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Purpose of Q17 on LF351?

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

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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

Reply to
Ted Pavlic
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Ted Pavlic schrieb:

Forget the collector of Q17: that makes Q17 a diode between the base of Q 4 and the collector of Q5. Since Q5 is just an emitter follower of Q4 the diode prevents saturation of Q5. The result is an improved switching speed when the opamp is overdriven. The same trick can be found in many opamps.

Martin.

Reply to
clicliclic

That's beautiful. When Q5 starts to saturate, Q17 takes control and robs Q4 of base current. I'm happy with that. Terrific!

Thanks, again -- Ted

Reply to
Ted Pavlic

Additionally, depending on how Q5 is implemented, saturation could be dangerous to the IC. Yet another benefit of Q17, right?

Thanks, again -- Ted

Reply to
Ted Pavlic

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