close loop gain of a feedback amplifier (Avf)

Discussion in 'Homework Help' started by BlackMelon, Oct 2, 2016.

  1. BlackMelon

    Thread Starter Member

    Mar 19, 2015

    I'm asked to find the parameter. The problem and my solution are on the attachments. My answer is Avf=10.9759, but my book says 10.174. Did I make anything wrong?

    PS. I even try the common collector's gain formula (not to approximate the gain of the common collector circuit to 1). The answer is around 10.9 something but not 10.174

    Thank you
  2. MrAl

    Well-Known Member

    Jun 17, 2014

    I get 10.97 no matter what i try. That includes lowering the gain of the transistor as the Early voltage might lead to if we included that, but still not much difference. Never gets down as low as 10.2 even. i even varied the Vbe from 0 to 0.7 with not much difference, as we might expect from a feedback amplifier :)
    If the Early voltage did take an effect here then i would expect to see a non constant gain anyway, so that seems out since they did not specify an operating point, or did they?

    Either we are both missing something or the book is wrong. I've run into books that are wrong in multiple cases, sometimes simple errors like this and sometimes more complex where whole parts of formulas are missing. Editors must miss stuff now and then. Is it really a book error? The only way to know for sure is to find the author and ask them, or else someone can come forward with a different method or something, but i dont see how when nothing reasonable seems to change the gain much, even forcing that internal gain change just to see what would happen.
    BTW this is most likely a large signal problem.

    If this is going to count as a part of a grade, then you have to contact the instructor or something like that.
    Last edited: Oct 3, 2016