Thus each transistor will conduct between 180 o and 360 o of the time depending on the amount of current output and pre-biasing. This action is achieved by pre-biasing the two transistors in the amplifiers output stage. Then the Class AB amplifier output stage combines the advantages of the Class A amplifier and the Class B amplifier while minimising the problems of low efficiency and distortion associated with them.Īs we said above, the Class AB Amplifier is a combination of Classes A and B in that for small power outputs the amplifier operates as a class A amplifier but changes to a class B amplifier for larger current outputs. One way to produce an amplifier with the high efficiency output of the Class B configuration along with the low distortion of the Class A configuration is to create an amplifier circuit which is a combination of the previous two classes resulting in a new type of amplifier circuit called a Class AB Amplifier. The advantage here is that the DC-to-AC conversion efficiency is much higher at around 75%, but this Class B configuration results in crossover distortion of the output signal which could be unacceptable. However if we modify the Class A amplifier circuit to operate in Class B mode, (where each transistor conducts for only 50% of the time) the collector current flows in each transistor for only 180 o of the cycle.
We have seen that the power output of an amplifier is the product of the voltage and current, (P = V*I) applied to the load, while the power input is the product of the DC voltage and current taken from the power supply.Īlthough the amplification of a Class A amplifier, (where the output transistor conducts 100% of the time) can be high, the efficiency of the conversion from the DC power supply to an AC power output is generally poor at less than 50%. The purpose of any amplifier is to produce an output which follows the characteristics of the input signal but is sufficiently large enough to supply the needs of the load connected to it.
Importance of transistor biasing full#
Both the Class B amplifier and the Class AB amplifier have an output stage comprising of two transistors whose outputs are configured in such a way as to reconstruct the full 360 o input waveform with or without distortion.
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