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From: Roy McCammon
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Subject: Re: SPICE simulation goes awry?
References: <firstname.lastname@example.org> <email@example.com> <firstname.lastname@example.org>
Date: Sat, 14 Dec 2002 15:12:34 GMT
NNTP-Posting-Date: Sat, 14 Dec 2002 09:12:34 CST
Organization: Road Runner - Texas
Paul Burridge wrote:
> At first sight I thought that was an ammeter. How the heck can you
> have a theoretical concept like a current source in a real-world
it is easy enough to build a good enough current source.
A current mirror or just a large value resistance might
be good enough.
> What's it supposed to be doing there and why are there two
> separate voltage sources for the supply rail instead of just one
> common one?
It allows for studying the transient behavior of the
oscillator without getting snarled up in the details of
powering and biasing.
> And why only 1volt?
maybe behavior versus V1 was the thing of interest
> See above. AFAICS, there's no way this schematic can be translated
> into the real world on account of this "current source."
There are no ideal current sources and there are no
ideal voltage sources, but the ideal sources are
convenient for building models. Some circuits are more
easily modeled with current sources. There are many
ways to implement approximate current sources. In this
case, the designer is basically saying that he knows a
suitable way to pull 1mA out of the emitter such that the
emitter current will be close enough to a constant 1mA
under the actual circuit operating conditions.
A simple example would be a 1 Mega-ohm resister connected
o a -1000V supply. The current will be less that 1%
different from 1mA. Of course it wound waste a lot of power
and there are other ways. A current mirror is often used.
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