From: Jim Thompson
Subject: Re: Determining bjt noise parameters for Spice models?
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Date: Tue, 10 Dec 2002 21:56:53 GMT
NNTP-Posting-Date: Tue, 10 Dec 2002 16:56:53 EST
Organization: Cox Communications
On Tue, 10 Dec 2002 21:50:06 +0100,
In Newsgroup: sci.electronics.cad,
Entitled: "Determining bjt noise parameters for Spice models?",
Wrote the following:
|I wanted to predict noise properties of some bjt-based circuits using
|Spice. Then I've noticed that the bjt models (almost) never include the
|noise parameters (AF, KF ). How can those be determined from the data
|I've tried the advice found in a Spice manual: set AF=1 and then find
|KF such that it gives the requested noise figure; I've tried this for
|2SC3329. Ok, I've found something like KF=5e-15 to get the specified
|NF at 1 kHz with Ri=20k. Then I've varied the collector current but have
|not observed the specified NF changes! I've then looked at the NF at
|10Hz. Apparently, AF=1 does not fit this transistor; I've tweaked it
|to AF=1.13 to get the noise figures ressembling those in the datasheet,
|but without knowing what I am doing!
|- 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
|- 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)?
AF and KF only affect low frequency (1/f) noise performance.
Bipolar transistor equations for noise.........
Noise is calculated assuming a 1.0-hertz bandwidth, using the
following spectral power densities (per unit bandwidth):
parasitic resistances thermal noise (AC terms on the left of the =
sign, DC terms on the right).....
RC: Ic^2 = 4·k·T/(RC/area)
RB: Ib^2 = 4·k·T/RB
RE: Ie^2 = 4·k·T/(RE/area)
base and collector currents shot and flicker noise
IB: Ib^2 = 2·q·Ib + KF·Ib^AF / FREQUENCY
IC: Ic^2 = 2·q·Ic
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| Jim-T@analog_innovations.com Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
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