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Subject: Re: Determining bjt noise parameters for Spice models?
Date: Mon, 16 Dec 2002 22:01:32 +0100
Organization: Wanadoo, l'internet avec France Telecom
References: <3DF6537E.B0FC9DBE@worldonline.fr> <3DF90203.C9691374@worldonline.fr> <3DF94AE1.firstname.lastname@example.org> <7V3L9.email@example.com>
NNTP-Posting-Date: 16 Dec 2002 21:01:33 GMT
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"John S. Dyson" wrote:
> It is probably best to consider both the en and in type effects, and not
> prejudicially pronounce 'rbb and 1/f' as being the key.
> Rather, I tend to prefer these ideas for midband:
> en^2 = 4 * k * T * rbb(eff) + 2 * q * Ic * re^2
> in^2 = 2 * q * Ib
Of what order should the Spice RE be expected? It is also very often missing from the models...
If comparable to RB, it would contribute comparable thermal noise (voltage term) as well...
> The input noise voltage is then limited by the
> en > re * sqrt(2*q*Ic) and en > sqrt(4*k*T*rbb(eff)) terms.
> The input noise current is purely defined (assuming no parasitic
> noise sources) by the operating current and Beta. (Again,
> ignoring 1/f.)
> So, the input noise power is inversely related to the sqrt(B)
> and contributions between rbb(eff) and the operating point
> physical limitations are of the same kind (often of the same
> order in discrete parts, at normal 100ua to 10ma bias.)
> NF is kind of 'bogus', but if properly designed, it is useful for
> getting a qualitative view of 'low noise'. The noise factor
> for an ideal transistor is about:
> nfactor (ideal rsource) = 1 + sqrt(1 + 2 * rbb / re) / sqrt (B).
> Once the rbb drops much below re, then the only thing that you
> can do is to increase beta so as to decrease total noise. rbb
> is of diminishing returns.
> When using rbb as a criteria for selecting a low noise device,
And low RE?
> even for low impedances where the noise is still significant, choosing
> increased Beta can still decrease the noise. This is still important
> when trying to decrease the en by running at high Ic for a low
> rbb device. Increased beta will help to counter the effect of
> high Ic on the base noise current.
> I don't mean to claim that low rbb components aren't often desirable.
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