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Op-amp task

Discussion in 'Electronics Homework Help' started by kamikaze1993, Jul 8, 2016.

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  1. kamikaze1993

    kamikaze1993

    14
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    Mar 25, 2016
    Hello I was wondering how the approach is to solve this exercise?

    I wonder how to find u+ = 2.5 V?

    Any suggestion ?
     

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  2. Bluejets

    Bluejets

    4,505
    958
    Oct 5, 2014
    No one is going to tell you outright but a tip is...
    .............where is the voltage located?
    have a good look.
     
  3. LvW

    LvW

    604
    146
    Apr 12, 2014
    Before solving it one must know the task.
    So - what is the question?
     
    Arouse1973 likes this.
  4. duke37

    duke37

    5,364
    769
    Jan 9, 2011
    R5 and R6 are connected in series with a positive voltage at on end and a negative voltage at the other end.
    You can find the current from the total voltage and the total resistance.
    Then calculate the voltage across each resistor and find U+ relative to one end and then shift the reference to the common.

    The values are chosen so the sums can be done in your head.
     
    Harald Kapp likes this.
  5. Harald Kapp

    Harald Kapp Moderator Moderator

    11,013
    2,506
    Nov 17, 2011
    Or use the common equation for a voltage divider.

    This is so obvious:rolleyes:
     
  6. Harald Kapp

    Harald Kapp Moderator Moderator

    11,013
    2,506
    Nov 17, 2011
    The important thing to consider is the impedance of the Opamp's input in comparison to the resistors' values...
     
  7. kamikaze1993

    kamikaze1993

    14
    0
    Mar 25, 2016
    When I used the voltage divider law:

    (7,5+12,5)*3/20+10 = 2 V

    I know this is wrong, could someone explain too me why?
     
  8. Herschel Peeler

    Herschel Peeler

    401
    65
    Feb 21, 2016
    Break it down into simple steps.
    Total resistance of R5 and R6.
    Total voltage across R5 and R6.
    Current through R5 and R6.
    Voltage drop across R5. subtract that from +10 V.
     
  9. Herschel Peeler

    Herschel Peeler

    401
    65
    Feb 21, 2016
    Why? It doesn't tell you what the op amp is so there is no way to know. Given an ideal op amp, the impedance of the op amp is of no concern.
     
  10. kamikaze1993

    kamikaze1993

    14
    0
    Mar 25, 2016
    Thank you for a good answer. Maybe this question is very easy anyhow, how do you find "Total voltage across R5 and R6". Why do you need solve only the voltage drop across R5, why not R5+R6?



    Rotal = 7,5+12,5 =20 ohm
     
  11. Herschel Peeler

    Herschel Peeler

    401
    65
    Feb 21, 2016
    See where R5 and R6 go. One side goes to +10 V the other side goes to -10 V, so a total of 20 V across a total of 20,000 ohms. Easy calculation, 1 mA. Your point of interest is 7,500 ohms down from +10 V, at 1 mA that puts it at+2.5V. It works out the same if you use the other resistor.
     
  12. Harald Kapp

    Harald Kapp Moderator Moderator

    11,013
    2,506
    Nov 17, 2011
    The input ompedance of an ideal opamp is infinity, that's why it is of no concern and that was where I was hinting at.
     
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