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What are FETs Used for?

Discussion in 'Electronic Basics' started by Nelson Win, Oct 30, 2003.

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  1. Nelson Win

    Nelson Win Guest

    I'm attending an amplifier course at a community college. The course
    has finished with Op Amps and BJTs and is moving onto FETs. I'm
    thinking of dropping out of class as nearly all the circuits I've seen
    use Op Amps and NPN transistors. I'm interested in sequencing,
    flashing and fading LEDs. Grateful for any advice whether FETs could
    be of any use to me later on. Don't want to regret it.

    Nelson
     
  2. Tim Kettring

    Tim Kettring Guest

    A FET is a high impedance input that does not load-down the circuit feeding
    it like a NPN or PNP typically would .

    I have an old voltmeter with a FET input , one of the best in those days .

    FETs basically replace tubes in modern electronics .

    tim
     
  3. Tim Kettring

    Tim Kettring Guest

    FETs have a very high impedance input , they are great where you dont want
    to load the circuit feeding it down .

    tim
     
  4. I think you should stick with it. As Tim already told us (a couple of
    times ;-), FETs have high input impedance and can be quite useful.
    There are different types and some FETs have interesting characteristics
    like being able to switch an AC circuit or pass current in either
    direction. I believe you'll also find that some of them have extremely
    low DC resistance when saturated. This is quite useful when you wish to
    switch large currents and don't want a huge heatsink. If you think any
    of this might be useful in the future, you should check them out.

    michael
     
  5. I have one word of advice.

    Don't.

    To expand on that: Stay with the class! FETs are incredibly useful
    as high-impedance amplifiers, and certain types of FETs (GaAsFETs) are
    even more useful in low-noise high-gain RF amplifiers.

    I made the mistake of dropping out of my early electronics classes
    when I was well on my way to getting my A.A.S. degree. That was in 1978.
    It was not until 1997 that I started back up again, and I regret letting
    it go that long.

    Stay with the class. You won't regret it in the long run.
     
  6. Nelson Win

    Nelson Win Guest

    Stay with the class! FETs are incredibly useful

    Thanks, guys. I will stay with the class. The instructor will be
    suprised to see me next week as I told him I'm withdrawing...he's a
    good instructor, knows his stuff inside out and has a nice smile, come
    to think of it, I enjoy his class, don't understand everything. But
    who does?

    Cheers,
    Nelson
     

  7. FETs are great, but in particular the MOSFET (metal oxide semiconductor
    field effect transistor) is probably the single most important electrical
    component of modern electronics. Their popularity is well justified for
    they really do have some extremely valuable characterisitics. If you have
    any interest in electronics at all, I'm sure you won't regret knowing what a
    MOSFET is and how to use it.

    What are FETs in general used for? Anything and everything basically.
    Digital integrated circuits are typically almost exclusively built up from
    logic gates composed of a few MOSFETs each. The millions of transistors
    composing the CPU of the computer you posted with are not BJTs, but rather
    MOSFETs. In the analog world FETs continually have been gaining ground and
    popularity against the traditional BJT. Most of the latest high performance
    Op Amps coming out these days have rail to rail I/O made possible with FETs.

    In the discrete electronics world power MOSFETs offer truely impressive
    capability. For example the IRLMS2002:

    http://www.irf.com/product-info/datasheets/data/irlms2002.pdf

    Allows one to realistically switch currents in excess of 4Amps in a little
    tiny SOT23-6 package with very low loss.

    Or on the bigger side of things the IRF2804:

    http://www.irf.com/product-info/datasheets/data/irf2804.pdf

    Can switch seriously wicked current levels in a TO-220 package with little
    or no heatsinking. With no heatsink it can easily switch loads in excess of
    17 Amps. With a modest heatsink (of say 20W or less dissipation) the device
    can switch 75 Amps of juice with no problems. Try to compare that against
    any BJT offering and you will find there is no comparison.
     
  8. That'z exactly right. FETs can't compare with IGBTs.

    http://www.elec.gla.ac.uk/groups/dev_mod/papers/igbt/igbt.html
    "The operation of the IGBT can therefore be considered like a wide-
    base pnp transistor whose base drive current is supplied by the MOSFET
    current through the channel." (see pictures)

    IGBTs can switch thousands of volts at hundreds of amps.
    http://www.dynexsemi.com/products/igbt/


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  9. Oh yeah baby. That company produces some pretty sweet devices. I wouldn't
    mind having a few of those 3300V 1200A IGBTs in my junk parts bin.

    http://www.dynexsemi.com/assets/DNX_DIM1200ESM33-F000.pdf

    3300V @ 1200A works out to just under 4MW of maximum theoretical power
    handling capability. Of course a real implementation wouldn't likely
    achieve quite this much continuous power, but still. You could
    theoretically juice up a whole town with just one of those beasts. I wonder
    how much they cost each? Anyone have any ideas? Anyone know of
    substantially larger IGBTs?
     
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