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Homework Help: BJT active region

  1. Jun 13, 2014 #1
    1. The problem statement, all variables and given/known data


    The transistor in the circuit of Fig. P5.69 has a very
    high 0. Find the highest value of VB for which the transistor
    still operates in the active mode.
    2. Relevant equations

    IC=IE (since IB is approximated to zero due to high beta)

    EDIT: IE= [VE-0]/1k (VB - VE = VBE = 0.7V ---> VE = VB-0.7V)

    3. The attempt at a solution

    This question is taken from sedra/smith 5th edition, there's some worked solution floating around the internet, however the solution provided is confusing. The condition given is that VC-VB=-0.4V

    I cant agree with the solution given because that equation would mean that the voltage at base is higher than that of the voltage of the collector since the potential difference is negative.

    What I was thinking is that VC - VB = 0.

    Am I correct, or did I missed anything?
    Last edited: Jun 13, 2014
  2. jcsd
  3. Jun 14, 2014 #2

    rude man

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    Homework Helper
    Gold Member

    I think you're correct. Saturation is defined as Vc < Vb by any amount. Which is when the b-c junction becomes forward-biased.

    However, fact remains that saturation voltages well below Vb are typically attained. Well below 0.3V which would roughly correspond to their answer. But you don't have the information you need to determine that level.
  4. Jun 14, 2014 #3


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    Staff: Mentor

    The textbook is correct in making that reasonable assumption. For very low collector currents, VCE can go down to 0.1V or less for some transistors, so this means that (for NPN) the base can be at higher potential than the collector. Take a look at the data sheet for some common small signal transistors.

    Saturation occurs when the base current loses control of the collector current, i.e., when an increase in IB fails to elicit a corresponding increase in IC. Under these conditions, IC is being controlled by the external collector/emitter circuit, and not by the base.
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