Why is the following assumption valid?
For the positive side of v1 all load current is flowing through Qn and Qp is open.
In= IL and Ip=0.
And reversely  for negative side of vi:
Ip = Il and In=0.
The purpose of Vbb/2 is avoiding discontinuation of the output signal when v1&lt;0.6 V.
It's the enhanced version of the pur AB class amplifier.

Don't know if this help you.