Next report - BJT polarization

Discussion in 'Homework Help' started by PsySc0rpi0n, Apr 11, 2014.

  1. PsySc0rpi0n

    Thread Starter Well-Known Member

    Mar 4, 2014
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    Let me try to explain what I need now:

    I need to plot in the same window the 3 curves regarding the 3 DC Bias points that I have calculated!

    Then I need to draw the Load Line and point the values of Ic and Vce for each curve of each calculated DC Bias point.

    I also need to know how to calculate Ic and Vce when the temp changes but this one is not needed to plot anything!
     
  2. PsySc0rpi0n

    Thread Starter Well-Known Member

    Mar 4, 2014
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    Morning...

    Anyone around to help?

    I also need to know if I can do to a current source the same that was made to the voltage source in youtube video that jony130 posted earlier!!!
     
  3. ericgibbs

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    Jan 29, 2010
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    You can use a Current Source in the same way and measure the Temperature response of your circuit.

    Post your LTS.asc file and I will check it out.
     
  4. PsySc0rpi0n

    Thread Starter Well-Known Member

    Mar 4, 2014
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    I managed to do it but I still couldn't do what I need to do.
    I'll explain again what I need:

    1 - Simulate a circuit with with 2N2222 or 2N2222A, with fixed polarization, Rb = 720k and Rc = 2.2k and Vcc = 15V that is what I have in 2.1.png and 2.1.asc and find the DC Bias point.

    2 - Now we are asked to change β to 100 and plot again the same curve which is 2.3.png and 2.3asc and find the new DC Bias point.

    3 - Now we are asked to change temperature to 100ºC and plot again the same curve and find the new DC Bias point (not done yet because I still don't know if it is possible to calculate new DC Bias point with calculus).

    4 - Finally we are asked to run a simulation as we did in the previous job/report we did and to mark the 3 DC Bias points previously calculated and draw the Load Line (not done yet too by the reason below)...

    But I think the 4th task is not logic as we will try to compare different BJTs and draw only one Load Line... I don't know what is being asked and how to do it!
     
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  5. ericgibbs

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    Jan 29, 2010
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    The way I read your post is,

    1. Use 2N2222, find DC point
    2. Use 2N2222B, find DC point
    3. Repeat 1 and 2 but Temp at 100C [ Just add .step temp list 25 100 to your circuit]

    4. Repeat 1 and 2 and get the 3 DC points and draw the Load line for each ie: 2N2222 and 2N2222b.

    image of your sims with .step temp list 25 100 added
     
  6. PsySc0rpi0n

    Thread Starter Well-Known Member

    Mar 4, 2014
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    It's almost that... But I need 3 curves, all with different settings, and in the same plot so that I can mark each DC Bias Point and draw the Load Line.

    1. Use 2N2222(A) and calculate DC Bias Point.
    2. Use 2N2222(A) with β = 100 and calculate new DC Bias Point.
    3. Use 2N2222(A) with temp = 100ºC and calculate new DC Bias Point.
    4. Use 2N2222(A) and running the same kind of simulation as in previous job/report (where we got like 4 or 5 Ic(Vce) curves, mark the 3 previous DC Bias Points and draw the Load Line from those 3 DC Bias Points!

    Some friends of mine did what is in the image I posted a few posts ago!
    I need 3 Ic(Vce) curves in the same plot in the 1., 2. and 3. conditions and mark in that plot the 3 DC Bias Points and draw the Load Line with those 3 points!
     
    Last edited: Apr 15, 2014
  7. PsySc0rpi0n

    Thread Starter Well-Known Member

    Mar 4, 2014
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    Hi mr EricGibbs!!!

    Would you be able to help me? Is it possible to do what I need to do in the last post?
     
  8. ericgibbs

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    Jan 29, 2010
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    hi Psy,
    I would like to help, so could you please post the asc files and images you did for the 'previous job report'. Mark your images with the 3 DC points you calculated and I will relook.

    The question being asked by your tutor is not very clear.:confused:
     
  9. PsySc0rpi0n

    Thread Starter Well-Known Member

    Mar 4, 2014
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    Hi EricGibbs... Sorry for being absent these last days but my work has been killing all my spare time!

    Ok, I'm going to explain here, sorted, the questions from our teacher!

    1 - Simulate the circuit 2.1 (BJT 2N2222), and calculate the DC Bias point. (This one is done!).
    2 - Change β to 100 and recalculate DC Bias Point. (Also done).
    3 - Change Temperature to 100ºC and recalculate DC Bias Point. (Not done yet).
    4 - On top of the Ic(Vce) curves, that were obtained with the method used in the previous job/report, draw the Load Line, marking the previous calculated DC Bias Points.

