bjt switch

Discussion in 'Homework Help' started by ihaveaquestion, Dec 12, 2010.

  1. ihaveaquestion

    Thread Starter Active Member

    May 1, 2009
    hi guys,

    when asked to make a simple switch using a bjt, i chose this setup (the circle with the red lines coming out of it is the load):

    from my understanding, this setup supports only forward current (not negative) on the output terminal in the 'on' state, and positive voltage (not negative) across the output terminal in the 'off' state. Is that right?

    I'm also asked to find a design of mosfets/bjts that supports all four capabilities, forward AND reverse current on the output in the 'on' state, and positive AND negative voltage across the output in the 'off' state.... any ideas?

    thanks in advance...
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    When the current through the 1K resistor goes through the NPN transistor's base, the polarity forward biases the PN junction and current flows in the collector circuit, lighting the lamp - the transistor has worked as a switch. See our Ebook -

    As far as the other conditions go, you can see if some device that will do all that turns up.
  3. ihaveaquestion

    Thread Starter Active Member

    May 1, 2009
    why do the arrows in my photo point in the opposite direction as those in your link?
  4. tyblu


    Nov 29, 2010
    Who knows (could be arbitrary, could be majority carrier/electron flow, could be loop current sign convention), but the WikiP picture shows the correct current flow convention.
  5. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
    The book uses "electron flow" instead of "current" to determine the directions of the arrows. No matter which convention you pick, as long as you are consistent throughout the problem you will get the correct answer.

    Electron flow is in the opposite direction of current.