Seminar on "Power Amplifiers"

Seminar on "Power Amplifiers"

Amplifiers Types of Amplifiers : Small Signal Amplifier Large Signal Amplifier 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, FETs BJTs etc.

Lee De Forest 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. Power Amplifier is generally used in Public Address System. 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.

Small Signal Amplifier It amplifies Voltage. It is small in size. No Power Transistors required. Used in preamplifier or voltage amplifier.

Large Signal Amplifier It amplifies Current or Power. It is Bulky. Power Transistors are required. Used in Last stage in P.A. System & other

circuits.

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

Class Class Class Class

A B AB C Position of Q point Center of load line.

In cut off region. Just above cut off. Below cut off. Class A Class C

Class B Class AB 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. 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. Circuit Diagram

Waveform Advantages: Simple Construction. No Distortion at output. Disadvantages: Very low efficiency(25% to 50%).

Large Power dissipation in power transistors. 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. Why zero power dissipation? As Transistor remains off in negative half cycle.

Why Higher efficiency? Due to no Power dissipation. Advantages: 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. 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. Advantages:

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%

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% ). Class C Amplifiers generally use a tuned circuit as load. It is also used as a collector modulator to produce amplitude modulated signal.

Advantages: Very Higher efficiency (95%). Less Power dissipation. Disadvantages: Output waveform can be distorted.

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