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
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    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:
    [​IMG]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.:eek: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
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    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
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    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

    which is a wrong answer

    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
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    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. :rolleyes: 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
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    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.

    [​IMG]
     
    Last edited: Dec 21, 2011
  9. Wendy

    Moderator

    Mar 24, 2008
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    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.
     
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