References: <3DF6537E.B0FC9DBE@worldonline.fr> <3DF90203.C9691374@worldonline.fr> <3DF94AE1.email@example.com>
Subject: Re: Determining bjt noise parameters for Spice models?
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Date: Sun, 15 Dec 2002 11:21:33 -0500
NNTP-Posting-Date: Sun, 15 Dec 2002 10:54:52 EST
"Kevin Aylward" wrote in message news:E3XK9.162$cy2.14496@newsfep2-gui...
> John S. Dyson wrote:
> > "Kevin Aylward" wrote in message
> > news:MZOK9.1335$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.
> >> Summary:
> >> A low noise transistor is essentially 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.
> Ahmmm. Did you read my post? I did address the hfe issue, to wit:
I did, but you said in an unqualified way that rbb (and the obvious
low 1/f noise) as being the key... Remember your 'end of story'
> A rough optimum being to set re=Rs/sqrt(hfe)
> Noting from this that the noise variation due to spreads of hfe is
> rather low, so hfe can usually be ignored.
Maybe, where you cannot use high beta parts, you can ignore it, but
you aren't always limited to medium beta parts.
> For instance, if the source is highly inductive, this can make the
> effect of base current noise much more significant.
This makes low rbb insufficient in many cases, doesn't it? Remember,
your 'end of story' comment?
> Sure if the source is >> rbb.
It only has to be >rbb (not >>rbb) in cases where doubling the Beta
> But that does not really change the statement that a low noise
> transistor is essentially a transistor with low rbb. If you chose *any*
> transistor *type*, it will usually have a hfe from 100 to 400 say. Its
> not often you really want to use a superbeta transistor because of ease
> of its availability.
Your permise about availability is wrong. Transistors with high beta
(not really superbeta) are easily available (I buy them by the 1000's easily,
at very low price, typical of LF bipolars today.) It makes almost no
sense to purchase BJTs from suppliers anymore in small quantity
(at 2-8 cents apiece), because of UPS/FedEX charges.
There are MANY cases of an rbb (the effective rbb for noise) being 50
instead of 10 is better if the beta is 600 instead of 200. rbb of 50
isn't all that uncommon, nor is a minimum beta of 400. Super
low rbb of 5-10 doesn't buy that much noise performance, even with
common low-z audio source of 150-200 ohms. For the normal cases
of super low noise applications, you'll often do better with paralleled
sets of higher beta transistors. Some transistors do have excessively
high (effective) rbb, but I am not speaking of those either -- just as
I am not speaking of BJTs with Beta of 50.
Super-beta is a term that applies to parts that generally have very
high beta, but serious tradeoffs in behavior are also made (e.g. the
ZTX689 might be marginally considered a discrete superbeta), but a very
common part like an MPSA18 isn't really a horrid set of tradeoffs. It isn't
perfect, and I wouldn't necessarily use it on a low-z mic preamp
(without a transformer, or significant parallleling), but a device with
even with an rbb of 10 and Beta of 100-400 isn't going to be able to
compete where the MPSA18 type part does well. (Assuming
good 1/f performance.)
There are indeed some especially 'nice' parts out there specified
for low impedance noise, but claiming that many are readily available
is rather specious.
For availability, I buy the MPSA18 by the bag, just like the 2n3904. There
are some parts that are much less available (not using normal suppliers, or
huge quantities) that have interesting low noise characteristics for
high current/low impedances, but those are yet another class of low noise
that is optimized for different applications.