    Can you understand all the steps?

    Now I have a couple of questions:

    Is it possible to plot the 3 first situations in the same plot or do I need to come up with some kind of graphic trick to overlay the 3 simulations and draw the Load Line and mark the 3 DC Bias Points previous calculated?

    Is it possible to calculate a DC Bias Point when we only change the temperature?

    Thanks
    Psy

    PS: image 2 is the simulation for β = 100
     
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  10. ericgibbs

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    Jan 29, 2010
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    hi Psy,
    I have downloaded your asc files/plots, I will look thru the questions as soon as possible.

    As its Easter and hopefully the fine weather will continue, I will be busy outdoors.:)

    Hopefully a fellow member will have some free time to help you.:rolleyes:

    Eric
     
  11. ericgibbs

    Moderator

    Jan 29, 2010
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    hi,
    This is a quick way of combining the circuit plots.

    I guess you know you have Ibase fixed at 19.86uA, the 720K has no effect on the Base current.?

    E
     
  12. PsySc0rpi0n

    Thread Starter Well-Known Member

    Mar 4, 2014
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    Ok, Happy Easter for you and your family.

    Just tell me something.

    I wan to simulate a circuit with BJT at a Colector-Base polarization.
    The teacher asks us to calculate an Rb so that the circuit is polarized approximately half the Load Line.

    I have done my calculations and got an Rb of 514kΩ. If I just set this Rb with this value, the Ic(Vce) curve is very different than if I place the current sorce and the Rb in the circuit... Why and which one is correct?

    Shouldn't be the same if I place only the calculated Rb or only the current source or both???

    I'll upload the circuit in a minute!

    Edit;
    Ok, screens are up...

    Why isn't the circuit giving the same response?
     
    Last edited: Apr 18, 2014
  13. PsySc0rpi0n

    Thread Starter Well-Known Member

    Mar 4, 2014
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    I'm not talking about temperature.

    I also made everything from the very beggining and used a new set of parameters for 2N2222.

    The question is why do I need to inlcude the current source in the BJT base to be able to get a similar Ic(Vce) as the previous ones?
     
  14. ericgibbs

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    Jan 29, 2010
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    hi,
    Happy Easter to you and yours.:)

    The Curve will be different because you have 'negative feedback' from collector to base, via Rb.

    So as Vcol falls due to increasing IC, it will cause Ib to decrease until the transistor reaches a steady state.

    Its a common method of biasing transistors.


    Your asc file link is reporting an error, I cannot download, it maybe due the way the file name is formatted.
     
  15. PsySc0rpi0n

    Thread Starter Well-Known Member

    Mar 4, 2014
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    I think I've deleted the .asc file in the meantime.

    But which screen is the correct? The one with the current source or the one without the current source?
     
  16. ericgibbs

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    If I understand correctly you were using that 720K and the current source for the Base when you were plotting the Early voltages etc.?
     
  17. PsySc0rpi0n

    Thread Starter Well-Known Member

    Mar 4, 2014
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    Here are the .asc files with and without current source
     
  18. PsySc0rpi0n

    Thread Starter Well-Known Member

    Mar 4, 2014
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    Ahh sorry.. My mistake...

    This is already another one... The one from 720K at Rb was a fixed polarized BJT.

    I'm now working at a Colector-Base polarization!

    Forget the other one. It's already done!
     
  19. PsySc0rpi0n

    Thread Starter Well-Known Member

    Mar 4, 2014
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    Morning.

    I want to ask you guys who are helping to forget what I asked behind... It's done.

    Now I'm analyzing a circuit with the Thevenin equivalent. This is supposed to be the most stabilized circuit but I'm not being able to get that stability when I change temperature.

    I'm going to post here 3 simulations.

    One is the normal circuit, the other is with hFE = 100 and the other is with temp = 100ºC. As far as I know they all should be more or less equal, but the last one is far from being close (sounds weird :confused:) to the other two simulations.

    I need to know If am I doing anything wrong to get results so far from the two previous ones.
     
    Last edited: Apr 19, 2014
  20. ericgibbs

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    Jan 29, 2010
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    hi,
    The Base current on the last circuit is set to 36.63uA and the other two circuits its only 14.66uA, they will be very different.

    The resistor divider chain on the Base has NO effect on the Base current.

    You could just connect the left side of the Ib current source to Common/0V.!
     
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