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Calculate Transistor Parameters

Discussion in 'Electronics Homework Help' started by electronicsLearner77, Nov 29, 2020.

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  1. electronicsLearner77


    Jul 2, 2015
    I need to calculate IB,IE and IC with Alpha(DC) = 0.98 for the below circuit
    I generally did problems with some base resistor and collector resistance and here i don't find both. I did the following attempt
    Since VBB > 0.7V the BE is forward biased (current required?)
    If i consider the resistance RE as coming in series with the BE, then the current
    IB = (2 - 0.7)/1kΩ = 1.3mA
    From alpha = 0.98, beta = (alpha / (1-alpha)) = 0.98/0.02=49.
    IC = beta * IB = 49*1.3mA = 63.7mA;
    IE = IC + IB = (1.3 + 63.7)mA = 65mA.
    Vcc - Vce = IE*RE ; Vce = Vcc-IE*RE = 10 - 65 = -55. Vce seems to be very high!
  2. Nanren888


    Nov 8, 2015
    Your instinct that something is wrong is correct.
    If your first equation gives you the voltage on the emitter resistor, then this is the voltage, hence relates to the total current through it. This tells you the total current throught the resistor being a combination, addition of the base-emitter current and the collector emitter current. That is, all that current does not come from the base. Your gain calculations tell you the ratio of currents from each source.
    If you prefer, the two loop currents which both flow through the emitter resistor, must add to the emitter current you calculate from the voltage.
    Let us know how you get on.
    electronicsLearner77 likes this.
  3. electronicsLearner77


    Jul 2, 2015
    Thank you, that really helped. So,
    VE= 2-0.7V= 1.3V;
    IE = 1.3/1k = 1.3mA;
    IC = Alpha * IE = 0.98*1.3mA = 1.274mA
    IB = IE - IC = 0.026mA
    Vce = 10 - 1.3 = 8.7V
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