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are pull ups/downs needed on bjt's?

Discussion in 'Electronic Design' started by [email protected], Apr 24, 2007.

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  1. Guest

    do I need to put a pull down on the base of your garden variety
    2n2222? if the base is left floating, no base current = transistor
    off, right?
     
  2. Eeyore

    Eeyore Guest

    It depends what you're doing with it !

    Aside from leakage currents and storage issues (tiny for a 2N2222) yes.

    Graham
     
  3. Tim Wescott

    Tim Wescott Guest

    No base current = transistor off, yes.

    If you charge up the base, then leave it floating, the base capacitance
    must discharge through the base, which takes time. It will be faster if
    it discharges through the current-limiting resistor that you may find on
    the output of a CMOS logic device, but it'll discharge faster yet if you
    put in that resistor to ground.

    If your base is sitting there at 0V with lots of capacitance to the
    world (or just the wrong conductor) your transistor may turn on from
    EMI. The resistor to ground will help prevent this.

    If you're running the thing close to it's rated voltage, a resistor from
    base to ground will help keep it from leaking excessive collector current.

    There are more caveats yet, but these are the biggest ones that I can
    think of.

    --

    Tim Wescott
    Wescott Design Services
    http://www.wescottdesign.com

    Posting from Google? See http://cfaj.freeshell.org/google/

    Do you need to implement control loops in software?
    "Applied Control Theory for Embedded Systems" gives you just what it says.
    See details at http://www.wescottdesign.com/actfes/actfes.html
     
  4. It depends on how completely off you need, and how quickly
    it has to achieve off.

    Any transistor with collector to emitter voltage has some
    (possibly very small) current leaking into the base from the
    collector. The open base transistor amplifies that leakage
    current with its current gain. When a turn on base current
    goes to zero, the charge stored in the base can take quite a
    while to drain away while the transistor slowly turns off.
     
  5. Maybe?!
    BJT's ARE controlled devices so depending on quality of _control_ you
    get the responses.
    They are not telepatic and don't read your mind.

    HTH

    Stanislaw
     
  6. Mike Monett

    Mike Monett Guest

    This is a Bad Idea. It can make your circuit sensitive to stray charges and
    temperature effects. There are several extremely sensitive circuits on the
    web that use transistors with floating base.

    Never leave inputs floating. Always use the appropriate resistance or
    recommended termination method on unused inputs. CMOS is especially
    sensitive due to the high input impedance.

    Many op amps share internal bias supplies. If you use only one section of a
    dual or quad, the other sections can upset the bias supply and make the one
    being used behave strangely. To prevent this, tie the postive inputs of
    unused sections to the appropriate voltage, and connect the negative inut
    to the output pin.

    Always check the manufacturer's recommendation on floating and unused
    inputs to prevent hours wasted troubleshooting silly problems.

    Regards,

    Mike Monett
     
  7. whit3rd

    whit3rd Guest

    Maybe.
    The base wire is one source of base current. Three others
    are:

    (1) dV/dT on the collector induces current into the base through
    the CB stray capacitance (Miller effect depends on this)

    (2) avalanche breakdown of the collector/base junction (this is
    the normal operation of an avalanche transistor)

    (3) photoelectric effect can cause current in the depeletion region
    of the
    transistor (yes, the 2N2222A is in a metal opaque case, but cosmic
    rays
    and X-rays can still do it).
     
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