the ACsignal can swing a full cycle.

Sadhiqin 2011 18

• If the Q-point is not centered on the ac load

line, the output signal will be limited

Sadhiqin 2011 19

a) Amplitude limited by cutoff b) Transistor driven into cutoff

with an increase of the input

amplitude

• Here is shown when the Q-point is moved

away from center towards saturation.

Sadhiqin 2011 20

a) Amplitude Vce and Ic limited

by saturation

b) Transistor driven into

saturation by a further

increase in input amplitude

Class A Amplifier

• 2 load lines

• Power Gain

• Output Power

• Efficiency

• Examples

Sadhiqin 2011 21

Outline

2 Load Lines

Sadhiqin 2011 22

Class A Amplifier

• 2 load lines

• Power Gain

• Output Power

• Efficiency

• Examples

Sadhiqin 2011 23

Outline

Power Gain

• A power amplifier delivers power

to the load.

• The power gain of an amplifier is

the ratio of the output power

(power delivered to the load) to

the input power.

Sadhiqin 2011 24

A p is the power gain

P L is signal power delivered to the load

Pin is signal power delivered to the amplifier

Class A Amplifier

Power Gain• DC Quiescent Power

• Output Power

• Efficiency• Examples

Sadhiqin 2011 25

Outline

DC Quiescent Power

• The power dissipation of transistor with no

signal input is the product of its Q-point

current and voltage.

Sadhiqin 2011 26

CEQCQ DQ V I P

Class A Amplifier

• Output Power

•Efficiency

• Examples

Sadhiqin 2011 27

Outline

Output Power

• The maximum power from a class A amplifier

Sadhiqin 2011 28

EQC CQout(max) V I P 5.0

Class A Amplifier

• Output Power

•Efficiency

• Examples

Sadhiqin 2011 29

Outline

Efficiency

• The ratio of the output signal power supplied

to a load to the total power from the dc

supply

Sadhiqin 2011 30

%25250

Increase Efficiency with Transformer

Coupling

• By using the transformer coupling, the

efficiency can be increased by 50% but there

are several drawbacks such as:

– Size of transformers

– Cost of transformers

– Potential distortion problems once the

transformer begins to saturate

Sadhiqin 2011 31

Class A Amplifier

• Power Gain

• Output Power

•Efficiency

• Examples

Sadhiqin 2011 32

Outline

Example 1

• Determine the voltage gain and the power

gain of the class A power amplifier below:

Sadhiqin 2011 33

Example 2

• Determine the efficiency of the same power

amplifier

Sadhiqin 2011 34

Example 3

• Figure below shows aCE power amplifier inwhich the collectorresistor serves also asthe load resistor.Assume

a) Determine the dc Q-

point (ICQ and VCEQ)b) Determine the

voltage gain and thepower gain.

Sadhiqin 2011 35

100 ac DC

c) The power

dissipated in the

transistor with no

d) The total power

from the power

supply with no loade) The signal power in

the load with a

500mV input

Sadhiqin 2011 36

Example 4

• Determine each Q-point for each amplifier

below:

Sadhiqin 2011 37

Example 5

• Find the maximum

output signal power

to the load and

efficiency for theamplifier in the figure

below with a 500 Ω

resistor.

Sadhiqin 2011 38

Example 6

• Calculate the input power, output power, and efficiency of the

amplifier circuit below for and input voltage that results in a

base current of 10mA peak.

Sadhiqin 2011 39

End of Class A Amplifier

• Power Gain

• Output Power

•Efficiency

• Examples

S dhi i 2011 40

Back to outline

BENE 3143_poweramp_classAamplifier (1)

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