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2N4261 modern days replacement

Discussion in 'Electronic Design' started by Sergey Kubushyn, Jun 26, 2013.

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  1. Can anybody point to something available now to replace 2N4261 transistor?
    It will be used in its primary role, as fast switcher.
  2. Tim Williams

    Tim Williams Guest

    BFT92 might be close. Don't think the t_r, t_f is spec'd, but I doubt
    they use the same test circuit as the original, either (I'm looking at an
    old Motorola scan for reference here). Volts, amps and frequency are
    comparable. Have fun with the package though.

  3. OK, thanks, will give it a try. It might be _too_ HF and start oscilating
    and saturation voltage is not known and it is not clear how fast it will
    go into saturation and back but it is definitely worth to try.

    As of the package -- SOT23 is a big one, no problems whatsoever :)
  4. josephkk

    josephkk Guest

    The device actually seems to actually be available (arrow) in to72 package
    made by microsemi and semicoa (albeit only in expensive military grades).
    Doesn't seem all that special from the data sheets i found.

  5. It is "available" but they have none in stock. You can definitely place an
    order with them for a box or more and eventually you might even get it in a
    year or so.

    The only real guys who do have those in stock are Freelance Electronics.
    BTW, highly recommended -- they have lots of obsolete and rare parts and
    those are really in stock so you get actual price right away and can
    purchase them online and get them shipped next business day. Not like all
    those sellers of hot air (and now many of them even show up on Octopart)
    that you have to ask for a quote, give them your target price and they might
    start looking for those parts usually coming out empty.

    Quest has 3 (three) in stock, Freelance has more (100+) but those are pretty
    expensive. I would like to buy something like 50 of those but not going to
    do this for $10 a pop...

    As for not being all that special -- such an expression usually means there
    are many similar parts around so one can just use something else. So if they
    are not all that special please point me to those similar transistors and I
    will happily purchase those instead.

    The closest one I'm aware of is BSR12. It works just fine in place of e.g.
    2N4258 that is also used in many older Tek instruments but 2N4261 is at
    least 5 times faster and I can not find anything that would've matched its
    speed among modern transistors. If you know something suitable please tell
    what it is.
  6. josephkk

    josephkk Guest

    Well a quick bit of hunting brought up BFT92 and BFQ75 both in surface
    mount packages. Both of have higher ft though.

  7. BFQ75 is even a bigger rarity than 2N4261. BFT92 I do know and was going to
    give those a try but they are NOT switching transistors. They MIGHT work OK
    for fast saturating switching but that is not guaranteed -- nobody tested
    them in such applications and high Ft does not guarantee they would be as
    good switchers as they are amplifiers.

    NXP switching transistors are BSxxx (for BipolarSwitching ?) and their best
    PNP switching transistor is BSR12 that is way slower than 2N4261. And
    neither Semicoa nor Microsemi do even specify such a parameter as Ft; they
    only give turn-on and turn-off times that are not specified for BFT92 (BF
    standing for Bipolar ampliFier?)
  8. Wimpie

    Wimpie Guest

    El 01-07-13 5:42, Sergey Kubushyn escribió:
    Is it really used as a saturated switch? The test circuit below the
    switching time graphs of an old Motorola datasheet (can be found on
    the web) shows a long-tailed pair. This suggest non-saturated
    switching times.

    If it is used as saturated switch, is it really that fast as shown in
    some short-form datasheets? All short form datasheets don't show the
    reverse base current for measuring turn-off time.
  9. Look at Semicoa or Microsemi datasheets. They clearly say "Saturated Turn On
    Switching Time to 90%" and off to 10% (Vcc = 17V, 50 ohm pulse generator).

    And yes, it is used as saturated switch.

    I can not find that old Motorola datasheet. I can only find various
    Semicoa/Microsemi datasheets (some shorter, some more complete) and one
    really old from New Jersey Semiconductor Products, Inc. dated June 1973. I
    confess to not digging past Google 5th page but it is mostly junk "sellers"
    ready to sell you any part, even one that never existed.
  10. Frank Miles

    Frank Miles Guest

    There's a 3-page datasheet in the Moto "Small Signal Semiconductors" (C) 1987, starting
    on page 4-172. My copy has some obvious printing defects, but definitely
    gives turn-on and turn-off delays.
  11. Unfortunately Google won't find your copy :) I do not have a paper catalog
    so I'm limited to only what I can find on the Net...

    BTW, it is a good idea to start looking for a paper catalog. It might be
    extremely handy for older equipment repair 'coz many of those older
    semiconductors never made it to the Net...
  12. Too late :) I've already bought my own copy of "Small Signal Semiconductors"
    so I will be able to see the original once it hit my door...
  13. Wimpie

    Wimpie Guest

    El 02-07-13 0:26, Sergey Kubushyn escribió:
    I know the notes on saturated switching on the short form data sheets
    without any details. The old motorola sheet shows a long-tailed pair
    and then it becomes suspicious.

    I mailed you the Motorola datasheet showing the long-tailed pair. When
    I look to the circuit and component values mentioned, the transistor
    doesn't go into saturation.
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