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Amplifier classes-class A,class B,Class AB,Class C

This page provides comparison between amplifier classes viz.class A,class B,Class AB and Class C.

Amplifier class-A operation

• In this class-A, amplifier operates in active region at all the times(2*π).
• Stage efficiency = PL(max)/ PS x 100 %
Where PS is DC input power.
•  Load Power = PL = (VL)2 /RL = (VPP)2/8*RL

Amplifier class-B operation

• In this class-B, current flows for half of the input cycle(π).
• Q point is located at cutoff the DC and AC load lines.
•  AC load power of class-B push pull amplifier,
PL = (VPP)2/8*RL

Amplifier class-AB operation

• In this class-AB, amplifier operates between half and full input cycle(between π to 2*π).

Amplifier class-C operation

• In this class-C, amplifier operates for less than half of the input cycle(less than π).
• It is basically a tuned amplifier.
•  AC load power for class-C amplifier is,
PL = (VPP)2/8*RL
Where, VPP is peak to peak load voltage and RL is load resistance

Following table mention difference between Class-A,Class-B,Class-AB and Class-C with respect to important parameters.

Amplifier Class type Q point Iq Conduction Angle Maximum Efficiency
Class A 0.5 0.5 2*pi 50%
Class B 0 0 pi 78%
Class AB 0-0.5 0-0.5 pi-2*pi 50-78%
Class C <0 0 0-pi approaches to 100%

In order to operate transistor (FET) for a certain class, the gate and drain DC voltages have to be biased carefully to the certain operating point which is called as Quiescent point or Q-point.

Class A offers least distortion, while Class C offers maximum distortion of the output signal.


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