PSpice Model of diode, BJT, MOS

Discussion in 'Homework Help' started by mau80, Oct 24, 2010.

  1. mau80

    Thread Starter New Member

    Oct 24, 2010
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    Hi to all!

    I'm trying to solve some didactical circuit which includes ( diodes, BJT, MOS ). For solve this exercise I've to use a simplified characteristic of devices.
    For example for:

    • diode use piecewise linear model Ideal diode in series with voltage source
    • BJT Piecewise model

    See attached file att.jpg for characteristic formulas and circuit that I’ve to solve

    I'd like to be able to solve this problem with PSpice in order to better understand how to solve this kind of exercise and be able to inspect various nodes of circuit.

    My problem is that I'm not able to use such model in PSpice, I've tried to search in the forum but I didn't find anything.

    I'd like to ask if you can provide to me some information about.

    Thank you very much for you help.


    Regards
    Maurizio
     
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  2. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    Well for this circuit and DC analysis you can use simply
    voltage source instead of Diode. And voltage source + current-controlled current source instead BJT.
     
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  3. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    And of-course we can quickly verify the results, assuming β=∞.
    So voltage on emitter Q1 is equal:

    Ve1 = (9V - 0.7V) * 40/(80+40) = 2.76666667V

    then Ie1 = Ve1/ Re1 = 1.38333334mA

    Ie1 = Ic1 = 1.38mA

    Ie2 = (Ic1 * Rc1)/Re2 = ( 1.38mA * 2K )/100 = 27.6666668mA

    And voltage on Q2 collector

    Vc2 = Ie2 * 100 = 2.76666668V
     
  4. mau80

    Thread Starter New Member

    Oct 24, 2010
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    Thank you very much for you help!
    I'll analyze your mail in dept this afternoon.
    I'd like to ask if you could attach the schematics to the message.

    Futhermore I've to analyze circuit like that in attachement for Vi from 0 to Vdd, I'd like to ask if the substitution made in Pspice could be applied also to that circuit.

    Thank you again for your great help!

    Bye
    Maurizio
     
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  5. mau80

    Thread Starter New Member

    Oct 24, 2010
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    I've study the circuit that you post. Using your substitution, I can reach the correct results.

    Using your circuit the result it's correct, furthermore I've noted that I can apply Kirchhoff KCL e KVL after that substitution, but if I want to analyze a circuit with an input signal varying by time, I've to redraw the circuit substituting the devices with it's corresponding model for different state which device can operate: AD, LIN, AR, SAT in case of BJT. So I'd like to ask if there is a way for have a model of devices working piecewise in PSpice. This for avoid to redraw every time the circuit considering. I've attach an example of this required analysis.

    Thank you very much to everybody for your great help!

    Maurizio
     
  6. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    Well simply use "normal" model in PSpice.

    But for this simply circuit yo don't need a PSpice simulation.

    [​IMG]


    For expel for Vin = Vcc = 5V
    T1 will be in saturation If : Ib1*β >> Ic2+IR3

    Ic1 = β*(5V - 0.75V)/20K = 21.25mA

    Ic2 = Ib2 *β

    Ib2 = (Vcc - Vbe - Vce_sat)/ ( Rb + (β+1) * Rc ) = ( 5 - 0.75V - 0.2V ) / ( 5K + 101*500) = 4.05V/55.5K = 72.97uA

    Ic2 = 7.297mA

    IR3 = (5V - 0.2V)/2K = 2.4mA

    So
    Ib1*β = 21.25mA and Ic2+IR3 ≈ 9.7mA
    So for sure for Vi = 5V T1 is in saturation and Vu = 0.2V
    And T1 will start to go out from saturation for Vin < then
    9.7mA/β = 97uA---> Vin = 97uA*20K + 0.75V = 2.69V.
    And Vu = 2.5V for Vin equal
    Ic1 = Ic2 + IR3
    Ic2 for Vu =2.5V is equal --->
    Ic2 =β* (Vcc - Vbe - Vu)/ ( Rb + (β+1) * Rc ) = (5V - 0.75V - 2.5V)/55.5K = 100* 31.53uA = 3.153mA
    IR3 = 2.5V/2K = 1.25mA
    So Ic1 = 4.403mA we have Vin = 1.6306V
    Summarize
    If Vin = 0 V then Vu = 5V
    Vin = 1.6V then Vu = 2.5V
    and for Vin greater than 2.69V output voltage is equal Vu = 0.2V
     
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  7. mau80

    Thread Starter New Member

    Oct 24, 2010
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    I've anlyze your solution.
    I've try to resolve it and I was able to find the solution.
    I'd like to thank you very very much for your precious help and your time!

    Bye
    Maurizio
     
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