Interesting biasing schemo Q

[Samuel Groner]

I recently studied the "equivalent schematic" of an Apex opamp: PA09U.pdf

The biasing circuitry seems to be interesting; note the clever arrangement for Q16 to get high output swing.

However, I do not understand the function of Q18 and it's associated resistors. At first it looks like it would provide constant current for D1, but that's already done with Q10/D3, right?

If I were to implement this in discrete form, I would like to add some bits to get rid of the zener noise; is bypassing them with some uF a good solution?

Thanks!
Samuel

 

[bcarso]

Methinks there is a drafting error---what is zener D3 doing? It's shown across Q10's base-emitter junction. OTOH if it were instead interchanged with the R directly above it, the overall impedance of the string of parts from rail-to-rail would be uncomfortably low.

Etwas ist nicht in Ordnung.

 

[Samuel Groner]

I quickly went through many other schematics of this company - chances are good that we'll find an error. Sometimes they show a zener "bypassed" with a wire - now that is fun!

I think Q10 needs an emitter resistor, no?

Samuel

 

[PRR]

> the function of Q18

Thermal shutdown.