buffer logic gate

Discussion in 'General Electronics Chat' started by omaroski, Apr 27, 2012.

  1. omaroski

    Thread Starter Active Member

    Jun 19, 2008
    34
    0
    http://www.allaboutcircuits.com/vol_4/chpt_3/3.html

    I don't understand Q1 very well. When input is HIGH there's no current through BE but only through BC which makes Q2 going in saturation. Why there's current going into Q1's collector?

    When input is LOW there's current between BE's Q1 but it is not going in saturation, shouldn't it?

    From Q3 and on it is clear, but the role of Q1 i don't get it still. Can you please explain me?
     
  2. WBahn

    Moderator

    Mar 31, 2012
    17,715
    4,788
    There's more than one circuit diagram on that page. Which one are you referring to?
     
  3. debjit625

    Well-Known Member

    Apr 17, 2010
    790
    186
    [​IMG]

    Very simple, if you look at the diagram you will find Q1 is a NPN transistor, now the base emitter acts like a simple junction diode, emitter is N type and base is the P type. When input is high i.e.. we are applying positive approx Vcc to the N type emitter and the P type base is already connected to Vcc through R1, so across PN junction their will no voltage difference as both the sides are Vcc ,as a result no current will flow through this PN (base emitter) junction.


    Now as the P type base of Q1 is connected to Vcc through R1 and the N type collector is connected to the base of Q2,and here with Q2 you will find another PN (base emitter) junction which is connected to ground through R3.So here current flows form Vcc to R1 then cross the Q1's PN (base collector) junction and the pass through base emitter junction of Q2 and at last passes through R3.

    In short the base collector of Q1 is forward biased as a result current flows.

    Just replace the transistors Q1 and Q2 with diode equivalents and the stuff will be more clear.

    Good Luck
     
  4. omaroski

    Thread Starter Active Member

    Jun 19, 2008
    34
    0
    Thank you,

    i refer to the one debjit625 is linking.

    I probably have some ''hole'' (don't mean positive carrier...) about transistors.

    So base-collector junction let pass current just like base-emitter does?
     
  5. debjit625

    Well-Known Member

    Apr 17, 2010
    790
    186
    Yes the base collector junction is a PN diode and when forward biased it will conduct just like base emitter junction.
     
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