Purpose of Q17 on LF351?

[Ted Pavlic]

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

 

[clicliclic@freenet.de]

Ted Pavlic schrieb:

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

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.

 

[Ted Pavlic]

http://cache.national.com/ds/LF/LF351.pdf#page=7
However, I don't understand the purpose of Q17. In fact, I can't
imagine a time when it would ever become active!

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.

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

 

[Ted Pavlic]

http://cache.national.com/ds/LF/LF351.pdf#page=7
However, I don't understand the purpose of Q17. In fact, I can't
imagine a time when it would ever become active!
Could someone help?

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.

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

Thanks, again --
Ted