Transistor noise

Discussion in 'General Electronics Chat' started by Steve1992, Jul 24, 2010.

  1. Steve1992

    Thread Starter Senior Member

    Apr 7, 2006
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    Im familiar with the audible noise that can be generated by transformers.

    Is it possible for transistors/FET to produce an audible high pitch noise, in particular in high power circuits if the component is under stress - electrons crossing the junctions at much higher speed??
    Is it called shot noise?
     
  2. tom66

    Senior Member

    May 9, 2009
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    You can get Johnson-Nyquist noise. That's the big one I've heard about.

    If you're not dealing with really high frequencies (i.e. audio level) the noise is in the nanovolts to microvolts range so it generally is not an issue.

    Shot noise... a quick look on Wikipedia says for 1 Hz the noise at 100mA is 0.18nA (nanoamps)... I'm not sure how to extrapolate this but I'd imagine it doesn't increase too much with audio frequencies.

    In general RF interference and power supply noise would overpower any of these, which are tiny.
     
  3. Steve1992

    Thread Starter Senior Member

    Apr 7, 2006
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    Thanks.

    I remember testing circuits that contained bipolar transistors handling high frequencies. Problems occurred because this caused internal capacitances at the transistors junctions.
     
  4. Ghar

    Active Member

    Mar 8, 2010
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    Are you talking about an audible sound coming from the transistor itself? As in, you put your ear to it and you can hear it?

    If so, what Tom is talking about is noise in the signals. It's only audible if you put in a speaker.

    I would imagine that given enough current lots of things can hum like a transformer but I don't have any experience with that in particular.
     
  5. tom66

    Senior Member

    May 9, 2009
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    Ah, yes, the microphonic effect. A power device will expand and shrink due to changing temperatures which can cause a small amount of audible noise. However, the effect is more significant the other way; if you take a transistor in a multimeter on the Hfe setting and squeeze it, the meter reading will change by about 5 or 10 counts, usually going down. I used a pair of plyers to avoid my body temperature effecting the result, don't squeeze too hard or you'll break the transistor!
     
    Last edited: Jul 24, 2010
  6. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Certain early transistor types were prone to producing real noise when they acted as pressure transducers. They could also act as 'microphones' you will find plenty of regferences to this from the 1950s and 1960s.

    This was due to vibration of the internal whiskers in point contact types.

    Going back further in time some valves also exhibited this undesirable characteristic.

    However I have never heard of FETs or junction transistors being prone to this.
     
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