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From: "John S. Dyson"
References: <3DF6537E.B0FC9DBE@worldonline.fr> <3DF90203.C9691374@worldonline.fr> <3DF94AE1.email@example.com>
Subject: Re: Determining bjt noise parameters for Spice models?
X-Newsreader: Microsoft Outlook Express 6.00.2800.1106
Date: Sat, 14 Dec 2002 19:00:25 -0500
NNTP-Posting-Date: Sat, 14 Dec 2002 18:52:45 EST
"Kevin Aylward" wrote in message news:MZOK9.1335$TY1.firstname.lastname@example.org...
> It is the fact that transistors have a large spread in rbb' from device
> to device that actually differentiates low noise devices from any othe
> device. Its inherent that if one tries to chose a low noise device, then
> one is selecting principly for *low* rbb. Nothing else matters much.
> A low noise transistor is essentailly a transistor with low rbb, and low
> 1/f noise. End of story.
I'd agree with you given the following qualification: you are speaking of
low/middle freqs, and also hfe(Beta) does count for high source resistance
applications, especially in cases where the Ic might be chosen to be high
for bandwidth or other reasons. There are cases where an rbb decrease
from 50 down to 10 will cause essentially nil difference in noise, but a doubling
of beta will make a significant difference. There are also cases vice/versa.
What you say for maybe a 100 ohm source is generally true, but that isn't
always the case.
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