Help constructing a Biploar Transistor switch in multisim

Discussion in 'Homework Help' started by moataz, Sep 22, 2006.

  1. moataz

    Thread Starter New Member

    Sep 21, 2006
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    I need help constructing a Bipolar Transistor switch in multisim. In my lab, I constructed the circuit shown here. But when I got home, I realized my measurements were all wrong. So, I'm trying to use multsim to contruct the circuit, and get the right measurements.

    The measurements i've been getting are.
    Vbe = .680 V
    Vcb = 2.727 V
    Vce = 1.470

    Measurement I should be getting
    Vce should be 1 to 4 volts
    Vbe should be .6 to .8 range
    Vcb 1 volt or so

    I posted a pic of what I have so for in multisim.
     
  2. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    Look at your multisim diagram again.

    You have your battery polarity reversed.

    Those measurements you list as your getting, is that in the lab or in multisim?
     
  3. moataz

    Thread Starter New Member

    Sep 21, 2006
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    those were the measements from the lab.
     
  4. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    what transistor were you using in the lab? a 2N4400?
     
  5. moataz

    Thread Starter New Member

    Sep 21, 2006
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    we were using a MPSa20 (417-801)
     
  6. moataz

    Thread Starter New Member

    Sep 21, 2006
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    I just realized I was using the wrong transistor in multisim. I'm using the MPSa20 now, but i'm still getting the same measurement I was with the other transistor. Should I be connecting a switch to the 220k resistor?? Thats the only difference I see between the circuit in my lab book and my multisim diagram.
     
  7. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    If your applying 5 volts as input to the 220k resistor, you are saturating the transistor. Spice models vary on how they achieve similar results in the linear region and will vary widely in saturation.

    The two models I have [Zetex 3f5 Spice] and one that came with the simulation program, show widely differring V b-c and V c-e.

    What did you use for an input at the lab? Did you use seperate supplies for Vcc and Vin?

    Take a few moments and learn how to sweep parameters in multisim, you then can repeat your lab experiment using the input values you swept and do the same comparison. They should be much closer.

    On edit ....

    The simulation models can and often have different results. Even within the same program the results can be different. Attached are the results from two MPSA20 models and one Spice 3F5 model. One produced some unusual results for a switch.
     
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