1. Amplifiers Types of Amplifiers : Small Signal Amplifier
Large Signal Amplifier

2. History
Power Amplifier was invented in 1909 by Lee De Forest who also invented Triode vacuum tube.
Triode vacuum tube was used to make first AM Radio.
Early Power Amplifier was based on vacuum tubes but Modern Power Amplifiers are based on Transistors, MOSFET, FET’s BJT’s etc.
Lee De Forest

4. Definition of Power Amplifier
Amplify :
The term Amplify means to increase the strength of weak signal without changing its shape.
Power Amplifier :
“Power Amplifier” is that amplifier which amplifies low power audio signals to a level suitable for driving loud speakers.
Simplest Power Amplifier circuit is “emitter follower” circuit.
I/P to Power Amplifier is few micro watts but O/P of Power Amplifier is in thousands of watts.

5. Block diagram of an AF Amplifier
Power Amplifier is generally used in Public Address System.

6. Applications of Power Amplifier
Main application is Public address system.
Theatrical and concert sound reinforcement.
In domestic sound system such as stereo or home theatre system.
In Instrument amplifier including guitar amplifier.

7. Comparison between Small Signal & Power Amplifier
Small Signal Amplifier
Large Signal Amplifier
It amplifies Voltage.
It is small in size.
No Power Transistors required.
Used in preamplifier or voltage amplifier.
It amplifies Current or Power.
It is Bulky.
Power Transistors are required.
Used in Last stage in P.A. System & other circuits.

8. Features of Power Amplifier
Impedance matching with load.
Power Transistors are required.
Power Amplifiers are bulky.
Harmonic distortion is present at o/p.
Capable of handling large power.

9. Types of Power Amplifier
Class A
Class B
Class AB
Class C

10. Position of Q point Types Position of Q point Class A Class B Class AB
Class C
Center of load line.
In cut off region.
Just above cut off.
Below cut off.

11. Output waveform in different Power Amplifier classes
Class A
Class B
Class C
Class AB

12. Features of Class A Power Amplifier
Transistor conducts for full cycle duration i.e. 360 degree.
Q Point is at center of load line.
There is no any distortion at the output.
Large Power dissipation.
Very low efficiency as compared to other Amplifier.

13. Class A Power Amplifier
Why Large Power dissipation?
Due to use of Transistor continuously in active region.
Why Less Efficiency?
As large Power is dissipated in form of heat Class A Power Amplifier has less efficiency.

14. CKT Diagram & Waveform of Class A
Circuit Diagram
Waveform

15. Advantages & Disadvantages of Class A
Simple Construction.
No Distortion at output.
Disadvantages:
Very low efficiency(25% to 50%).
Large Power dissipation in power transistors.

16. Features of Class B Power Amplifier
Transistor conducts only in one half cycle i.e. 180 degrees.
Q point is adjusted to be at cutoff region.
Crossover distortion is present at o/p.
No Power dissipation.
Higher efficiency.

17. Class B Power Amplifier
Why zero power dissipation?
As Transistor remains off in negative half cycle.
Why Higher efficiency?
Due to no Power dissipation.

18. Waveform & CKT Diagram of Class B

19. Advantages & Disadvantages of Class B
Higher efficiency (78.5%).
Zero Power dissipation.
Impedance matching is possible.
Disadvantages:
Crossover distortion is present at o/p.
Efficiency is low as compared to Class C.

20. Class AB Power Amplifier
Features of Class AB Power Amplifier
Transistor conducts between 181 to 359 degrees.
Q Point is adjusted just above the cutoff.
No any crossover distortion.
Efficiency is equal to Class B.
Power dissipation is between Class A & Class B operation.

21. CKT Diagram & Waveform of Class AB

22. Advantages of Class AB Power Amplifier
Elimination of Cross over distortion.
Configuration is preferred in audio system, radio, TV receiver.
Efficiency:
Efficiency is same as that of Class B Power Amplifier i.e. 78.5%

23. Class C Power Amplifier
Features of Class C Power Amplifier
Transistor conducts for less than 180 degrees.
Q point is adjusted below the cutoff.
Distortion can be present at the output.
Low Power dissipation.
Efficiency is very high (More than 95% ).

24. Application of the Class C
Class C Amplifiers generally use a tuned circuit as load.
It is also used as a collector modulator to produce amplitude modulated signal.

25. CKT Diagram & Waveform of Class C

26. Advantages & Disadvantages of Class C
Very Higher efficiency (95%).
Less Power dissipation.
Disadvantages:
Output waveform can be distorted.