Help With Circuit Analaysis

Discussion in 'General Electronics Chat' started by pizzaboy150, Jul 10, 2016.

  1. pizzaboy150

    Thread Starter New Member

    Aug 20, 2015
    Hi All,

    I have been working on this and am struggling with the analysis...

    With a Vin of 1 volt

    I worked out the base current like this..

    1v - 0.7 - (0.3 / 1500) = 0.0002amps as the base current?

    so I am assuming the current from the 15v to gnd would be 0.0002 x (b100 + 1)

    which would give me an emitter current of 0.0202 amps.

    then to work out the voltage drop across the 1.5k resitor 1500 * 0.0202 = 30.3v....???

    So completely wrong, but my thought here is that the transistor is getting its 0.7 volts to turn on so 0.3 volts should drop across the resistor.
    but looking at my math I am getting around a 100 times more current in the base to emitter circuit than what i should be?

    Can anyone point me in the right direction?
  2. benta


    Dec 7, 2015
    You need to work the other way round.
    If your input voltage is 1 V, the emitter current will be (1 - 0.7)/1500 = 0.2 mA
    The base current will adjust itself to approximately 0.2 mA / beta

    That's it.

  3. pizzaboy150

    Thread Starter New Member

    Aug 20, 2015
    I never thought of it that way round and now it makes perfect sense! Thank you!!!