What is a Class AB Amplifier?


As its name suggests, the Class AB Amplifier is a combination of the “Class A” and the “Class B” type amplifiers. The AB classification of the amplifier is currently one of the most commonly used types of audio power amplifier design. The class AB amplifier is a variation of a class B amplifier. However, both devices conduct at the same time around the waveforms crossover point. This eliminates the crossover distortion problems of the previous class B amplifier.


Two Transistors


The two transistors have a minimal bias voltage, typically at 5 to 10% of the quiescent current to bias the transistors just above its cut-off point. Then the conducting device, either bipolar or FET, will be “ON” for more than one-half cycle. However, much less than one full cycle of the input signal. Therefore, in a class AB amplifier design, each of the push-pull transistors is conducting for slightly more than half a cycle of conduction in class B. However, much less than the full cycle of conduction of class A. In other words, the conduction angle of a class AB amplifier is somewhere between 180o and 360o depending upon the chosen bias point, as shown.

The advantage of this small bias voltage is that it overcomes the crossover distortion created by the class B amplifier characteristics. It also lacks the inefficiencies of the class A amplifier design. So the class AB amplifier is a good compromise between class A and class B in terms of efficiency and linearity. Conversion efficiencies reach about 50% to 60%.


Overcoming Cross-Over Distortion


An alternative approach to overcome the cross-over distortion is to use the AB amplifier. Class AB amplifier uses an intermediate conduction angle of both Classes A and B. Thus; we can see the property of both Class A and Class B amplifier in this AB class amplifier topology. Same as class B, it has the same configuration with two active devices that conduct during half of the cycles individually. Still, each device is biased differently, so they do not get entirely OFF during the unusable moment (crossover moment). Each device does not leave the conduction immediately after completing half of the sinusoidal waveform. Instead, they conduct a small amount of input on another half cycle. This biasing technique dramatically reduces the crossover mismatch during the dead zone.

However, in this configuration, efficiency is reduced as the linearity of the devices is compromised. The efficiency remains more than the efficiency of a typical Class A amplifier. But it is less than the Class B amplifier system. Also, the diodes need to have the same rating. What’s more, they need to be as close as possible to the output device. In some circuit construction, designers tend to add a small value resistor. It provides a stable quiescent current across the device to minimize the distortion across the output.


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