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Another transistor question...

Discussion in 'Electronic Components' started by Dave, Sep 29, 2004.

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  1. Dave

    Dave Guest

    Anybody have any idea what the upper frequency limit is on a 2N2222?
    Datasheet lists "typical freq (min)" as 200 MHz, but that's all. Any help
    is appreciated...

  2. Jamie

    Jamie Guest

    the data book i use list transitors at their unity gain point.
    meaning if a common emmitter amp would have a gain of lets say
    200 at near 0 freq, at 200 mhz the gain will be 1
    but that is how my manual spec's them.
  3. Depends on who made it. Like for TI for instance, the chip used in the
    2N2222A (2N2222 is obsolete and hard to find, IIRC), is the N24 chip.
    This is a 19 X 19 mil, epitaxial, direct-contact chip. The TI manual
    says that it has a low of 100, typical 400 MHz, with test at 10V and 50
    mA, 100 MHz. That sounds reasonable. But that's TI, and TI and
    Motorola and a zillion other companies made 2N2222As, each maker having
    made a different choice for the chip.

    What datasheet were you referring to? What company? What date code?
    Older ones could have a wider variation in Ft, so hand picking one could
    result in a higher Ft.
  4. Tim Shoppa

    Tim Shoppa Guest

    Well, if you put 1GHz in you'll get *something* out... transistors
    are usually specced by their "transition frequency" fT where AC small-signal-
    current gain has declined to one. Generally fT has to be bigger than one if
    you expect to get gain. fT itself is a function of several parameters
    (notably collector current). And fT is usually implied by the gain-
    bandwidth product at a frequency a few times lower than the measured fT
    (with good reason, you usually want a gain of at least a few...)

    If you're using the transistor as a saturating
    switch then other parameters become more important, particularly base
    capacitance. Many manufacturers publish "turn-off time" curves related
    to this. In certain switch applications a jellybean NPN wouldn't be good
    above a few MHz.
    For current PN2222A parts, fT = 250MHz or 300MHz at collector current
    of 20mA. It declines for both higher and lower currents. I don't know what
    the original JAN specs said.

  5. The specs aren't JAN, they're JEDEC. The PN2222A is the plastic cased
    equival;ent of the 2N2222A.
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