Class B BJT Amplifiers

High-power PNP transistors tend to be scarcer and more expensive than high-power NPN transistors, a fact which complicates the construction of a high-power complementary push-pull amplifier circuits. An ingenious solution to this problem is to modify the basic Darlington push-pull circuit, replacing the final PNP transistor with an NPN transistor, like this:

The cascaded combination of an NPN and PNP transistor is called a Sziklai pair, or complementary Darlington pair. In this case, the small PNP transistor controls the larger NPN power transistor in the Sziklai pair, performing the same basic function as a PNP Darlington pair.

Modify the circuit shown here to use diodes in the biasing network instead of just resistors. The solution is not quite the same for this circuit as it is for a conventional Darlington push-pull circuit!

A popular variation of the Class B amplifier is the Class AB amplifier, designed to eliminate any trace of crossover distortion. What makes the difference between a Class B and a Class AB amplifier? Why do Class AB amplifiers have less crossover distortion than Class B amplifiers? And, is there any disadvantage to changing from Class B to Class AB operation?

An interesting addition to the basic Class B push-pull amplifier circuit is overcurrent protection, in the form of two more transistors and two more resistors added to the circuit:

This form of overcurrent protection is common in voltage-regulated DC power supply circuitry, but it works well in amplifier circuitry, too. Explain how the additional transistors and resistors work together to protect the main power transistors from damage in the event of an overload.