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Uncle Krunkus
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 Posted: Wed Mar 07, 2012 3:02 am Post subject:
Questions about noise
Subject description: Doing some theory research |
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Hey all,
Just wanted to find out where I could find some theory about why a reverse biased transistor puts out noise, and how close to "real" white noise is it?
Any ideas?
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Dougster
Joined: Sep 20, 2005
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Location: Tucson, AZ, USA
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| Posted: Wed Mar 07, 2012 6:49 am Post subject:
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Interesting subject...
I've been on a quest to learn more about transistors, so I've got a few books and references on the subject. I don't have a definitive answer for you, but there seem to be a couple sources of noise:
Johnson noise
Shot noise
Avalanche noise (zener diode)
Simplistically, a lot of noise seems to be caused by thermal agitation of the electrons in the device.
Some discussions of noise:
http://www.odyseus.nildram.co.uk/RFIC_Theory_Files/Noise_Tutorial.pdf
http://www.maxim-ic.com/app-notes/index.mvp/id/3469
Horowitz and Hill
TI Transistor Circuit Design chapter 21
GE Transistor Manual chapter 18
Cowles Sourceboook of Modern Transistor Circuits chapter 7
MobileReference Electronics and Circuit Analysis Study Guide chapter 10
Regards,
Doug
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Uncle Krunkus
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| Posted: Wed Mar 07, 2012 1:15 pm Post subject:
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Thanks Doug,
a bit of extra background reading always helps to round out the understanding. 
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frijitz
Joined: May 04, 2007
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Location: NM USA
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Posted: Wed Mar 07, 2012 9:31 pm Post subject:
Re: Questions about noise
Subject description: Doing some theory research |
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| Uncle Krunkus wrote: | Hey all,
Just wanted to find out where I could find some theory about why a reverse biased transistor puts out noise, and how close to "real" white noise is it?
Any ideas? |
Here's a pretty good online reference:
http://www.qis.ex.nii.ac.jp/qis/documents_YY/y3_02chp6_txt.pdf
Ian |
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JingleJoe
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| Posted: Wed Mar 14, 2012 5:27 am Post subject:
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As far as I'm concerned it is real white noise; it's totally random  It's due to electrons migrating accross the pn junctions in the transistor and you can't quite predict them, there are too many variables.
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Uncle Krunkus
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| Posted: Wed Mar 14, 2012 7:58 pm Post subject:
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I know it's totally random, and I even have a good idea why, but doesn't "White Noise" have a very specific definition or something? Does it match that?
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Acoustic Interloper
Joined: Jul 07, 2007
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Location: Berks County, PA
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| Posted: Wed Mar 14, 2012 8:22 pm Post subject:
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| Quote: | Johnson–Nyquist noise (thermal noise, Johnson noise, or Nyquist noise) is the electronic noise generated by the thermal agitation of the charge carriers (usually the electrons) inside an electrical conductor at equilibrium, which happens regardless of any applied voltage. The generic, statistical physical derivation of this noise is called the fluctuation-dissipation theorem, where generalized impedance or generalized susceptibility is used to characterize the medium.
Thermal noise in an idealistic resistor is approximately white, meaning that the power spectral density is nearly constant throughout the frequency spectrum (however see the section below on extremely high frequencies). Additionally, the amplitude of the signal has very nearly a Gaussian probability density function. |
from here. It's the constant spectral density that makes it white. Pink noise, for example, is filtered. I guess the effects of thermal agitation on electrons have constant spectral density 
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Last edited by Acoustic Interloper on Wed Mar 14, 2012 8:24 pm; edited 1 time in total |
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frijitz
Joined: May 04, 2007
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Location: NM USA
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| Posted: Wed Mar 14, 2012 8:23 pm Post subject:
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| Uncle Krunkus wrote: | | I know it's totally random, and I even have a good idea why, but doesn't "White Noise" have a very specific definition or something? Does it match that? |
Yes, of course. See Eq 6.77 in the link I gave you. S is the spectrum and it doesn't depend on frequency. Therefore it is white noise by definition.
Ian |
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frijitz
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| Posted: Wed Mar 14, 2012 8:26 pm Post subject:
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Yes, yes. But that doesn't say anything about p-n junctions, which is what the question was.
| Acoustic Interloper wrote: | | Quote: | Johnson–Nyquist noise (thermal noise, Johnson noise, or Nyquist noise) is the electronic noise generated by the thermal agitation of the charge carriers (usually the electrons) inside an electrical conductor at equilibrium, which happens regardless of any applied voltage. The generic, statistical physical derivation of this noise is called the fluctuation-dissipation theorem, where generalized impedance or generalized susceptibility is used to characterize the medium.
Thermal noise in an idealistic resistor is approximately white, meaning that the power spectral density is nearly constant throughout the frequency spectrum (however see the section below on extremely high frequencies). Additionally, the amplitude of the signal has very nearly a Gaussian probability density function. |
from here. It's the constant spectral density that makes it white. Pink noise, for example, is filtered. I guess the effects of thermal agitation on electrons have constant spectral density |
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JingleJoe
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| Posted: Thu Mar 15, 2012 10:17 am Post subject:
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Perhaps it's an idea to think of the pn junction like a barrel full of water over a hole in the ground. The electrons which jump accross the junction are the occasional drips of water which leak out of the barrel into the hole, as it contracts and expands due to temperature change.
White noise theoretically contains all frequencies, of course in reality it doesn't, it's frequency would depend on the components of the circuit and thier filtering effects.
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