[sdiy] Moog ladder theory questions

[David G. Dixon]

I think Jim was only noting the fact that one almost always assumes that
collector and emitter currents are equal (implying that base current is
virtually zero, and therefore, beta is virtually infinite).  Of course, for
most purposes, this is fine.  However, for impedance calculations and so on,
one must take a number for beta (typically 100 for the common small signal
transistors we always use, I think).

David G. Dixon
Professor
Department of Materials Engineering
University of British Columbia
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-----Original Message-----
From: synth-diy-bounces at dropmix.xs4all.nl
[mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of Cornutt, David K
Sent: Wednesday, January 14, 2009 1:57 PM
To: synth-diy at dropmix.xs4all.nl
Subject: RE: [sdiy] Moog ladder theory questions

 
> From: Jim Patchell [mailto:patchell at cox.net] 

> Assuming that beta in infinite is just a way of making 
> analysis easier in some cases.  

Interesting... I've never seen any writeup of how transistors
work, or how to analyze transistor circuits, that assumes that
beta is infinite.  It seems like it would break most of the math --
if beta is infinite, then emitter current is infinite for any
non-zero value of base current, and so forth.  Maybe that works
for analysis of switching circuits?  (ECL, anyone?)

Now, of course, op-amp tutorials often start out by stating
that a theoretical op-amp is capable of infinite gain.
If I didn't know better, I'd be tempted to say that someone
confused the two concepts.

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