Quiescent current

Discussion in 'Homework Help' started by yifei87, Oct 30, 2008.

  1. yifei87

    Thread Starter Member

    Sep 7, 2008
    hey guy,
    Given Ib= 250uA, Rb= 5k, Is for Q1= 10^-15A, Is for Q1= 10^-16A.
    assume Vo= 0, beta= infinite.

    what is the quiescent current?
    if possible pls show me step by step and explanation. thank you
  2. Ratch

    New Member

    Mar 20, 2007

    Through where?

  3. steveb

    Senior Member

    Jul 3, 2008
    Since it's a homework problem, we should not give you a step by step approach, but instead provide some hints. When I look at this, I feel the tricky thing about the problem is that you can't use the approximate linear DC equations that are often employed. I'm talking about assuming Vbe is 0.7 volts and then using ohms law, voltage loops and current node equations. That just doesn't work in this circuit. There would be no way to get the collector currents. Note that since beta is infinite, each transistor has Ic=Ie.

    Instead you have to use the nonlinear exponential transistor equation that treats the transistors as voltage controlled (i.e. Vbe) current sources. Two things give this away. First, they provide the reverse saturation currents, and second, there is no other way to do it.

    They simplify the problem by letting you assume that beta is infinite, so that you can ignore the input base currents. Also, assuming Vo is zero is equivalent to saying you have grounded the output.
    Last edited: Oct 30, 2008