Noise from a back biased NPN junction - how to predict RMS?

Discussion in 'Analog & Mixed-Signal Design' started by pgo1, Jun 27, 2016.

  1. pgo1

    Thread Starter New Member

    Nov 7, 2012
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    A common way of getting a noise source is to use a back biased NPN junction, but the RMS output varies greatly from transistor to transistor. Does the RMS value depend on any other measurable/predictable value (beta, maybe?)
     
  2. joeyd999

    AAC Fanatic!

    Jun 6, 2011
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    See this article on shot noise.
     
  3. DickCappels

    Moderator

    Aug 21, 2008
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    You can measure it, typically noise is charactarized as x volts a a -3db bandwidth of y Hz. In other words, if you change the bandwidth, you change the RMS noise.

    For your reading pleasure, here is a short but informative article on zener noise:
    http://users.cosylab.com/~msekoranja/tmp/04236738.pdf
     
  4. pgo1

    Thread Starter New Member

    Nov 7, 2012
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    OK, late reply here - thanks for the help, shot noise and advice on measuring noise received, but what I want to do is to say - if i buy 100 of this particular zener diode (or transistor and use it back biased) what range of noise voltage can I expect between them? They are all different, if I want a type in which the noise varies very little from diode to diode, how do i know which ones to choose? There is no measure of it on the datasheet, so can I infer it from some other measurement that is on the datasheet?
     
  5. OBW0549

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    Mar 2, 2015
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    I doubt the semiconductor processing and testing for those devices controls for that characteristic, and I don't think the noise level can be inferred from anything else on the datasheet; to get repeatable noise voltages I think you'd have to resort to specialized components like noise diodes.
     
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  6. pgo1

    Thread Starter New Member

    Nov 7, 2012
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    hhmm. interesting. thanks for the tip, i'll look into those diodes
     
  7. Kermit2

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    Feb 5, 2010
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    More reading
     
  8. Sensacell

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    Jun 19, 2012
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    how about a "noise AGC"?
     
  9. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    I would think a CC source feeding a temp controlled junction device of your choosing would stabilize the noise amplitude sufficiently to allow one to "grade" the device and perform a crude bin sort of multiple devices.
     
  10. DickCappels

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    Aug 21, 2008
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    Or a cermet trimmer pot?
     
  11. pgo1

    Thread Starter New Member

    Nov 7, 2012
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    yep, these are things I had considered -

    grading the devices is time consuming and not really suitable as i will not be assembling this myself. it doesn't really improve on a trimmer which is the current solution.

    the noise AGC is - i think - an improvement on the trimmer
     
  12. OBW0549

    Well-Known Member

    Mar 2, 2015
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    This app note from Linear Tech shows one approach to doing that, in Fig. 17.
     
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