8.1 Introduction
B amplifiers have long been a mainstay of linear amplifier design. Compared to class A operation, class B amplifiers offer much greater power efficiency. That is, a much larger percentage of the applied DC power can be turned into useful AC output to the load. This also means that the power dissipation requirements for the transistors are lowered. Further, unlike class A operation, class B designs do not continuously draw full power from their DC supplies. Instead, they draw current as it is needed, and therefore run relatively cool at idle and at low output power.
The downside for this improved efficiency is added complexity. For starters, two transistors are required for linear class B operation. Also, the biasing can be a little trickier which requires modifications to the relatively straightforward class A biasing circuits we have already examined. Also, class B amplifiers suffer from a unique form of distortion that class A amplifiers do not.
In this chapter we shall also examine the use auxiliary device configurations and sub-circuits to improve performance. These include the current mirror, Sziklai pair, VBE multiplier, and overload protection and prevention circuitry. As with the class A amplifier, the common collector or voltage follower configuration tends to be the most widely used configuration for class B circuits, hence, we shall focus on followers and not on voltage amplifiers.
