What is a Class B Amplifier?

A Class B amplifier is similar to a Class A amplifier but is created using two transistors, which have a unique push-pull configuration. Each transistor conducts half of the input cycle (180°) and provides a combined current drive for the load. A push-pull amplifier is a type of electronic circuit that uses a pair of active devices (diode, transistor, etc. ) to supply current to the connected load alternately. This design results in high efficiency and reduced power dissipation compared to Class A amplifiers, thus making them ideal for applications where energy efficiency and cost-effectiveness are important factors. They’re commonly used for RF signal amplification, in audio systems, and for signal processing due to simple thermal management and minimal DC power consumption. 

Working Principle of a Class B Amplifier

A sinusoidal input signal consisting of two halves – positive and negative is given as an input. The Class B amplifier uses a push-pull configuration with two complimentary transistors (NPN and PNP) or a pair of MOSFETs. Each transistor amplifies one-half of the input signal. The NPN transistor (or N-channel MOSFET) conducts during the positive half of the input signal, whereas the PNP transistor (or P-channel MOSFET) conducts during the negative half of the input signal. During each cycle, the input is amplified and is produced at the output. The output signal resembles the original waveform but with a higher amplitude.

The silicon transistors must have at least 0.7V of forward base-emitter bias in order to conduct the signal. As the forward bias in the Class B amplifier is produced by the input signal, as long as the input is below 0.7 V, the transistors will be in a non-conducting state. This forms a “dead band” in the input and produces a crossover distortion in the output. In other words, when the input signal transitions from positive to negative or vice-versa, ideally, another transistor should turn on precisely as the former one turns off. However, due to the delay in the switching mechanism, there’s a brief period when none of the transistors is conducting, which creates a distortion in the output at the cross-over point. 

Cross-over distortion 

Advantages of Class B Amplifiers 

Class B amplifiers have more efficiency than Class A amplifiers because each transistor conductor for half of the input cycle. This reduces the power consumption and helps in increasing the overall efficiency. Also, less power consumption leads to less heat generation, which helps in simplifying thermal management and reduces the need for extra heat sinks. The push-pull configuration used helps cancel out the even-order harmonic distortions and improves the signal quality.

Disadvantages of Class B Amplifiers 

The most notable disadvantage of Class B amplifiers is the crossover distortion at the zero-crossing point during the switching of the transistors. It impacts the signal quality, especially for low-power signals. In order to remove the crossover distortions, a complex biasing is required, which again becomes a disadvantage of the Class B amplifier.