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Determinant Zero, Saturated Bipolar Transistor

  1. Feb 27, 2013 #1
    1. The problem statement, all variables and given/known data
    I am looking for the base, collector, and emitter voltages in the following circuit:
    circ.jpg

    2. Relevant equations
    KCL
    KVL
    offset voltage = VB - VE
    saturation voltage = VC - VE

    3. The attempt at a solution
    First I made a matrix without assuming any values for offset or saturation voltages. Doing so gave me this:

    1 55.56 14.7 | 678.72 KCL with transistor as node
    9/500 1 9/34 | 12.216 KVL right hand mesh
    17/250 34/9 1 | 46.149 KVL left hand mesh

    The determinant for this matrix is 0, and seems to have infinite solutions. So that was not a good approach here. I checked the circuit with CircuitLab, and sure enough the voltages work with my matrix. Its just that the matrix cant produce them unfortunately... (10.07, 9.539, 9.428 for Vbase, Vcollector, and Vemitter). First, I was wondering if this always happens with a saturated bipolar transistor. Or, this isn't a textbook problem, is that the only reason the matrix didn't work out?

    A much easier solution was to just assume that the offset voltage was 0.7 V and saturation voltage (Vce) was 0.2 V. Those 2 equations

    Vbase - Vemitter = 0.7
    Vcollector - Vemitter = 0.2

    along with the first equation in the matrix gave solutions consistent with CircuitLab to within a hundreth of a volt. So second, I'm just kind of wondering what makes the first assumption valid. I was told offset voltage is not a constant. The Vce assumption makes sense just looking at a datasheet for the 2n2222. But why can I say that Vbe is 0.7 with confidence here?

    Thanks.
     
  2. jcsd
  3. Feb 27, 2013 #2
    I think I see what happened, I used the same equation in rows 1 and 2 of my matrix.

    Actually they are all the same...
     
  4. Feb 28, 2013 #3

    CWatters

    User Avatar
    Science Advisor
    Homework Helper

    If the transistor is ON then Vbe will be roughly 0.7V. Look at the diode model of a transistor.

    It might seem obvious for this problem but it would be best to check the transistor is saturated by comparing the base and collector currents. For example most small signal transistors have a gain >50 so multiply the base current by 50 and if the answer is more than the collector current the transistor will be ON/saturated.
     
  5. Feb 28, 2013 #4
    If you want the super simplified model, you consider the transistor is ON, That VBE=0.7V and that the current gain is 50 (approximate value for a 2n2222 at Ic=1mA)

    That gives you the following matrix:

    [itex]
    \left ( \begin{array}{ccc}
    10680 & 680& 0\\
    680 & 860 &1\\
    -50 & 1 &0
    \end{array} \right ) \cdot \left (
    \begin{array}{c}
    I_{b} \\
    I_{c} \\
    V_{CE}\end{array} \right ) = \left ( \begin{array}{c}
    11.3\\
    12\\0\end{array}\right )[/itex]

    If you want a model that's slightly more complex, you can use the hybridparameters. (See here)

    (Edit, correct Vbe to Vce)
     
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