Does anyone know what this diode is for? [SOLVED]

[DiligentMinds.com]

.

[w2aew]

My first thought would be that it is used as part of temperature compensation. The tempo of the NPN b-e junction somewhat compensates for the Zener tempo.  Perhaps the diode is helping to minimize variations in the opamp q-point vs. temperature.

[DrGeoff]

I'd say it is supplying the power to the LTZ1000 block.

[HackedFridgeMagnet]

the blurb from linear tech says:

External circuitry is used to set operating currents and to temperature stabilize the reference. This allows maximum flexibility and best long term stability and noise.

Apart from seeing that, I cant help.

[elal1862]

My best guess would be: To prevent the opamp's output from sinking current.
Having a source/sink voltage source sounds nice, but they can have some serious stability issues.

So the diode is (most likely) there to prevent instability -or even oscillation- under certain load conditions.

[w2aew]

Yeah, I thought about that too, but the op-amp output will do whatever it has to do to keep the (+) and (-) inputs the same-- thus, the LTZ1000 is in control, not the op-amp.  The temperature compensation is controlled by the NPN, and the ratio of the transistor current to the zener current.  The 30K resistor controls this ratio (and thus the temperature compensation).  The 120-ohm resistor controls the zener current.  The diode (when installed) only serves to cause the op-amp to have to output a slightly higher voltage than it would without it.  In temperature sweeps in LT-Spice, the diode has no effect (other than what was already stated).  The designer must have put it there for a reason, but for the life of me, I can't figure out what it is...

Anybody?

I still suspect it's part of fine temperature compensation.  Even though its inside the feedback loop, it will have a small influence on the opamp's output voltage vs. temperature, thus have a very small influence on the input voltage difference, and therefore on the input bias currents vs. temperature.  Note that the input bias current changes vs. temperature might not be included in the LTSpice model.

[muvideo]

Hello, while waiting for Linear's answer I give my thought.
Probably is useful to startup correctly the reference,
upon startup both opamp inputs are zero, so depending
on the unit's input offset, the out will try to go either up,
following starting the reference, or down locking it.
The diode will keep the opamp without feedback until
the out will not forward-bias the diode, so it will amplify
the noise on inputs with a gain of few million times.
So basically it's waiting that the noise will steer the out
the correct way (up) to start the reference.

Disclaimer: here is late, so I could have written strange
or plainly wrong things
So I asked also advice also on volt-nuts mailing list,
they are the ones on these sort of things.

Fabio.

[muvideo]

Well done muvideo!  You are the winner! (No prize, just recognition, respect, and thanks).

Not bad from a non EE 

By the way that's all the best I'm happy to win on a friendly forum,
the problem is when that is all my employer wants to pay,
and is becoming too frequent here lately     

Fabio.

[ecat]

I do know this much,

Albert Einstein:
He graduated from high school at the age of 17 and enrolled at the Swiss Federal Polytechnic School in Zurich. Albert graduated in 1900 with a degree in physics.

Bill Gates:
A dumpster diver, bullshitter and to all intents a thief - allegedly .

Nikola Tesla:
Earned degrees in physics, mathematics, and mechanical and electrical engineering from the Austrian Polytechnic Institute... Before he moved to the US and became famous?

[muvideo]

@Fabio,
Ahhh, but you are in good company.  Jim Williams did not have a degree. 
[cut...]
Like Steve Wozniak says:  "Don't get a degree, start a company instead."... And he should know...

So it's even worse, I have a degree (ME), so no hope for me, I'm doomed...   

[free_electron]

you know how einstein was always bad at maths ? well, i'm even worse ! 

[ecat]

Actually, Nikola Tesla was going to he Austrian Polytechnic Institute, but his father died and he could not complete his studies due to lack of funds.  All of the "degrees" he received later were "honorary"-- and honestly, Mr. Tesla knew far more about electromagnetics and the Aether than we do today-- so to have him get an official degree would be silly-- it would be like the student trying to teach the professor.  I have been studying his work over the years (when I have the time), and the more I read, the more I am astonished at his brilliance.

Contrary to popular opinion, Albert Einstein flunked high school (probably because he was bored).  He never finished his studies at the Swiss Federal Polytechnic School.  All degrees he received were also "honorary".  The version you are quoting is the "for public consumption" version.  The official records and historical accounts were modified and the books were cooked to reflect the "public version" of events.  (Yes, there are people that rewrite history to suit their agenda.)

I agree with what you said about Bill Gates-- but you were too kind.  The reason I mentioned him before, is that it shows that you can become successful even without a degree.  Perhaps he was a bad example, because he made his money on the backs of others.  A better example might have been Ray Dolby, who quit school to found Dolby Labs, where his famous audio noise reduction system was born.

I'll give you the Tesla history, I had a vague memory there was some complication. Still, I'd bet money he would have passed

Einstein: I thought his lack of any degree was the 'popular opinion', which is why I jumped in - A recent T.V. program went out of its way to make this point. Could have been a mathematics and physics teaching diploma?

[ecat]

"... A recent T.V. program went out of its way ..."

Yep.  Just what I was saying.  There is a very strong influence to make the public believe the Einstein story.  What they don't want you to find out, is that Einstein was not entirely 100% correct.  None of this has anything to do with this thread, and so I will not go into this further, but I will say that, just because something is shown on TV does not automatically make it correct or true.  If you keep pounding disinformation into the minds of the public, there will come a point where they will start to believe the lies. 

Most definitely, but this was the BBC 

If you start a new thread on this subject, I will try to expound on what I know to be the truth. 

No, you're ok. At the end of the day all we can do is wave our willies in the direction of that which we each believe to be the most credible source while skirting the issue of Norma Jeane jollies.

... I apologize for straying off-topic.

ditto