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Power Amplifiers & Heat SinksPresented By :

S.V. Rahul Nitin Miglani

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Contents

Presented By : S.V.Rahuli. Voltage v/s Power Ampli

fiersii. Power Amp : Introductio

niii. Power Amps : Class Aiv. Transformer Coupled Cla

ss A Power Ampsv. Power Amps : Class B

Presented By : Nitin Miglanivi. Crossover Distortionvii. Power Amps : Class ABviii. Power Amps : Class Cix. Heat Sinksx. Techniques used for Hea

t Sinks

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Voltage v/s Power Amps

Voltage Amplifiers• Small Signal Amplifiers• Small Power Output• Less Collector Current• Low Magnitude Input Signal Voltages• Low Current Devices with high Output

Resistance• Very Less Heat Generated.

Power Amplifiers• Large Signal Amplifiers• Large Power Output• High Collector Current• Higher Magnitude Input Signal

Voltage• Low output resistance & Large supply

currents required• Considerable Heat Generated, Heat

sinks required.

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Power Amplifier : Introduction

Power Amplifiers:• Transistor Amplifier which

Raises power level of I/p Signals

• Large amount of power may Be fed to load

• Generally used in last stage Amplification of a multi-Level amplifier

• Classification according the Angle of conduction

Classification

Pow

er A

mpl

ifier Class A

Class BClass ABClass C

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Power Amps : Class A• Conduct over the full

input cycle• Highly Linear Relation

between output & input signals.

• Max. Efficiency 25%• Absence of Crossover

distortion

Q

I C

I BQI CQ

VCC

VCEVCEQ

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Transformer Coupled Class A Amps

• Transformer Used to Couple Output Signal from Amp to Load

• Impedance Matching done at the Transformer

• Maximum Power Transfer When o/p Impedance Equals Load RL

• Efficiency Raised to 50 % Due to the Absence of Rc

VCCRL

R2

R1 RE

BC

E

vo

I B

vin

n:1

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Power Amps : Class B• Amplification Done only

for a Half Cycle• Large Distortion in

Output Signal• Max. Theoretical

Efficiency 78.5%• Practically Implemented

in Push Pull Circuits.• Disadvantage: Crossover

Distortion is Introduced

VCC

RC

R1 RE

BC

E

vo

I B

I C

vin

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Crossover Distortion• In push-pull configuration,

one device must begin operation exactly as Other stops

• Mismatch at Junction can Cause Distortions at Output

• This is called Crossover Distortion

• Improvement : Biasing Devices such that they are not completely off after a half cycle

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Power Amps : Class AB

• Transistors Operate over a Half Cycle & also over Small region of Other Half.

• Max Efficiency < 78.5%• Efficiency of Class B is

Traded-Off for increased Linearity of Output.

• Lesser Crossover Distortion.

VCC

RCR2

RE

BC

E

voI BQ

I CQ

vin

D

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Power Amps : Class C• Device Operated as

Switch to Reduce Resistance Loss

• Very Short Conduction Period

• Power loss Greatly Minimized

• High Amplification Efficiencies 90% possible.

• High Output Distortion

Q

I C

I BQ< 0VCE

I CQ= 0

VCC

VCC

RC

R2

RE

BC

E

voI BQ< 0

I CQ=0

vin

-VBB

• All Components Need Specific Operating Temperatures.

• Large Amount of Power is Dissipated while Power Amplifier Operates

• Excess Heat Causes Increase in device Temperature

• Heat sink is a Passive Component that Cools a Device by Dissipating excess Heat Into the surroundings. 

Heat Sinks

Speedup of natural convection done using fans

Surface Area increased using fins speeding up all forms of heat loss

Components are usually made black to radiate the most heat.

Techniques used for

Heat Sinks

ReferencesOnline Resources

Lecture Notes by Dr. K W Whites : whites.sdsmt.edu/

Wikipedia: en.wikipedia.org/

All about circuits: allaboutcircuits.com/

Offline Resources Lecture Notes by

Dr. S. Nagpal Integrated

electronics by : Jacob Millman & Christos C. Halkias