1
Power Amplifiers & Heat SinksPresented By :
S.V. Rahul Nitin Miglani
2
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
3
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.
4
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
5
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
6
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
7
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
8
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
9
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
10
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