NPN BJT switch

Discussion in 'General Electronics Chat' started by Anestis88, May 29, 2015.

  1. Anestis88

    Thread Starter New Member

    Aug 9, 2011
    19
    2
    Hi
    I made a simple switch with NPN transistor. When measure at Vout, I get High as I excpect, but when the signal is low, the Vout oscillates as seen in the picture. What can cause these oscillations ? Any idea ?
    Thanks
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    Noise pickup. With the NPN turned off (input low), V(out) is a very high-impedance node. Even a 1V noise source coupled through 1pF is sufficient to make the node jump around...

    73.gif

    Notice you do not need the base resistor.
     
  3. crutschow

    Expert

    Mar 14, 2008
    13,028
    3,237
    In your circuit the transistor is not a switch, it is an emitter follower with the output being about a volt below the input.

    Not sure why it's oscillating. May be something to do with the LED you are using.

    Try placing the LED and resistor in series between the power supply and the transistor collector, and ground the transistor emitter. That will make it into a switch which should not oscillate.
     
  4. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,542
    1,251
    Notice that the oscillation, noise, fur, whatever, is present on both the high and low voltage levels in the first image.

    ak
     
  5. DerStrom8

    Well-Known Member

    Feb 20, 2011
    2,428
    1,328
    In reality the resistor and LED should be between the second power supply and the BJT collector, and the emitter should be connected directly to ground. Not sure if that would cause the oscillations you're seeing but it's a habit I would recommend getting into.
     
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