emergency and badly needed help

Discussion in 'Homework Help' started by zamansabbir, Jun 2, 2008.

  1. zamansabbir

    Thread Starter Member

    May 27, 2008
    this is sabbir
    i am facing a problem to prove that BJT biasing with emitter resistance yields an improved level of stability in response to the variation of Beta.
    is there any mathematical solution to prove that problem.
    I am looking for your help.

  2. silvrstring

    Active Member

    Mar 27, 2008
    First, I believe you want to start with a large resistance for Rb--say, 1 MOhm.
    You can adjust it later if you want to reach Q point, but you want to start high so I(B) is low. That way, you don't saturate the BJT.

    Second, the fact that Beta is temperature dependent is the reason for instability in a fixed bias circuit; And if Beta changes, Ic will change, and if Ic changes, V(Rc) will change--which means Vce will change. To check it, evaluate V(Rc) and Vce of your circuit when Beta is 100, and compare it to your result if Beta were to change (due to temperature) to 150, or 200 to make a point.

    If you use a bias emitter circuit (i.e., one with emitter resistance), RC/RE = V(RC)/V(RE), and only V(BE) will change with temperature--and the amount is very small in contrast to V(RE), V(RC), and V(CE). Therefore, your Vce and Vc will be more stable than if RE were not there.

    For the fixed bias circuit, I(B) = (Vcc-.7)/RB.
    For the emitter biased circuit, I(B) = (Vcc - .7)/(RB+(Beta+1)RE).
    Last edited: Jun 2, 2008
  3. zamansabbir

    Thread Starter Member

    May 27, 2008
    Thanks for your help.