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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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Power Amplifiers 3
A Simple Power Amplifier
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Power Amplifiers 4
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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Power Amplifiers 5
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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Power Amplifiers 6
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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How much maximum power can beobtained from a source ?
Transformer impedance matching :
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Solution :
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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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Power Amplifiers 15
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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Power Amplifiers 20
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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Power Amplifiers 21
Th t t i
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Power Amplifiers 22
The output power is
Since, ICQ= 146 mA,
Thus, the output circuit efficiency is
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Power Amplifiers 23
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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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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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.