Basic ELECTRONICS

(2+1)ENGR MUHAMMAD SHAKEEL AKRAM

ELECTRONICS ENGINEERING FICT, BUITEMS.

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CHAPTER 7

Power AMPLIFIERS

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POWER AMPLIFIERS

• Those amplifiers that have the objective of delivering power to a load.

• Rated more than 1 W• Leads to power dissipation problem

• Resolved using heat sink• In case of transistor collector is critical

terminal

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CLASS A POWER AMPLIFIER• Transistor always operates in linear region • Current flows for 360˚• Output is the amplified replica of input signal• Large signal type

• When output signal is large and approaches the limits of ac load line

• Both large and small signal type comes under class A if the operate throughout in linear region

• Efficiency nearly equal to 25%

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CLASS A POWER AMPLIFIER• Centered Q- point

• Point where DC and AC load line intersects• For Class A it must be at the center

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CLASS A POWER AMPLIFIER• Centered Q- point

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CLASS A POWER AMPLIFIER• Centered Q- point

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CLASS B AND AB PUSH PULL POWER AMPLIFIER

• Class B• Biased at cutoff so that operates for only 180˚

• Class AB• Biased to conduct slightly more than 180˚ but

less than 360

• Advantage • Higher Power gain as efficiency is nearly

equal to 79%• Dis advantage

• Difficult to implement

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CLASS B AND AB PUSH PULL POWER AMPLIFIER

• Class B operation • Biased at cutoff so that operates for only 180˚• Q point at cutoff ICQ = 0 and VCEQ = VCE(Sat)

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CLASS B AND AB PUSH PULL POWER AMPLIFIER

• Class AB push pull operation • Two class B amplifiers. One operate in positive cycle

and other in negative• Two approaches to do that

• Transformer coupling

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CLASS B AND AB PUSH PULL POWER AMPLIFIER

• Class AB push pull operation • Two class B amplifiers. One operate in positive cycle

and other in negative• Two approaches to do that

• Complementary symmetry transistor

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CLASS C POWER AMPLIFIER• Operates for less than 180˚ of input cycle• Ideal maximum collector current is Ic(sat) and ideal

minimum collector voltage is Vce(cutoff)

• Provides more power gain as compared to Class A, B and AB as its efficiency in nearly equal to 100%

• Out amplitude is non-linear • Used in RF applications , oscillators, modulator

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CLASS C POWER AMPLIFIER• Basic class C operation

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