Transistor as a switch

Discussion in 'General Electronics Chat' started by alexmath, Jun 1, 2014.

  1. alexmath

    Thread Starter New Member

    May 2, 2014
    17
    0
    When setting up a transistor as a switch is there any difference between putting the load on the collector or emitter?
     
  2. MrChips

    Moderator

    Oct 2, 2009
    12,429
    3,360
    Yes.

    With the load on the collector, the configuration is called "common emitter amplifier".
    This circuit provides both voltage gain and current gain.
    A base-emitter voltage (Vbe) greater than 0.7V will turn on the transistor as a switch.
    The standard practice is to use a current gain (beta) = 10. Hence if you want to switch 100mA through the load, make sure the source driving the base current can provide 10mA, i.e. Ib = Ic/beta.

    A "common collector" or "voltage follower" configuration is used when you need to match to a low impedance load.
    There is no voltage gain and Vb (voltage at the base) must exceed the voltage out at the load.
     
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