Applying 0 volts to base raises Vc?

Discussion in 'General Electronics Chat' started by rougie, Oct 10, 2012.

  1. rougie

    Thread Starter AAC Fanatic!

    Dec 11, 2006

    Please consider a voltage divider bias circuit in the attachment below. There is something weird going on here. This configuration as it is, gives a Vc output of approximately 1.2 VDC. Why is it that when Vs power supply is "off" and when "S1" closes, Vc raises to 3.35 VDC ???

    Even if I turn Vs "on" but keep it's voltage output to 0 volts, Vc still raises to 3.35VDC???

    The only way I can get rid of this unexplained issue is if I keep "S1" open.

    All help appreciated!
    Last edited: Oct 10, 2012
  2. Audioguru


    Dec 20, 2007
    Your circuit is WRONG!
    A transistor is turned on with base current, not base voltage. 2V between base and emitter will create a very high base-emitter current that will DESTROY the transistor.

    Resistors Rb and R2 feed a low current to the base which turns on the transistor. If you want to turn off the transistor then short the base to ground (or feed 0V to the base like you did) which stops the base current.

    When the transistor is turned on then it conducts and its collector voltage drops.
    When a transistor is turned off then it does not conduct and its collector voltage rises to the supply voltage if there is no load.
  3. rougie

    Thread Starter AAC Fanatic!

    Dec 11, 2006
    Hi Audioguru:

    Oh no!!! my intention was never to put 2V to that base. My only question was why when Vs was 0 volts and "S1" was on does Vc shoot up to 3.3VDC....

    I figured out that, if S1 is closed and Vs provides 0 volts, then the base sees a grounded potential which puts the transistor in cutoff mode and Vcc reflects at Vc.

    Yes, exactly... I just realized that even when the positive prom of Vs is 0 volts... it too acts as a ground!

    It's weird though that even if Vs is *OFF*, and I close "S1", I still get the same effect!!!
    I guess the base still sees a ground at the positive prom of Vs, even though Vs is off.