Output impedances in the eBook

Discussion in 'Feedback and Suggestions' started by The Electrician, Aug 15, 2009.

  1. The Electrician

    Thread Starter AAC Fanatic!

    Oct 9, 2007
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    On this page:

    http://www.allaboutcircuits.com/vol_3/chpt_4/13.html

    just before the figure, the text says "However, R. Victor Jones develops expressions for output resistance.", and then there is a hyperlink.

    When I click on the hyperlink, the page I get appears identical to the page I just left, and I don't see any output resistance expressions.

    Are there output resistance expressions somewhere?
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    Interesting, looking at it I would guess someone meant to put more in the URL, but hasn't gotten around to it. This is a fairly new section, so it could easily be a work in progress.
     
  3. Dcrunkilton

    E-book Co-ordinator

    Jul 31, 2004
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    Examination of the the affected AAC Page
    http://www.allaboutcircuits.com/vol_3/chpt_4/13.html


    Leads me to compare to the corresponding ibiblio page#target
    http://www.openbookproject.net//electricCircuits/Semi/SEMI_4.html#xtocid674716

    Going down the page we find the offending link at AAC:
    There do not appear to be any simple formulas for the output impedances. However, R. Victor Jones develops expressions for output resistance. [RVJ]

    And the Corresponding link at ibiblio:
    There do not appear to be any simple formulas for the output impedances. However, R. Victor Jones develops expressions for output resistance. [RVJ]

    We find that the ibiblio [RV] link redirects us to the Bibliography at the end of the chapter:

    Bibliography



    1. [AGT] A. G. Thiele in Loyd P. Hunter, “Handbook of Semiconductor Electronics,” Low Frequency Amplifiers, ISBN -07-031305-9, 1970
    2. [GET] “GE Transistor Manual”, General Electric, 1964.
    3. [RVJ] R. Victor Jones, “Basic BJT Amplifier Configurations”, November 7, 2001. at http://people.seas.harvard.edu/~jones/es154/lectures/lecture_3/bjt_amps/bjt_amps.html
    4. [TK1] Tony Kuphaldt,“Lessons in Electric Circuits”, Vol. 1, DC, DC Network Analysis, Thevenin's Theorem, at http://www.openbookproject.net/electricCircuits/DC/DC_10.html#xtocid102679
    5. [FAR] “PN22221 Datasheet”,Fairchild Semiconductor Corporation, 2007 at http://www.fairchildsemi.com/ds/PN/PN2222A.pdf
    #3 is the bibliography entry of interest; It points to the web link for Jones' article. The Output Impedance equation is near the end of his page.


    Summary: AAC does not have a Bibliography, ibiblio does. Morover, the Bibliography at the end of the ibiblio chapter would correspond to a Bibliography page at AAC. AAC cannot link from one page to another. And I don't know an easy way to do it.
     
  4. The Electrician

    Thread Starter AAC Fanatic!

    Oct 9, 2007
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    I didn't realize that this AAC page was essentially a copy of the imbiblio material. Is the entire AAC eBook based on imbiblio?

    I contacted Tony Kuphaldt once upon a time about an error in another section of the imbiblio project.

    I see some more tiny errors in the page under discussion here. For example, the expression Rin = β RE should be Rin = (β+1)RE and similarly for some more expressions on that page.

    The page under discussion says "There do not appear to be any simple formulas for the output impedances.", but then refers to R. Victor Jones's expressions for output resistance which seem to me to be about as simple as one could wish.

    Why doesn't the AAC (or imbiblio) page just borrow Jones's expressions and give credit?
     
  5. Dcrunkilton

    E-book Co-ordinator

    Jul 31, 2004
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    The ibiblio page requests that error reports be sent to this forum at AAC.

    I try to keep it simple by substituting β for (β + 1). This works quite well for small signal transistors, β>100.

    This text was originally directed toward 2-year college technicians. Keeping it simple at that level has made it suprisingly popular amoung 4-year engineering students. I prefer to not make the text overly complex. Since it is not a simple expression, I prefer to reference it, giving Jones credit, rather than include it.
     
  6. The Electrician

    Thread Starter AAC Fanatic!

    Oct 9, 2007
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    But it's incorrect. Using (β+1) where it's correct works quite well when β>100, and it also works well when β<10, say for power transistors.

    Using β instead of (β+1) in the gain expression for a common emitter stage without external emitter resistor leads to the conclusion that the stage has voltage gain even if β = 0. It also implies that Ie = Ic rather than that Ie = Ic + Ib. Do you think it's a good thing to let students believe these things?

    The decrease in complexity going from (β+1) to β is really negligible given the danger of wrong results by doing so.

    How do you know that it's the simplicity that has made it popular, rather than the ready accessibility? If you made it slightly less simple, but correct, how do you know it wouldn't be just as popular? Was there a time when it was more complex and not popular, and then when it was made less complex, it got popular? Has that comparison ever been put to the test?

    When word gets around that there are incorrect formulas in it, what will that do to the popularity?

    The expressions aren't really that complex. Even two year technicians are required to know algebra, aren't they?

    Why contribute to the dumbing down of America; why not combat it?

    What will the reader do when Jones' web site disappears?
     
  7. steveb

    Senior Member

    Jul 3, 2008
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    I have to say I agree on this one. The Electrician has caught me a couple of times making that approximation without thinking, but it is better to use the correct formula. Adding one to any number is no problem at all, and the conceptual misunderstandings, the inadvertent mistakes or even the numerical inanaccuracies that can result from the approximation are not worth the needless risk.

    Generally, correctness has value and (to some people) is even worth paying a high price for, but when the price is so low, I say we should all take the deal.
     
  8. The Electrician

    Thread Starter AAC Fanatic!

    Oct 9, 2007
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    I have absolutely no objection to using approximations; I use them myself. But, I think they should be identified as such, and their domain of applicability should be given.

    I've seen questions in AAC forums where a student has tried to use a formula outside its domain of applicability because his text didn't explain that it was just an approximation.

    One way to deal with the situation is to give approximations, identified as such, in the main body of a text, and then give the full, exact, formula in an appendix, possibly even showing the derivation of the exact formula and explaining how limiting the domain of applicability leads to a simplified approximation.
     
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