Purpose of Q17 on LF351?
Ted Pavlic
schematic:
http://cache.national.com/ds/LF/LF351.pdf#page=7
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
Winfield Hill
Imaging Q5 were to become saturated as the LF351 amplifier
circuit attempted to make its output more negative (a minus
offscale situation), what's to limit how much rail-to-rail Q4
current flows from the positive-supply into Q5's base? That's
what Q17 does, it shuts off Q5's base drive when its collector-
emitter voltage drops to about 0.6 volts. This also has the effect
of limiting the LF351's most negative output pulldown to about
1.3 volts above the negative supply rail...
Ted Pavlic
> > schematic:
> >http://cache.national.com/ds/LF/LF351.pdf#page=7
> > imagine a time when it would ever become active!
> circuit attempted to make its output more negative (a minus
> offscale situation), what's to limit how much rail-to-rail Q4
> current flows from the positive-supply into Q5's base? That's
> what Q17 does, it shuts off Q5's base drive when its collector-
> emitter voltage drops to about 0.6 volts. This also has the effect
> of limiting the LF351's most negative output pulldown to about
> 1.3 volts above the negative supply rail...
Thanks, Dr. HIll. That makes perfect sense. That probably makes the
LF351 a lot faster getting out of its negative output rail too.
At risk of sounding trite, thanks for one of the most valuable
textbooks in my office as well.
Best wishes --
Ted