This document discusses different classes of power amplifiers, including: - Class A power amplifiers, which operate linearly for 360 degrees and have an efficiency of around 25%. - Class B and AB push-pull amplifiers, which operate for more than 180 degrees but less than 360 degrees, and can have an efficiency near 79%. - Class C power amplifiers, which operate for less than 180 degrees but can have an efficiency near 100%, making them useful for applications like RF and oscillators.

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CHAPTER 7

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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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