Reply-To: "Kevin Aylward"
From: "Kevin Aylward"
References: <3DF6537E.B0FC9DBE@worldonline.fr> <3DF90203.C9691374@worldonline.fr>
Subject: Re: Determining bjt noise parameters for Spice models?
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Date: Fri, 13 Dec 2002 07:25:15 -0000
NNTP-Posting-Date: Fri, 13 Dec 2002 07:25:19 GMT
> 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
>> 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?
AF is the exponent, which is what I wrote. f^AF means f to the power
>> This noise is added to the normal white noise of the transistor.
>>> - 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:
KF needs to be set at a low frequency, say 10 Hz.
> - 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
> - 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.
I dont understand whatvyou say here. Is the spice value too high or too
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