Class AB Amplifiers

CLASS AB AMPLIFIERS

To overcome the cross over distortion in class B amplifiers, we are going for class AB amplifiers. The output signal is obtained for more than 180° but less than 360° of the ac input signal.

The Q point is adjusted to be slightly above the cutoff region.

Construction and Operation

Class AB amplifier is a combination of Class A and Class B amplifiers. To eliminate cross over distortion in class B push pull amplifiers, biasing is required. Q point of the transistor lies between that of Class A and Class B type amplifiers. A complementary symmetry Class B amplifier can be converted to Class AB amplifier by selecting resistor values such that V_BE of both T₁ and T₂ makes transistor away from the cut-off point i.e V_BE > 0.6 V.

The conduction angle of class AB amplifier is between 180° and 360°. There is no instant where both transistors are OFF simultaneously and thus cross-over distortion is eliminated. The efficiency of class AB power amplifier is around 70%.

Q point is not positioned at cut-off and there will be a small amount of collector current at zero input.

Advantage

1. No cross over distortion

2. Bulky coupling transformers are not required

Disadvantage

1. Less Efficiency compared to Class B configuration.

2. There will be some de components in the output as the load is directly coupled.

3. Capacitive coupling can eliminate de components but it is not practical for heavy loads.