E-book question

Discussion in 'General Electronics Chat' started by lendo1, Jul 14, 2011.

  1. lendo1

    Thread Starter Active Member

    Apr 24, 2010
    Volume 4, Chapter 3, Section 2 (The NOT gate)


    The picture shows current flowing from the base of Q4, but where is that current coming from, and why would it flow if the output was not at a different potential? <- (I realize that the voltage says "approximately equal to" 0, but if it were approximately zero, how could it overcome the 0.7V to forward bias the base-collector junction of Q4?)

    Also, why is Q3 not saturated? The section explains that there wouldn't be enough voltage to forward bias both Q3 and the diode, but isn't that arbitrary based on the values of the resistors? This is a tough chapter for me, thanks in advance.
  2. debjit625

    Well-Known Member

    Apr 17, 2010
    I didn’t understood all your questions...anyway
    First of all you should know that in this e-book the flow of electrons from negative to positive is taken as current.

    From negative terminal of the battery or from the ground of the power supply, then it enters emitter and flows to base...

    Which output you are talking about I have no idea. The base and emitter region needs to be at a different potential, it is at a different potential of approx 0.7V,base and emitter of Q4, so its conducting.

    I think you assumed the working of transistor in a wrong way, you will find approx 0V at the output i.e.. collector of Q4 because when a transistor start conducting or more correctly for this case when it gets saturated it drops a very little voltage across emitter and collector, you can assume the emitter and collector path as a single wire. Now if you measure voltage across a wire what should you get?

    No, only one transistor can be conducting between Q3 and Q4,but to explain that you need to understand how transistor works. Above all your questions seems you have misunderstood the concept...as per me start from PN junction section of the e-book.

    Good Luck
  3. Wendy


    Mar 24, 2008
    A quick comment, D1 isn't ususally in TTL chips. I'm not sure why it is in this diagram, but it's function is as a protection component. If the input voltage goes over 5V (the power supply) it conducts and limits this. For analysis just ignore it.

    The book is showing a transistor as two diodes. Personally I find this pretty far off, but it helps analyze the current paths.

    As debjit says it is important to understand that transistors base emitter is isolated from the collector emitter. The CE can be very close to 0V (I would say it was 0V) while the BE is dropping 0.6V. Not intuitive, but correct.