I'm looking at an audio amplifier integrated circuit and the output stage seems a bit unusual. Has anyone seen this style of output stage, and does it have a name? And can anyone fill in the details on how it operates?

Here's my understanding of how it works: Transistors Q12 and Q14 form a push-pull output stage for a class AB amplifier. Q8 takes the output from the differential input stage and amplifies it. Q16, Q17, and Q18 are part of a current mirror (with Q13 providing 3 times the current). Q17 (and Q16?) provide base current for Q14, turning it on and pulling the output high. If Q8 conducts, the current from Q17 and Q16 flows through Q9 and Q8, turning off Q14. If Q8 conducts, Q11 will mirror the current through Q10, turning on Q12 and pulling the output low. Otherwise, Q13 will "dump" the current from Q18. Thus, the input current results in an inverted, amplified output.

I understand it at a hand-waving level, but I don't get why they built it this way. The transistors are carefully sized (e.g. x3 indicates 3 transistors in parallel), so there's something clever going on with the currents that I'm missing.

Partial schematic


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