Purpose of Q17 on LF351?

On page 7 of the (discontinued) LF351 datasheet, they have the schematic:

formatting link

Overall, the architecture looks pretty similar to every other OA. It's JFET input. There are current limiters at the output. etc. etc.

However, I don't understand the purpose of Q17. In fact, I can't imagine a time when it would ever become active!

I have a feeling it's there to protect the JFETs somehow, but it's unclear to me how it could.

Could someone help?

Thanks -- Ted

It prevents Q5 from saturating.

Cheers, James Arthur

I see. So, when Q5 starts saturating, Q17 robs Q4 of its base current, and everything calms down.

Keeping Q5 out of saturation improves the speed of the OA (and prevents any dangerous conditions setup by Q17's implementation).

That's terrific. Thanks!

--Ted

That is...

=2E..conditions setup by Q5's implementation.

Yep, that's it.

You're most welcome.

Cheers, James Arthur