# 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

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

just wondering if its a mistake in the video.

thanks

2. ### wayneh Expert

Sep 9, 2010
12,394
3,246
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

r

4. ### wayneh Expert

Sep 9, 2010
12,394
3,246
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
12,394
3,246
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.

Dec 11, 2006
410
2
okay....