Subject: Re: Determining bjt noise parameters for Spice models?
Date: Thu, 12 Dec 2002 22:39:15 +0100
Organization: Wanadoo, l'internet avec France Telecom
NNTP-Posting-Date: 12 Dec 2002 21:39:16 GMT
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Kevin, Thank you.
Kevin Aylward wrote:
> This won't really work. KF is the flicker noise co-coefficient, or 1/f
> noise contribution. A low noise transistor will have a 1/f corner such
> that it won't contribute much noise at 1Khz. Secondly, you should use a
> zero source resistance. So, look at the data sheet graph, and set KF to
> match the data sheet at a low frequency, say 10 Hz. Do this in voltage
> noise terms (nv/sqrthz), NF is pretty much useless.
> And... AF is the flicker noise exponent. i.e.
> Vn^2 = KF/f^AF in V/sqrthz
I read the formula given by Jim Thompson as AF being the exponent for Ib,
not the frequency, could you confirm? Is this formula to be applied as is
to obtain the equivalent input voltage noise source in series with base?
> This noise is added to the normal white noise of the transistor.
> > So:
> > - would it be possible that someone explains the meaning of AF and KF
> > (refer to an on-line doc, etc.) and how to calculate them from the
> > datasheet?
> > - in general, is the bjt model in Spice capable of precisely enough
> > modelling the noise behaviour of all the bjts out there (in
> > particular: 2SC/2SA Toshiba/Hitachi low-noise series)?
> Just about. You need to make sure the base resistance is set right. This
> is the main variable that distinguishes transistor noise form exh other.
> Other noise contributions are inherently fixed by Ic and hfe. So, after
> the 1/f curve flattens out, set rbb until the flat portion reads
> correctly. Do this at a highish collector current, e.g 10ma, also with r
> source set to zero.
I've tried to do this at 10kHz in the following way:
- common emitter;
- collector connected to a voltage source in series with a resistance;
- base directly connected to a voltage source (VSRC, small AC);
- the base voltage source adjusted to get about 10mA collector current;
- equivalent input noise calculated as V(ONOISE) on the collector divided
by the gain calculated (by Spice)
on the input AC.
Doing that with the standard 2SC3329 model and varying RB from 2 to 10 Ohm
could not reach the 0.6nV/rtHz specified by the datasheet - much lower
levels obtained; the datasheet announces rb about 2Ohm.
Did I do something wrong?
> Kevin Aylward