Post on 15-Feb-2017
transcript
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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