# Uncorrect calculation

Discussion in 'Feedback and Suggestions' started by Thevenin's Planet, Dec 20, 2011.

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1. ### Thevenin's Planet Thread Starter Active Member

Nov 14, 2008
183
1
The calculated base resistor of 83k is much lower than the previous 883k. We choose 82k from the list of standard values. The emitter currents with the 82k RB for β=100 and β=300 are:
What is going on with the denominator of the fraction in Beta=100 and Rb =82k,

Rb/B+Re.The quotient is not approximated to 1.01 Ma. Are we adding the Beta and Re or what.If done as is, then,.0009036 Amp.,instead of 1.01 Ma.http://www.allaboutcircuits.com/vol_3/chpt_4/10.html

Last edited: Dec 20, 2011
2. ### debjit625 Well-Known Member

Apr 17, 2010
790
186
No its all right the calculation is like this ,may be it could be corrected in the book
$I_E = {V_{BB} - V_{BE} \over (R_B / \beta) + R_E}$

Good Luck

3. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
You skipped over a lot of ground between 82K and 870KΩ.

Off the top of my head I don't see the problem. The point is the initial value was too high, creating major variation due to β differences between transistors (which is normal). The point it is making is you need to have much lower values to increase the stability.

So where is the problem again?

4. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
OK, I went through the exercise, the math works on my calculator. You did notice that VBB was 2V? The rest of the equation was the same, as it should be, and my answers matched the books.

5. ### debjit625 Well-Known Member

Apr 17, 2010
790
186
For the denominator he added $\beta$ with $R_E$ and then divided $R_B$ with that result.

He did this
$I_E = {V_{BB} - V_{BE} \over R_B / \beta + R_E}$

$I_E = {2 - 0.7 \over 82000 / 100 + 470}$

$I_E = {1.3 \over 82000 / 570}$

$I_E = {1.3 \over 143.86}$

$I_E = 0.0090 Amp$

As I already said this on my earlier post ,the book may use brackets for the equation like this

$I_E = {V_{BB} - V_{BE} \over (R_B / \beta) + R_E}$

$I_E = {2 - 0.7 \over (82000 / 100) + 470}$

$I_E = {1.3 \over 820 + 470}$

$I_E = {1.3 \over 1290}$

$I_E = 0.00101 Amp$

@Thevenin's Planet
Always use BODMAS method for order of operation
http://en.wikipedia.org/wiki/Order_of_operations

Good Luck

6. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
OK, but in this case the book is not incorrect.

Not Verified.

7. ### Thevenin's Planet Thread Starter Active Member

Nov 14, 2008
183
1
The point is that you gave the wrong direction to the goal.Since people in this field depends on number representation, correct formulas,graphs and equations to get a clearer understanding of what is happening to the device or component not placing parenthesis,or brackets,ect can lead to a dispute. With that said,I assume your are expressing the idea that the higher the collector current,meaning lower values of resistors,the better the stability?

Last edited: Dec 21, 2011
8. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
Not really, I worked the math. Order of operations is pretty basic, and has been discussed on this site. Debijt has shown your error is excruciating detail, if you can but open your mind long enough to look at it.

The denial is all yours. I understand transistors well, having derived the equations from scratch several decades ago to get them down. So if the equation is correct and the math is correct there is no problem.

Debjit is correct in that parenthesis might make it clearer for beginners, but they are not needed, since the math is correct either way.

Consider you have two people how have independently gone over your assertion, and have concluded you are wrong. Why not show your math as Debjit did if you are still not convinced?

Side note, I went through the exercise of calculating the real base resistors. I am not a fan of showing a smaller power supply as shown in the book.

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Last edited: Dec 21, 2011
9. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
It is interesting to note you changed your verbage after my reply. My comment about denial was in direct response to a comment you made about denial. It is now gone.

Changing history is easy, but there is always a time stamp left behind. Next time I will quote you to pin what you say down.

I think this thread has run its course.