class AB amplfier help
Let's say that Q9 gets removed, and the bases of Q1 and Q16 get tied together to the op amp output. This is called a class B amplifier. The two halves (upper and lower) of the output circuitry get turned on or off depending on the polarity of the op amp output.

There's a problem, though. For op amp voltages near zero, neither half is conducting. (Why? hint - think about base voltages) So during the period that the op amp is transitioning from plus to minus or vice versa the output voltage won't respond at all. This is called crossover distortion.

Now add Q9 back in. It's just a current source, and it keeps both halves just a little bit on, so there is no dead zone during crossover. Technically, you could make a case for the amp being class A, since both halves are always conducting, but the "always on" current is very low compared to the peak current, so this is not done. Instead, the circuit is considered a hybrid of class A and B, or just a class AB amplifier.