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3n138 equivalent

Discussion in 'Electronic Components' started by Steve, May 30, 2007.

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  1. Steve

    Steve Guest

    Anyone know of an equivalent for a 3N138? I've found them online, but
    ~$40.00 for a single fet seem a bit steep.

    Thanks,
    Steve
     
  2. legg

    legg Guest

    3N128 is 20V and a wider Gm spread.
    3N139 has slightly higher capacitances.
    3SK33 is 25V and lower cap.
    40467/40468 are 20V were considerably less rare a few years ago..
    Pro-Electron subs would have different pin-out.,
    but BFS28 at 20V would ~fit.

    What are you trying to do? If this is for a repair of useful
    equipment, it's cost is probably less than the labour involved or
    (likely) the cost of replacing the equipment.

    If it's for a new project or an app note, then perhaps another n-mos
    depletion-mode device or analog switch would work just as well,
    particularly at low frequency.

    These parts are expensive probably because they are no longer mfred,
    and are static sensitive - diifficult to store (for 20 years), ship
    or install. If it arrives in a simple plastic or paper container, send
    it back.

    RL
     
  3. A dual gate mosfet? Or is it so old it only has one gate? I can't
    remember.

    To a large extent, it seems like small-signal RF mosfets have faded
    out. They were big at one time, but then people learned more about
    transistors and realized some of the problem with bipolars were they
    weren't being used properly. Once they got over the mentality that
    the key to transistors were low power, it seems like bipolars got
    better at handling strong signals, and the need for mosfets (that
    were once touted as being so great for strong signal handling) faded.

    So basically you are looking for an RF dual gate mosfet, and one
    that won't cost a lot. It's like small signal germanium diodes
    at this point, the key being germanium rather than part number.

    One place that has a small selection of mosfets, and is set
    up for dealing with hobbyists, is "Dan's Small Parts", and
    you might look over his webpage at
    http://www.danssmallpartsandkits.net
    at the very least, it might give you an idea of what might
    be available to search for those numbers at some other parts
    place.

    Michael
     
  4. Steve

    Steve Guest

    It is for a repair, and you are correct, the labor and replacement
    cost far outweigh the cost of the FET. However, I don't know if it's
    bad, I don't have any experience checking these types. Pinout isn't
    important, the board has room for bent pins.

    Thanks,
    Steve
     
  5. Steve

    Steve Guest

    This is for an audio oscillator, single gate. I'll give the site a
    shot.

    Thanks,
    Steve
     
  6. Chris Jones

    Chris Jones Guest


    You can turn a dual-gate mosfet into a single-gate mosfet by connecting the
    two gates together. Of course all of the other parameters may not be the
    same. If you post a schematic of your application circuit then people may
    be able to suggest a substitute that will work there.

    Chris
     
  7. Steve

    Steve Guest

    Actually, I did some testing this afternoon, and the original fet is
    fine. The problem lies in one of the op amps, as I posted in the CIC
    replacement parts post. It would be nice to have a backup, but since
    it's still working, I think I'll just keep using it.

    I appreciate the help though,

    Thanks,
    Steve
     
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