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Noise performance of small package mosfets

Discussion in 'Electronic Design' started by Bruce Varley, Feb 28, 2007.

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  1. Bruce Varley

    Bruce Varley Guest

    Hi, I've started using small mosfets, such as the 2N7000, for discrete
    linear design. It's almost like designing with 12AX7s again. Can anyone
    advise what their noise performance is like, how would one go as a mike
    preamp input stage, for example?
     
  2. Ken Smith

    Ken Smith Guest

    It is likely to be too noisy for microphones. For low noise, high
    impedance applications, a JFET will be a better option. Look at the
    LSK170.

    A big part of the problem with using a 2N7000 for a low noise device is
    that there is no noise spec. on them. This means you can't reject a batch
    just for being noisy.

    I have wondered for a long time what the noise of a large power MOSFET
    would be like. I've never gotten motivated to measure it though.
     
  3. Below 30mA, this is in the subthreshold region for these MOSFETs.

    I measured two small TO-92 MOSFETs with my hp4470A: a 2n7000 and
    its older brother, a VN01. Both MOSFETs were very noisy. The
    VN01 was 50 to 100x more noisy than a quiet JFET like a 2sk170,
    and the 2n7000 was 2 to 5x worse yet, over 1uV/rt-Hz. This was
    at 100kHz - the noise was much higher at lower frequencies and
    offscale below 3kHz. Also, this was at 10mA - the noise was
    much higher at lower currents, like 1mA or 300uA.
    Me either. Very high gate capacitance, ugh.
     
  4. John Larkin

    John Larkin Guest

    Nice data. It wouldn't shock me if you found popcorn noise, too.

    John
     
  5. A board we use for measurement purposes uses 8 channels of current
    measurement amplifiers. Each one has a 2N7000 as a source follower running
    at 3mA. It has always been a problem to get it through production because of
    excessive noise. There is no money to redesign it. But I found that
    Siliconix/Vishay versions were about 10 times less noisier than any others,
    eg. ST. Guess what - they're now discontinued. The boss will have to get
    them redesigned now.

    Graham Holloway
     
  6. Rich Grise

    Rich Grise Guest

    Well, when you're PWMing a 25 HP motor, I don't think a few millivolts
    of noise is going to be noticeable.

    And you certainly wouldn't use one for a mic preamp! ;-)

    FWIW, I just put "mosfet noise" WQ* into google, and got "about 603" hits;
    my impression from reading the comprehensible ones ( ;-) ) seems to
    reinforce something I seem to have picked up sometime in the mists of the
    past - mosfets are noisy. :)

    Cheers!
    Rich
    *WQ = With Quotes
     
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