Class AB Amplifier

What Is a Class Ab Amplifier?

As its name suggests, the Class AB Amplifier combines 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 simultaneously around the waveform 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 their cut-off point. Then, the conducting device, either bipolar or FET, will be "ON" for over half of the cycle. However, there is much less than one complete input signal cycle. Therefore, in a class-AB amplifier design, each push-pull transistor conducts for slightly more than half a cycle of conduction in class-B. However, it is much less than the entire 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 regarding efficiency and linearity. Conversion efficiencies reach about 50% to 60%.

Overcoming Crossover Distortion

An alternative approach to overcome the crossover 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 properties of Class-A and B amplifiers in this AB class amplifier topology. It has the same configuration as class-B, with two active devices that conduct 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 that of a typical Class-A amplifier but less than that of the Class-B amplifier system. Also, the diodes need to have the same rating.

Moreover, they must 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.

Source Texts

• Amplifier Classes
• Classes of Power Amplifiers