Class AB amplifiers are a combination of Class A and Class B amps. They are also a specific variation of a Class B one. Both Class A and Class B conduct, at the same time, around the waveform’s crossover point. This action removes the crossover distortion conflicts of the latter. For its convenience, the Class AB amp has been one of the most used types of audio power amplifier designs until now.
The Transistors in Class AB Amplifiers
Both Class A and Class B transistors have a minimal bias voltage. This is usually between five and 10 per cent of the inactive current, precisely to bias the transistors above its cut-off point. After said point, the conducting unit would be on for much of the cycle of the input signal. So, in essence, the conduction angle of a Class AB amplifier ranges between 180º and 360º, depending on the selected bias point. This conducting unit can be either FET or bipolar.
On Efficiency and Linearity
Class AB amplifiers are a great combination between Class A and Class B in terms of efficiency and linearity. On the bright side, the tiny bias voltage can easily overcome the crossover distortion generated by the Class B amp. However, it lacks the inefficiencies of the Class A design. For overcoming the crossover distortion it’s important to use a Class AB amplifier. The reason is it uses an intermediate conduction angle of both amps. When using this biasing technique, this amplifier is able to reduce the crossover mismatch resulting during the dead zone. Each unit is biased differently to prevent them from turning off upon unstable or crossover moments.
In addition, each one conducts a tiny amount of input on another half cycle. Regardless, they never leave conduction immediately after completing half of the sinusoidal waveform. This configuration reduces efficiency dramatically because it compromises the linearity of the units. The diodes need to have the same rating and they need to be as close as possible to the output device. Some circuit have a small value resistor which provides a stable inactive current across the unit in order to minimize the distortion across the output.
