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Level Shifting

Discussion in 'General Electronics Discussion' started by Raven Luni, Mar 3, 2014.

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  1. Raven Luni

    Raven Luni

    Oct 15, 2011

    Just wanted to be sure about this: Is it ok to have an NPN transistor's base voltage higher then the collector? It's for a simple logic level shift (9V - 5V). And the inversion doesnt matter (so no need for a second transistor)
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    The answer is: it depends.

    The important voltages are:

    1) Vbe (forward biased BE junction). For this to increase you have a high forward current -- bad juju.

    2) Veb (reverse biased BE junction) keep it small -- under 5V is safe.

    3) Vce, Vcb -- normal operation. Keep the voltage below the max specification

    And you're asking about

    4) Vbc (forward biased BC junction). I've never seen a specification for this, but it's petty rare to have the transistor operating in this regime.

    For typical operation the BC junction is never forward biased. In any case where it is, the base resistance which limits Ibe should also protect Ibc.

    If you keep all of the other voltages within range using the normal methods, then any forward biasing of the BC junction should be likewise protected.

    What sort of circuit are you using for this to be a possibility? (does it have no base resistor?)
  3. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Steve: it depends :)
    In saturation Vce < Vbe, therefore Vbc>0V, albeit only by a few 100 mV
  4. kpatz


    Feb 24, 2014
    Bipolar transistors are controlled by current, not voltage. In a typical circuit as you describe, you'd have a base resistor to limit base current to a few mA (depending on the circuit, your circuit's needs, and the gain of the transistor).

    With the resistor in place, you could feed 9V into the base resistor, a current would flow through the resistor and base, switching on the transistor. The actual voltage at the base would be the Vbe voltage drop, usually <1V (similar to a diode's voltage drop). The rest of the voltage would be across the resistor.
  5. Raven Luni

    Raven Luni

    Oct 15, 2011
    RIIIIGHT! Thanks! :D It's a headache trying to visualise this stuff sometimes.
  6. duke37


    Jan 9, 2011
    I thought that schottky digital circuits have a diode to stop the collector going well below the base and keep the transistor out of saturation to get faster switching.
  7. Arouse1973

    Arouse1973 Adam

    Dec 18, 2013
    Do you not just need a transistor switch?. See attached.

    Thanks Adam

    Attached Files:

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