Analyses for bipolar junction transistor

Discussion in 'General Electronics Chat' started by full, Oct 25, 2014.

  1. full

    Thread Starter Member

    May 3, 2014
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    Hello everyone

    I do Analyses for bipolar junction transistor (BJT) PNP on A ,B & C .

    for analysis A the current flow from emitter to base & collector .
    for analysis B the current flow from emitter to base & collector but there is source in emitter .
    for analysis C the current flow from emitter to base & collector but there is source in emitter .
    please check my analytic true?
    please If there is anything unclear told me.....

    thanks
     
    Last edited: Oct 25, 2014
  2. full

    Thread Starter Member

    May 3, 2014
    225
    2
    Analysis update :
    full3.PNG
     
    Last edited: Oct 25, 2014
  3. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    It helps to let people know that you are talking about PNP, PNP, NPN transistors.
     
  4. full

    Thread Starter Member

    May 3, 2014
    225
    2
    sir this is PNP

    thanks
     
  5. full

    Thread Starter Member

    May 3, 2014
    225
    2
    update :

    full4.PNG

    analyses :

    full5.PNG

    I think there is wrong in analysis B in emitter to base (Vth) but I do analysis should they (vth ) in positive not in negative .

    and there is wrong in analysis C in emitter to base (-Vre & -Vth) same problem in analysis B .

    I think there are problem in my loops :confused:

    thanks :)
     
  6. Jony130

    AAC Fanatic!

    Feb 17, 2009
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  7. MrChips

    Moderator

    Oct 2, 2009
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    Don't let the change of polarity confuse you. All three circuits have the same configuration and hence the analysis is the same.
    The analysis is still the same for a NPN transistor.
     
  8. full

    Thread Starter Member

    May 3, 2014
    225
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    update:

    full8.jpg
     
  9. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    Why you cannot stick to one convention when doing KVL ?
    For case A
    -VRe - Vbe - VRth + Vth = 0
    -VRe - Vec - VRc + Vcc = 0
    Also notice that case B and C are identical (Case B is turned 180 degrees case C). So KVL also must look identical.
    Also you made a error in case B, why Vth is negative ??

    Also notice that the only difference between case A and B&C is reference point (GND).
    So the current will stay the same but the voltage will change.
    Try analyze this circuit very careful.
    3.PNG
    And for example do you understand why the base voltage is -1.6V in case A and 7.4V in case B ?
     
    Last edited: Oct 27, 2014
  10. full

    Thread Starter Member

    May 3, 2014
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    3.PNG

    for first circuit :

    Vee -Vre + Vbe - Vth - Vth

    second circuit:

    Vee -Vre + Vbe - Vth - Vth

    the first circuit and second is same
    And for example do you understand why the base voltage is -1.6V in case A and 7.4V in case B ?
    no
    sorry for late because I have exams
    thanks
     
  11. full

    Thread Starter Member

    May 3, 2014
    225
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    I have another circuit :

    me Vth and DC analysis true?
     
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  12. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    Very good, but why you drew your circuit upside down ?
    First you need to understand what GND is, so please read this
    http://forum.allaboutcircuits.com/threads/ground-node.102921/
    Once you understand what GND is. Pleas notice where GND is in the first circuit. Ground is at positive battery terminal.
    And because the voltage at base is measured with respect to ground, this voltage is equal to the voltage drop across R2 resistor with minus sign.
    But if our ground is negative terminal then voltage at base is equal to VR1 voltage drop.
    On the other hand the collector voltage for the first circuit is a sum of a voltages Vc = VRe + Vce. But in second circuit Vc is simply -VRc.
    Ve for the first circuit is -VRe but for second circuit Ve = VRc + Vce
    And this is why this voltages are shifted by 9V depends on where the GND is.
    VB = 7.4V but if we change the ground the base voltage will be shift down to 7.4V - 9V = -1.6V
     
    Last edited: Oct 29, 2014
  13. full

    Thread Starter Member

    May 3, 2014
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    Hello again
    The Av is negative or positive?
    I think is positive
    Thanks
     
  14. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    If Av is a voltage gain for CE amplifier, the gain is negative but usually we ignore this "minus" sign. Because this only informs us about 180 degree phase shift.
     
    full likes this.
  15. full

    Thread Starter Member

    May 3, 2014
    225
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    If do not put "minus" on Av a voltage gain ,is mean wrong?
    actually when did exam test , didn't put minus on Av

    thanks
     
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