Reply-To: "Kevin Aylward"
From: "Kevin Aylward"
References: <3DF6537E.B0FC9DBE@worldonline.fr> <3DF90203.C9691374@worldonline.fr> <3DF94AE1.firstname.lastname@example.org>
Subject: Re: Determining bjt noise parameters for Spice models?
X-Newsreader: Microsoft Outlook Express 6.00.2800.1106
Date: Sun, 15 Dec 2002 08:29:17 -0000
NNTP-Posting-Date: Sun, 15 Dec 2002 08:29:24 GMT
John S. Dyson wrote:
> "Kevin Aylward" wrote in message
>> 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 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:
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.
It is why I said "essentially" and "usually". I always cover my arse:-)
For instance, if the source is highly inductive, this can make the
effect of base current noise much more significant.
> 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.
Sure if the source is >> rbb.
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. So, given that you set the circuit up for say a hfe
of 200, then slap in different transistors, the noise won't really
change much. The idea here is that in reality, you don't have much
choice in actually selecting a transistor for hfe in that most are all
in the same ballpark, however, rbb might vary from 10 to 500 ohms from
device to device. In fact, 2n3055s have been used for MC inputs as their
rbbs are around 2 ohms, with their hfes being rather low.
Again, its a *practical* selection issue, most common transistors have
about the same hfe, so hfe don't make much of a selection guide. Sure,
there are a few down at 30, and some at 800, but overall, hfe is not a
significant selection criteria imo, for most low noise design.
There is some other history to this. It is/was often quoted that the
BC109C is a low noise device because it has a relatively high hfe of
500. However, its rbb is around 400 ohms, making it quite a noisy device
for low source resistances.
> What you say for maybe a 100 ohm source is generally true, but that
> isn't always the case.
Well, for high impedance sources, one is usually much better using a
jfet, its certainly what I recommend for say, a guitar preamp.
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.