Re: Shot noise
From: Winfield Hill (Winfield_member_at_newsguy.com)
Date: 09/06/04
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Date: 6 Sep 2004 14:30:35 -0700
Mike Engelhardt wrote...
>
>>>>> I've seen Win have trouble with this idea in another post
>>>>> regarding the analog nature of the charge on the gate of a
>>>>> MOSFET. He went after the number of electrons it takes to
>>>>> charge the gate, but that quantization limit didn't apply
>>>>> to his example, again because electron current flow really
>>>>> is more like continuous flow of fluid-like current than a
>>>>> stream of discrete quantized charges.
>>>
>>> I thought that it was about being able to smoothly turn a
>>> MOSFET on or off due to the discreteness of electron charge.
>>> You went after the number of electrons it took to drive the
>>> gate voltage anywhere. The gate wasn't floating, so I just
>>> noticed that again you seemed to be applying charge quantization
>>> in a situation that it didn't apply, since you can basically
>>> charge a gate with any fraction of an electron you want. I
>>> just noticed it because it matched the erroneous statement
>>> in AoE. I don't know if you still have trouble with the
>>> concept or not.
Despite your repeated assertion, I have absolutely no trouble
with the concept, and have not for at least 20 years.
>> http://groups.google.com/groups?hl=en&threadm=av7rjl01663%40drn.newsguy.com
>> The gate _was_ floating ...
>
> Sure, if the gate is isolated, then you basically look directly
> at charge quantization. But that's not the thread I was talking
> about. I was talking about
>http://groups.google.com/groups?q=analog+wave+functions+overlap&hl=en&lr=&ie=UTF-8&selm=b83m6k%24r3b%40dispatch.concentric.net&rnum=1.
> In that thread you seemed to be still unaware that you don't
> directly see the effects of charge quantization in a conductor
> where the electrons' waveforms overlap. I don't know if you
> get it now or not.
As far as I can see the thread you reference gives you no reason
to make such a presumption about what I think. Especially when
your presumption flies in the face of my direct statements.
> BTW, I've thought about this defense you posted that that your
> mistake in the 1st Ed. of AoE was the mainstream thinking.
Well, fine, it well _may_ have been for _me_ 25 years ago, but
certainly it was not shortly after that. OK? I have no doubt
my coauthor, being the physics half of AoE, had a very clear
understanding of these issues 25 years ago when we first wrote
the book. What we say in the paragraphs on Johnson noise and
shot noise is correct, but we left out many details, some of
which we added into our second much larger edition. And some
more of which we'll try to squeeze into the third edition.
> Don't get me wrong, I liked AoE's discussion of stochastics.
> Things like considering the 1/f noise of resistors was a nice
> touch. Also, I really liked the discussion of equivalent input
> noise current and voltage of opamps. Though as I recall, you
> failed to mention that many opamps don't physically follow
> that model. For example, many JFET input opamps have noisy
> tail currents. If the input is perfectly balanced, that current
> does not get into the signal flow path. But there's an impedance
> imbalance on the two inputs, it does. That's a case where the
> En/In noise density picture falls down.
I wouldn't call that falling down, so much as being incomplete.
In the case you describe, the basic e_n and i_n parameters are
still working, but additional opamp model elements would be
appropriate.
As it happens, many or even most opamps and discrete parts are
poorly characterized at "high" frequencies, starting as low as
say 25kHz. Offhand, I'm not aware of any opamp specs that are
characterized for calculating input-impedance-imbalance noise.
Do you have some suggestions for a simple approach?
--
Thanks,
- Win
(email: use hill_at_rowland-dotties-org for now)
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