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Why this transistor?

Discussion in 'Electronic Design' started by Anthony Fremont, Apr 5, 2007.

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  1. I'm looking at this schematic and am wondering why the designer chose a
    2N2369 over a 2N2222 or 2N3904? I looked at the datasheet for the 2N2369,
    and I didn't see anythng amazing about jumping off the page. Did I miss
    something. The maximum frequency expected would be under 30MHz.
    http://homepage.eircom.net/~ei9gq/stab.html

    He also makes a big deal out of the particular op-amp chosen as (what
    appears to be) a simple voltage follower. Any reason that you all can think
    of?
     
  2. Andrew Holme

    Andrew Holme Guest

    Lots of NPN transistors, including the ones you mention, would work equally
    well there. He probably had a spare 2N2369 in his junk box.

    The op-amp needs to work from a single 5V supply, with (ideally)
    rail-to-rail input and output range - or as close as you can get to it.
     
  3. I think he just has a big bag of 2N2369s
    http://homepage.eircom.net/~ei9gq/2m.html


    martin
     
  4. Eeyore

    Eeyore Guest

    Because he had one already probably !

    Why did you expect to see him use a 2222 or 3904 ? Other devices do exist you
    know ! Most are quite considerably superior.

    Graham
     
  5. Ok then, I was just wondering if there was something special about it. It
    does specify a much faster Toff, but that is with a much lighter collector
    current (if that matters?).
    OK, well that's not a big problem either. I notice he only wants a range of
    1V - 4V out of it. Thanks. :)
     
  6. Jim Thompson

    Jim Thompson Guest

    2N2369 is GOLD-DOPED, thus fast recovery from saturation (if
    over-driven).

    ...Jim Thompson
     
  7. Try running a 2N2222 at 30MHz square wave/saturated and you see why he uses
    a 2N2369. Storage time! The alternative is a Baker Clamp.

    Don't know about the op amp. I'm sure something cheap and nasty would do the
    job.

    Graham H
     
  8. That seems to be the consensus.
    Generally, when it doesn't really matter, these parts tend to be the ones
    actually specified or listed as alternates. Why do you think I'm asking?
    I'd like to know about some decent/better alternatives myself. I was hoping
    that someone that had used the 2N2369 would speak up and say "ya, ist gut"
    or something like that. ;-)
    Ok, good to know. I'll be sure to stock up on the quality part numbers you
    provided. Maybe this is why 3904's and 2222's rule?
     
  9. Like this Eeyore.....this is the kind of thing I was hoping to see......an
    actual reason to use that part over a more common jelly-bean. I'm guessing
    this is where the Toff thingy comes in? Of course it's now a moot point
    since it's an obsolete part, but maybe the designer liked the metal cans.
     
  10. Jim Thompson

    Jim Thompson Guest

    I still have a bunch of them myself ;-)

    ...Jim Thompson
     
  11. Thanks Graham. :) The fT's aren't that far seperated, so I'd ass-u-med
    similar characteristics in that area, but I'm learning differently now. ;-)
    Ok, he sure makes a deal over it, but doesn't explain why.
     
  12. Is Toff the thing I should have been looking at to determine this? Gone are
    the days of loads of charts relating everything, now I mostly see a few
    paramaters specified that (at least AFAIC) leave no way to extrapolate
    anything useful. :-?
     
  13. Jim Thompson

    Jim Thompson Guest

    Yes. Though I've only "looked" at a 2N2369 in the past 20 years ;-)

    ...Jim Thompson
     
  14. Wimpie

    Wimpie Guest

    Hello Anthony,

    If I had to choose between the 2N2369 and the 2N2222 (for your
    application), I would choose the first one.

    The 2N2222 is designed for higher current operation (larger die size,
    higher capacitance). Because of higher Ft and lower reverse
    capacitance (2n2369 device), the gain of your circuit will be higher,
    so you need to pick up less signal from your local oscillator. Check
    the datasheet from Fairchild MMBT2369A (same chip in different
    package).

    At low current (for example 3 mA), the Ft of the 2N2222 device drops
    to below 200 MHz. It remains high for the 2N2369 device (they specify
    at 500MHz at 10 mA). When you overdrive (as Jim mentioned), the 2n2369
    device remains fast, so you get good pulses out of it. The 2N2222
    device will perform less (at your relative low currents).

    The 2n3904 is in-between the two.

    Regarding the opamp, I do not have the datasheet, maybe somebody knows
    whether it is equal to the OP-07.

    Best Regards,

    Wim
    PA3DJS
     
  15. Joerg

    Joerg Guest

    When using the 2369 or similar gold-doped transistors with fast
    switching times just keep in mind that their hfe is much lower than what
    you may be used to from the 3904. That's the price to pay for speed. IOW
    there is no free lunch:

    http://www.nxp.com/acrobat_download/datasheets/PMBT2369_4.pdf

    BTW the TO-92 version of this one seems to be on the way to memory lane,
    could become hardcore unobtanium soon. Unless Jim can spare some from
    his bag.

    The author might have used it in the circuit you posted because he
    needed the speed but not much in terms of current gain. Just one comment
    here: It isn't a good practice to drive a logic gate straight off the
    collector of a transistor. There should be a fast Schmitt first. If the
    input signal is smallish this counter might otherwise display next
    week's lottery numbers instead of the frequency.
     
  16. Lionel

    Lionel Guest

    That'd be my guess too. Most of us design with whatever we're
    comfortable with, & have in the parts drawers. In my case, I use a lot
    of BC5xx transistors because they're what I grew up up with, & I can
    buy them in bulk locally at dirt cheap prices.

    When you're working from a circuit like that one, as long as you've
    checked the spec's of the original part, there's no harm in
    substituting something equivalent that you already have in stock.
     
  17. Lionel

    Lionel Guest

    Yep. They're ubiquitous in American designs. Yurrupeen designs tend to
    have lots of BC548s & BC549s for the same reasons.
     
  18. Eeyore

    Eeyore Guest

    Not in my world. They're the very last parts I'd suggest.

    2N2222s are needlessly expensive simply because they have a metal can.

    They don't rule in any way shape or form.

    For general purpose medium voltage small-signal npn I'd use BC214 in the past
    and BC549 today.

    There are literally hundreds of transistors that would substitute perfectly for
    these parts.

    Graham
     
  19. Eeyore

    Eeyore Guest

    Yet quite irrelevant to this application most likely. The signal source isn't
    shown.

    Graham
     
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