8.4 Summary

Class B operation is defined as having transistor collector current active for 180° out of the waveform cycle. In order to amplify the entire 360° in linear fashion, two devices are required. Each transistor of a complementary pair is biased at cutoff to achieve a 180° conduction angle. This means that no-signal collector current is zero, leading to very low power consumption at idle and, unlike class A amplifiers, dynamic power consumption. Unfortunately, pure class B operation also results in notch or crossover distortion as the switch over from one transistor to the other is not seamless. This can be mitigated by biasing the devices slightly “on”, which is known as class AB operation. While simple resistor voltage dividers may be used for this purpose, a generally superior method uses diodes as they mimic the base-emitter current-voltage characteristic.

The compliance of a class B amplifier is based on its power supplies. Ideally, the peak-to-peak compliance of the amplifier will equal the total power supply differential. Worst case power dissipation and efficiency are far superior when compared to class A topology: Device power dissipation is only one-fifth of maximum load power and the theoretical efficiency at maximum load power is 78.5%. Maximum heating of the transistors occurs at approximately 40% of maximum load power.

A direct coupled drive stage is often used as it reduces parts count and improves performance. Amplifiers can also be extended through the use of Darlington pairs and current sharing schemes. Other refinements include the use of active current limiting for device protection, and V_BE multipliers in place of simple biasing diodes for greater flexibility.

Bridging is a technique used to increase output power. It relies on driving a floating load from both sides. Two amplifiers are needed for this configuration but it can offer a quadrupling of load power for the same power supplies.