Transistor Circuit Analysis

Discussion in 'Homework Help' started by blake, Mar 11, 2011.

  1. blake

    Thread Starter New Member

    Mar 11, 2011
    This is for a 300 level electronics lab class and I'm wondering if I'm close:

    The problem:

    If the following silicon transistor (V(BE) = 0.6 V) has a β value of 100, determine I(C) and V(CE).

    (See attached PDF)

    My attempt at a solution:

    First off, I'm wondering what sort of circuit this is exactly - at first I thought common-base, but with only the -15 V source, I'm thinking common-emitter.
    Then, does the capacitor have any effect as far as circuit analysis and solving this problem? I figured "no" and attempted to solve as follows:

    I found an expression for α (0.99) and noted that

    Then I used Kirchoff's laws for the BE loop and the "loop" from the -15 V down to ground:

    BE Loop:
    -15 + 2000I(C) + V(CE) + 1000I(E) = 0
    -15 + 2990I(C) + V(CE) = 0

    15 V to Ground:
    -15 + 40000I(B) +V(BC) + 2000I(C) = 0
    -15.6 + 2400I(C) + V(CE) =0

    I then solved these equations to get:
    I(C) = 1 mA
    V(CE) = 18.04 V

    Any tips would be greatly appreciated.

    Also, if anyone can recommend a good electronics text it would be a huge help. Mine (Principles of Electronic Instrumentation by Diefenderfer) is filled with errors and is short on example problems.

  2. Adjuster

    Well-Known Member

    Dec 26, 2010
    These results are incorrect. You have found VCE to be greater than the supply voltage, which is not possible. Also, Ic = 1mA is clearly wrong.

    Note that the base supply chain includes a 10kΩ resistance to ground. It does not seem to be included in your calculations, but it needs to be.
  3. saqib altaf

    New Member

    Apr 7, 2011
    its a type of voltage devider biased common emitter in DC analysis of this circuit RE I-e
    1k ohm resistance is included but in AC analysis this resistance is bypassed means it is removed from the circuit make ur calculations according to this configuration.
    for this
    first check the condition βRE>10R2 if it is

    than use the equation:

    VB=R2 Vcc/R1+R2




    for collector emitter loop


    VCE=-Vcc+Ic(Rc+RE) IE is nearly equal to IC as we know

    this the solution for your problem for more information see the book
    (Electronic Devices And Circuits Theory) 9th edition By Robert L.Boylestad

    this problem is on page#202 EX-4.27