Class B Amplifier

Class B Amplifier Advantages

The class-B Amplifier has a significant advantage over its class-A amplifier cousins in that no current flows through the transistors in their quiescent state (i.e., with no input signal). Therefore, the output transistors or the transformer dissipate no power when no signal is present. On the other hand, class-A amplifier stages require significant base bias, thereby dissipating lots of heat – even with no input signal present. So, the amplifier's overall conversion efficiency (η) is higher than that of the equivalent class-A, with efficiencies reaching as high as 70%, resulting in nearly all modern types of push-pull amplifiers operating in this class-B mode.

Using Two Devices

In a class-B amplifier, the active device conducts for 180 degrees of the cycle. This would cause extreme distortion if there were only one device. So, two devices are usually used, mainly at audio frequencies. Each conducts for one-half (180°) of the signal cycle, and the device currents combine to keep the load current continuous. When class-B amplifiers amplify the signal with two active devices, each operates over one-half of the cycle. Efficiency is much higher over class-A amplifiers. People also favor class-B amplifiers in battery-operated devices, such as transistor radios. Class-B has a maximum theoretical efficiency of π/4 (≈ 78.5%).

Preventing Distortion

At radio frequency, if the coupling to the load is via a tuned circuit, one can use a single device operating in class-B because the stored energy in the tuned circuit supplies the "missing" half of the waveform. Linear amplifiers use devices operating in class-B. The radio frequency output power is proportional to the square of the input excitation voltage. This characteristic prevents the distortion of amplitude-modulation or frequency-modulated signals passing through the amplifier. Such amplifiers have an efficiency of around 60%.

Drawbacks

A practical circuit using class-B elements is the push-pull stage. Complementary devices amplify the opposite halves of the input signal. After that, the input signal recombines at the output. This arrangement gives good efficiency but usually suffers from the drawback of a small mismatch in the crossover region – at the "joins" between the two halves of the signal, as one output device has to take over supplying power exactly as the other finishes. This is called crossover distortion. An improvement is to bias the devices so they are not completely off when not in use. This approach is called a class-AB operation.

Class B Amplifier