BJT CC Configuration

Discussion in 'General Electronics Chat' started by aashish, Feb 17, 2010.

  1. aashish

    Thread Starter New Member

    Feb 17, 2010
    13
    0
    Hi I have to simulate the input characteristics of BJT npn Common Collector (CC) Configuration.

    I used OrCAD PSpice for that,
    I am getting the output characteristics correctly but I am not getting the input characteristics {Ib vs Vbc for fixed values of Vec} resembling to (not even nearer to) what I have read in my text books.Here are the screen captures,
    [​IMG]
    Just look at the base currents (encircled red) are in A range but they should be in uA or pA range ?
    According to the texts,
    [​IMG]
    (Taken from "Electronic Devices and Applications By B. Somanathan Nair" page-198 fig-8.20)

    Please help me If possible please show me the correct circuit for the above simulation so that the simulated results resemble (at least near) to the actual one - as expected from a simulation.
     
  2. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
  3. aashish

    Thread Starter New Member

    Feb 17, 2010
    13
    0
    Yeh, you are right ! but I should not use any load R because my aim is only to get the behavior of the BJT - everyone knows that resistance limits the current then what has to be done by the BJT ?

    It is the property of BJT - the base current is very small (uA range) and collector and emitter currents are in mA range - without limited or reduced by the resistors.

    And while I was doing the same experiment to plot the characteristics of Common Base or Common Emitter configurations there was no need of introducing any resisters in order to reduce the current levels explicitly(by me) - the BJT by virtue of its internal working and characteristics automatically gave the reasonable current levels.

    So what is happening wrong to the Common collector config ?
     
    Last edited: Feb 18, 2010
  4. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    How about this?
     
  5. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    I forgot to include the .asc file, which will run on LTspice.
     
  6. aashish

    Thread Starter New Member

    Feb 17, 2010
    13
    0
    Thanks Ron,
    It is working in the same way as I wanted to have:) but you sweeped Ib and Vec - it is Vb vs Ib (not Ib vs Vb) for the values of Vec !!
     
  7. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    What??????
     
  8. aashish

    Thread Starter New Member

    Feb 17, 2010
    13
    0
    when I run your circuit I get,
    [​IMG]
    Actually I am new to LTSpice - how can I obtain Ib vs Vb ?
    Please tell me the steps.
     
    • 1.PNG
      1.PNG
      File size:
      46.7 KB
      Views:
      60
  9. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    Run the sim and plot i(Ib) vs i(Ib), as in the first attachment.
    Next hover the mouse over the horizontal axis (you will see a little scale, or ruler). Left-click. You will see the dialog window as in the second attachment.
    Change the quantity plotted to V(b) as in the 3rd attachment.
    You should now see the plot I posted originally, as in 4th attachment.

    The purpose of this exercise is, of course, to swap the axes. I don't know if there is a simpler way.
     
  10. aashish

    Thread Starter New Member

    Feb 17, 2010
    13
    0
    OK sir you have solved my problem Thanks....Thanks....Thanks....Thanks....Thanks....Thanks....Thanks....Thanks....

    But is there any way to get this by using a voltage source in the input side as shown in the text books e.t.c. ?
    I know that BJT is a current controlled device, but why couldn't I able to get his by using a voltage source in the input side?

    If it is so why do the book writers present the biasing arrangement in a bit wrong way ?

    Please clear my doubts.
     
  11. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    Unless the book writers can show you a simulation file showing how they got from the schematic you posted to the plot you posted, I think that this portion of the book was just poorly written. As far as I know, it is impossible to get the plot you posted by sweeping Vbc. The fact that the current axis has no scale is good evidence that there was a disconnect between the schematic you posted and the one they actually simulated.
     
  12. aashish

    Thread Starter New Member

    Feb 17, 2010
    13
    0
    OK thanks a lot
     
Loading...