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 = P_L(max)/ P_S x 100 %
Where P_S is DC input power.
• Load Power = P_L = (V_L)² /R_L = (V_PP)²/8*R_L

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.
V_CEQ = V_CC/2
• AC load power of class-B push pull amplifier,
P_L = (V_PP)²/8*R_L

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,
P_L = (V_PP)²/8*R_L
Where, V_PP is peak to peak load voltage and R_L 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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