Reply-To: "Kevin Aylward"
From: "Kevin Aylward"
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Subject: Re: Determining bjt noise parameters for Spice models?
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Date: Mon, 16 Dec 2002 21:39:18 -0000
NNTP-Posting-Date: Mon, 16 Dec 2002 21:40:02 GMT
> "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?
If by Re you mean the parasitic emitter resistance, then this is usually
less than 1 ohm, so can be ignored noise wise.
However, re is the dynamic resistance of the emitter circuit and is
given by re=1/40Ic at room temperature. re does not generate thermal
noise but it does generate a noise voltage due to emitter shot noise,
So, for approximate optimal noise set re=Rs/sqrt(hfe), then Ie=1/40re
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