Simple BJT Question

Discussion in 'General Electronics Chat' started by Mazaag, Feb 1, 2008.

  1. Mazaag

    Thread Starter Senior Member

    Oct 23, 2004
    Hey guys,

    For a NPN BJT, I know that in order to be running in the Forward Active Region ,we have to forward bias the Base-Emitter Junction, and Reverse Bias the Base Collector Junction

    my question is as follows

    does that mean the voltage at the base just has to be higher than that of the emitter ? or does it have to be higher that 0.7 across?

    what about the Base collector? does the collector voltage have to be higher than the base ? or higher by 0.7 ?

    thanks guys
  2. scubasteve_911

    Senior Member

    Dec 27, 2007
    I think you answered your own question.. You want to forward bias the base-emitter, then you need to ensure the voltage differential is at least 0.7V for a standard BJT. This means that if the emitter is 10V, then the base must be at least 0.7V. The same goes for the collector-emitter junction.

  3. Audioguru


    Dec 20, 2007
    The base to emitter is a silicon diode. The diode's voltage is 0.6V at very low current, 0.7V at medium current and up to 1.5V at high current. The base voltage for a turned on NPN transistor is more positive than the emitter voltage.

    The collector voltage drops lower than the base voltage when an NPN transistor is saturated. Then some current flows from base to collector.