2 Conceptual Questions

Discussion in 'Homework Help' started by CrktMan, May 16, 2006.

  1. CrktMan

    Thread Starter Active Member

    Nov 29, 2005
    Q-1: What does it mean when it says a transistor has a β = infinite?

    My understanding is that the transistor is in OFF state or not conducting.
    Please correct me!

    Q-2: When we have a horizontal bar sign at the top of a given voltage (V) or current (I), what actually does that mean?

    Thanks for comments.

  2. n9352527

    AAC Fanatic!

    Oct 14, 2005
    It is an ideal condition, where the effect of base current draw to circuit operation is neglected to simplify analysis. This can be used for examples, to ignore the input impedance of a transistor amplifier (infinite gain implies no base current is drawn and hence infinite input impedance) or to ignore the effect of base current in transistor current mirror (infinite gain means that no current flow through the base hence base current doesn't have to be subtracted from the mirrored current).

    Similar approaches are used in many analysis, e.g. assumptions of infinite input resistances and infinite gain of an op-amp. As long as these assumptions are reasonable enough, then the effects may be safely ignored. But, you have to be aware about where and how these assumptions are used, so that in occasions where they are no longer valid you could take steps to avoid any inconsistency.
  3. CrktMan

    Thread Starter Active Member

    Nov 29, 2005
    Can we say that collector current Ic = βIb = αIe = infinite; in other words one of the mirror transistors, which has infinite gain, is in OFF state?

    Any comments on my 2nd question?

  4. Papabravo


    Feb 24, 2006
    I think you may be missing the point entirely. The case of Ib=0 & β at infinity can't be equal to Ic, rather Ic is independent of Ib and β. Then we are left with α = 1 and Ic = Ie. This does not seem like OFF to me.

    The bar over V or I might indicate a phasor quantity. A phasor is a time varying quantity that can be espressed as a magnitude and a phase.

    In mathematics and physics the bar can also indicate the complex conjugate of a quantity.
  5. Dave

    Retired Moderator

    Nov 17, 2003
    To add to the above post, generally in Mathematics having a bar across the top of any variable (not just V and I) indicates that the variable is vectorised, i.e. has direction and magnitude. From Papabravo's explaination, it can be seen that phasor quantity implies a vectorised represenation of V and I where the voltage and current has both magnitude and phase.

    Nowdays in much of the literature and publications, it is more common to represent the complex conjugate using (*), e.g. V* is the complex conjugate of V. This is intended to remove any ambiguity with phasors in the world of mathematics and physics.