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Subject: Re: output impedance
X-Newsreader: Microsoft Outlook Express 6.00.2800.1106
Date: Fri, 08 Nov 2002 20:04:35 GMT
NNTP-Posting-Date: Fri, 08 Nov 2002 21:04:35 MET
"Mike" schrieb im Newsbeitrag
> "Reg Edwards" wrote in message
> > > The impedance is in parallel with the current source, not in series.
> > up
> > > the Thevenin-Norton theorem.
> > ==============================
> > I have had 60 or more years of understanding electric circuits and have
> > NEVER NEVER needed to refer to a "theorem". I have never even heard or
> > long forgotten most of them or what they are about. Nothing beyond
> > Law has ever been of any use.
> > Anybody who drags up a theorem in support of some explanation or other
> > understands very little of what he/she is waffling about and is merely
> > plagiarising and parotting an old wives' tale. To mention a theorem is
> > admission of ignorance. Or an admission that he/she has a memory and has
> > read something somewhere. Or who had a teacher who was none the wiser
> > Painting by numbers? The answer to your question is Theorem 19. Or is
> > 33.5 ? End of answer.
> > OK? Next question please.
> Yikes! Reg, I beg to differ!
> I've much less than 60 years experience, but I make use of a variety of
> theorems on a regular basis. I remember the names of a few of them, and
> would have to look up the names of others, but using the names of theorems
> isn't intended to take the place of understanding. Using names can be a
> convenient shorthand - it states the underlying mathematics without having
> to restate the proof.
> I actually used the reciprocity theorem in a design review a while back,
> when we were discussing the properties of an RC circuit. I might be able
> derive the proof of the reciprocity theorem, but I suspect it would take
> quite a while. On the other hand, knowing the name of the theorem _and_
> it applied meant that I didn't need to provide a proof - at least not at
> design review. It also meant that I could find a proof quickly, by looking
> it up in one of my circuits texts.
> I suppose I could have tried to explain conversions between equivalent
> voltage and current sources, but it's something most textbooks explain
> reasonably well, and I'm guessing that the original poster is a student,
> that he has a circuits textbook. I don't know how else it might be
> in all my books it's called either Thevenin's theorem or the
> theorem. Even if he doesn't have a textbook, he probably has web access,
> could Google for Thevenin-Norton. I know seems a bit like that line every
> student dreads reading: "It can be shown that...," which always seems to
> precede some remarkably complicated mathematical result, but that's really
> not the intention.
> -- Mike --
When doing electronics as a hobby I think it is already advanced to find
this question. Apparently there seems a lot of uncertainty even among the
engineers when it comes to current sources and different units like Siemens
(= mho= 1/ohm)for conductivity.
But it is nothing else then the voltage view, and sometimes one model is
better and sometimes another.
For example a voltage source can be described as a ideal source Uo with a
resistor Ri in series.
Now both elements describe the basic behaviour of a real voltage source.
Of course there are ICs, Transistors and passive components inside and you
will hardly find any Ri as a part.
| Ri |
+| | | |
(Uo ) |Uout | |Rload
-| V | |
You can design Ri to be positive, zero and even negative. All is possible
with a handful of electronic components.
Uout= Uo - Iout*Ri
When we look at the current source we see the similarity with the voltage
source and also the similar equation.
| | |
+| | | | | |
(Io ) | |Ri |Uout | |Rload
-| | | V | |
| | |
Iout= Io - Uout/Ri
Since the OP has asked about an equivalent resistor in a Thevenin theoretic
point of view, it is really the only appropriate way to answer explaining
electronic hardware designer
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