Rb calc is incorrect?

Discussion in 'The Projects Forum' started by rougie, Oct 24, 2012.

  1. rougie

    Thread Starter Active Member

    Dec 11, 2006
    410
    2
    hello,

    Quick question...

    i was just wondering how the fellow in the following video got 360k for Rb????

    please view 07:00 of 10:08 of the following video

    http://www.youtube.com/watch?v=Pqm5-axy7m8

    The math doesn't seem right....???

    just wondering if its a mistake in the video. :cool:

    thanks
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,100
    3,034
    It looks good to me and clearly explained by the screen at 7:13. Beta seems to be pulled out of the blue, but otherwise it's just applying Ohm's law.
     
  3. rougie

    Thread Starter Active Member

    Dec 11, 2006
    410
    2
    Well, first, Rb formula seems to be a little different... As per video is it:

    Rb = ((Vc - Vbb) / Ic) * 160

    or does he mean:

    Rb = ((Vc - Vbb) * 160) / Ic

    thanks for replying wayneh
    r
     
  4. wayneh

    Expert

    Sep 9, 2010
    12,100
    3,034
    Those are the same
     
  5. rougie

    Thread Starter Active Member

    Dec 11, 2006
    410
    2
    yes ... you are right

    but how is the following =360k???

    Rb = ((6.65 - 4.03) / 0.001) * 160

    bof...... I get 419,200 !
     
    Last edited: Oct 25, 2012
  6. HonT

    New Member

    Oct 25, 2012
    14
    2
    Have you tried calculating Vbb?
     
  7. rougie

    Thread Starter Active Member

    Dec 11, 2006
    410
    2
    no, I didn't work out the problem as per se, I only
    casually followed along as I was watching the video
    and noticed the numbers he plugged in didn't add
    up to 360k. everything else is pretty straight forwards
    except the 360k ?

    but off the top of my head, (3.3+0.7)?
    he plugged 4.03v as vbb! no?

    I guess there is something i am not seeing...
    why how do you see it?
     
    Last edited: Oct 25, 2012
  8. wayneh

    Expert

    Sep 9, 2010
    12,100
    3,034
    Of course you are right. I can only guess he made the same error I did - ballpark the first term at 2,000 and multiply by 160.
     
  9. rougie

    Thread Starter Active Member

    Dec 11, 2006
    410
    2
    okay....
    thanks for replying

    r
     
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