BJT Saturation Question(s)

Discussion in 'Homework Help' started by Ricky Spanish, Feb 17, 2013.

  1. Ricky Spanish

    Thread Starter New Member

    Feb 17, 2013
    Hi all,

    I did search and find a few other threads about saturation but I'm still confused or just not grasping it.

    Anyhow, the question for this problem is, "Determine if the transistors in the following circuits are saturated."

    Circuit A,


    For this one I have the following info,

    Vce = 5.1V
    Vbe = 0.7V
    Vcb = 4.4V
    Ic = 55mA
    Ib = 1.1mA
    Ie = 56.1mA


    Circuit B,


    For this one I have this info,

    Vce = -3.83V
    Vbe = -0.7V
    Vcb = -3.13V
    Ic = 10.7mA
    Ib = 85.2μA
    Ie = 10.8mA

    I'm pretty sure that should be enough info to determine saturation, if someone could point out the method, I'd really appreciate it. Thanks.

    Also, as a side note, those voltages were confirmed from the back of the book, since the answers are given for odd numbered questions,

    (my calculations didn't come up with the negative voltages for circuit B, I'm probably not seeing the obvious there...)

    My original question is an even numbered one related to those circuits.
  2. Ron H

    AAC Fanatic!

    Apr 14, 2005
    What have you learned about the definition of saturation?
  3. Ricky Spanish

    Thread Starter New Member

    Feb 17, 2013
    Thanks Ron, sorry if I'm not putting enough info in my post.

    Any other info you need, I'll gladly add what I can.

    Basically my understanding of saturation is when Ic can increase no further even if Ib increases.
  4. #12


    Nov 30, 2010
    I don't like that definition. It's too absolute.
    You need to look up some datasheets for common transistors like 2N3904 or 2N2222 and see what they say about Vce Sat. You will not only see that the answer isn't absolute, you will see the range of values that indicate saturation and learn more about what you're looking for.
    Ricky Spanish likes this.
  5. Ron H

    AAC Fanatic!

    Apr 14, 2005
    True, but if you have a mythical transistor with no specs other than beta, it's probably good enough. If he had calculated that Vce were 0.2v, or 0.4v, or..., you get my drift, then I would say we need to look closer at the definition of saturation.
    Ricky, how do you determine the maximum Ic possible in your problem circuits?
    Ricky Spanish likes this.
  6. Ricky Spanish

    Thread Starter New Member

    Feb 17, 2013
    Honestly, I'm not sure. I got Ic from,

    Ic = βDC*Ib

    Wouldn't you need a data sheet for that?
  7. WBahn


    Mar 31, 2012
    I think a reasonably simple yet useful description of the saturation process goes something like this:

    Think of a transistor as a device that tries to create a situation in which the collector current, Ic, is β*Ib. The mechanism by which it does this is by controlling the "resistance" of the collector-emitter path through the device. If the present resistance is such that the collector current is less than β*Ib, then the transistor reduces the resistance. This generally results in a lower collector-emitter voltage which means that the voltage dropped across the external components increases which generally allows more collector current to flow. The reverse happens if the present collector current is more than β*Ib in order to reduce it.

    But this process can only be carried so far. The transistor can only lower the collector-emitter voltage so far and, once that point is reached, the transistor no longer has any tools in its bag to increase the collector current up to the point it owuld like it to be, so the collector-emitter voltage stops dropping and the collector current is less β*Ib.

    Ideally this saturation voltage would be 0V and the collector-emitter junction would simply look like a short circuit allowing any current to flow as long as it doesn't go above β*Ib (since, at that point, the transistor will come out of saturation and start increasing Vce in order to hold the current to β*Ib).

    In practice, real transistors have a saturation voltage that is greater than 0V, but it is generally only a few hundred millivolts.

    Now, keep in mind that I have been talking about magnitudes and you have to be careful to allow for the different polarities involved when working with PNP versus NPN devices.
    anhnha and screen1988 like this.
  8. Jony130

    AAC Fanatic!

    Feb 17, 2009
    Simply change Rc from 180 to 390 in first diagram. And solve for Ic and Vce.
    And you will see the saturation.
    screen1988 and Ricky Spanish like this.