(21) Power Amplifiers

8/3/2019 (21) Power Amplifiers

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Power Amplifiers 1

Power Amplifiers


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Power Amplifiers 2

Need for a Power Amplifier

It is the last stage of almost all electronicsystems.

It is the power amplifier that drives theloudspeakers in an audio system.

Does a power amplifier actually amplifies

power ?

No. It only converts dc power to ac power,as controlled by the input signal.


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A Simple Power Amplifier


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How does a power amplifier differ from avoltages amplifier ?

It has different design considerations.

It handleslarge signals.

The effective dc input power is

where,

The total power drain from the dc supply is VCCICQ.The difference of the two powers is wasted as heatin RC and RE


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How to reduce this power loss ? Can wemake REzero ? No as stability will get

effected.But, we can do something about RC . Wecan replace it by a choke.


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LCCoupled load

Two advantages :1. No dc power loss.

2. No dc voltage drop; hence we can use lower

voltage supply, VCC. Still better would be to have transformer

coupled load


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Power Amplifiers 7

It gives added advantage ofimpedance matching.

This enables us to make use of Maximum powertransfer theorem.


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Power Amplifiers 8

Maximum Power Transfer

Theorem


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Maximum power is transferred when loadresistance is equal to source resistance.


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Power Amplifiers 10

How much maximum power can beobtained from a source ?

Transformer impedance matching :


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Power Amplifiers 11

Solution :


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Power Amplifiers 12

The power loss may occur :

Power Considerations


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Power Amplifiers 13

Current-carriers, while crossing thereverse-biased collector junction, lose

some energy.

This appears as heat.

If not dissipated, it may lead tothermal runaway.

To increase the heat dissipation

power, the body of the transistor isattached with heat sink.


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Power Amplifiers 14


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The dc power coming from the battery is

Power loss in the transistor is

It shows that PDdepends upon Po.

Worst case is when Po reduces to zero.


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Power Amplifiers 16

Output Circuit Efficiency

It is a measure of how well the power

amplifier converts the dc power from thebattery into useful ac output power.

Note that the greater the output power,

the greater is the efficiency.


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Power Amplifiers 17

Analysis of Single-Ended PowerAmplifier


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Power Amplifiers 18

Let the transistor have = 24, rin= 500 ,and PD(max) = 3.5 W.

The dc resistance of the primary is almostzero, and RE is very small (5 ).

The dc load line is almost vertical since Rc is

almost zero and the slope of dc load line isinverse of the RC+RE.

The Q- point should be fixed somewhere inthe middle, and below dissipation curve.

Passing through the Q-point, the ac load lineshould be drawn.


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Power Amplifiers 19


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The ac load on the collector, as seen lookinginto the primary is

2 2

' 1

2

38 72

1L L

NR R

N

First draw any lineAB. Then draw the ac loadline CD.

Now, suppose we apply Vi = 2 V (peak value).

Base current is 2/500 = 4 mA (peak value).


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Th t t i


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The output power is

Since, ICQ= 146 mA,

Thus, the output circuit efficiency is


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Maximum Theoretical Efficiency

The maximum swing ofQ-point takes placealong the ac load line, from point D( IB= 0) topoint C(saturation)


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Power Amplifiers 24


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In practice, the efficiency is about 35 %.

Based on the signal obtained at the output,we have


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Power Amplifiers 26


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Note the reduction in efficiency when thesignal is reduced.


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Power Amplifiers 28

Class-B Power Amplifier The transistor is biased just at cutoff. Thus, VCEQ= VCCand ICQ= 0.

Hence, under no signal condition, Pi(dc) = 0.

Let Vpbe the peak value of collector voltagevariations.


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and


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Power Amplifiers 31

Solution :


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Power Amplifiers 32

Harmonic Distortion in PowerAmplifiers

Due to large signals, this distortion is always

present.

For a given sinusoidal input signal, the outputcurrent is


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Power Amplifiers 33

Output ac power due to the fundamentalcomponent only:

This is the useful power at the output. But, the

actual output is


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Power Amplifiers 34

The total harmonic distortion (also calleddistortion factor) is

Thus,

Suppose a power amplifier gives 10 %distortion. The total output power is

The increase in power is only 1 %.


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Review Need for a Power Amplifier.

LC or Transformer Coupled load.

Maximum Power Transfer Theorem.

Heat Sink.

Output Circuit Efficiency.

Single-Ended Power Amplifier.

Class-B Power Amplifier.

Harmonic Distortion.

Distortion Factor.

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(21) Power Amplifiers