With a Class B amp, complementary transistors handle the positive and negative halves of the signal. One output transistor handles the positive half of the signal, another transistor handles the negative half of the signal. Each transistor runs half of the time, which is efficient; however; when one transistor hands off to the other one, the transition is not seamless. Distortion will result.
With a Class A amp, all output transistors operate all of the time. You don't get the distortion that occurs in Class B when one is handing off to the other. However, it is inefficient in that everything is operating all of the time. It produces a lot of heat and needs a good power supply.
A Class AB amp has transistors that hand off to one another for the positive and negative halves of the signal (like Class B), but there is a little bit of overlap in order to minimize the distortion that would otherwise occur when one hands off to the other. In this overlap region, both transistors are operating simultaneously (like Class A).
By the way, there are other classes as well.
With a Class A amp, all output transistors operate all of the time. You don't get the distortion that occurs in Class B when one is handing off to the other. However, it is inefficient in that everything is operating all of the time. It produces a lot of heat and needs a good power supply.
A Class AB amp has transistors that hand off to one another for the positive and negative halves of the signal (like Class B), but there is a little bit of overlap in order to minimize the distortion that would otherwise occur when one hands off to the other. In this overlap region, both transistors are operating simultaneously (like Class A).
By the way, there are other classes as well.