1. Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.
    Dismiss Notice

non-inverting OPAMP

Discussion in 'Homework Help' started by floflo, Oct 20, 2015.

  1. floflo

    floflo New Member

    Joined:
    Oct 20, 2015
    Messages:
    11
    Likes:
    0
    Hello everybody, I will have a work about op-amps. I must design a non-inverting amplifier with a voltage gain of 11. When the input voltage is larger than 10mV a LED must illuminate and when the output reach 11V a LED must illuminate also.
    Actually we didn't do op-amps as comparator in class yet, so I started to check by myself on the net and in my electronic book.
    But I am sure at all if I do understand. I tried to draw a design using 2 op-amps (lm741) as comparator for the input and another one op-amps (lm741) using the inverting as feedback to reduce the voltage gain at 11 and amplify.
    Is my drawing definitely wrong?
    thank you very much. :)
    20151020_232928.jpg
     
  2. JoeJester

    JoeJester Active Member

    Joined:
    Nov 6, 2009
    Messages:
    699
    Likes:
    105
    Location:
    Grand Prairie, TX
    At 11V is the same LED lit or is it a second LED being energized?

    Can you type exactly what your professor had in the problem statement?

    your drawing is an attempt at a window comparator feeding an amplifier with a gain of 11. you have your reference at ground and you have supply voltages of only 11V. You will need rail to rail opamps to get close to 11V output using an 11 V supply.
     
  3. crutschow

    crutschow Well-Known Member Most Helpful Member

    Joined:
    Mar 14, 2008
    Messages:
    10,592
    Likes:
    477
    Location:
    L.A., USA Zulu -8
    As Joe noted you need a higher supply voltage with a 741.
    The old 741 output can go no closer than about 2-3V of the positive and negative supply voltages before the output saturates.
     
  4. dave

    Dave New Member

    Joined:
    Jan 12, 1997
    Messages:
    -
    Likes:
    0


     
  5. floflo

    floflo New Member

    Joined:
    Oct 20, 2015
    Messages:
    11
    Likes:
    0

    thanks for your answer. Oh yes I read that this op-amps is old and not efficient, only school still using it, It means that I have to supply my op-amps at around 14V even 15V?
    Only my LED at the right far must lit at 11V, and my first LED at 10mV.
    design a non-amplifier with a voltage of 11 with the following features, when the input voltage is larger than 11mV p-p a red LED must lit to indicate signal presents, and when the output reaches 11V p-p a green LED must lit to indicate that the maximum voltage is reached. That's my problem statement...
    I thought to use a window comparator because the voltage at the input is very small to compare (10mV) with one op-amps, is it a bad idea?
    thank you.
     
  6. JoeJester

    JoeJester Active Member

    Joined:
    Nov 6, 2009
    Messages:
    699
    Likes:
    105
    Location:
    Grand Prairie, TX
    You have one amplifier with a voltage gain of 11. This has NOTHING to do with the supply voltage being 15V. In fact, with the 741, crutchow explained the output will be lower than the supply voltage by 2 to 3 volts.

    You can setup two comparators ... one for the > 10 mV and one for the > 11V. The signal feeding both comparators is the output of your x11 (voltage gain of 11) amplifier. Are you restricted to using the 741 as your comparator as well?

    Are there any constraints on the resistor series? (see attached file)
     

    Attached Files:

    Last edited: Oct 21, 2015
  7. floflo

    floflo New Member

    Joined:
    Oct 20, 2015
    Messages:
    11
    Likes:
    0
    ha okay sorry I do understand what you mean now with the output voltage.
    yes we only have to use the 741.
    we only use E12 series resistors.
    I tried to redraw my circuit, does it make more sens?
    Thank you very much
    20151022_151429.jpg
     
  8. JoeJester

    JoeJester Active Member

    Joined:
    Nov 6, 2009
    Messages:
    699
    Likes:
    105
    Location:
    Grand Prairie, TX
    That is one way. Your reference could be a single string of resistors, using three resistors. However, being restricted to the E12 series, it might be best using it the way you have it drawn.

    With my comments on the output voltage ... it was the output of the x11 amplifier. With 15V supplies, you will get 2-3 volts lower than 15 volts at the output of that amplifier.

    Go ahead and fill in the remaining values of the resistors. Repost your schematic when your done.

    Are you using a single or dual (+ and -) 15 volt supplies?
     
  9. floflo

    floflo New Member

    Joined:
    Oct 20, 2015
    Messages:
    11
    Likes:
    0
    I choose this method because my electronic book only shows up 3 methods : voltage divider reference, battery reference, zener diode set reference. I didn't know about a single string resistors thank you :).
    I am using a + and - 15 volt supplies, I suppose it's a dual supply?... ( sorry english is not my first language).
    I repost the schematic with my resistors value, and also the list of the resistors that I am allowed to use.
    Thank you very much.
    20151023_165147.jpg 20151023_174549.jpg
     
    Last edited: Oct 23, 2015
  10. atferrari

    atferrari Well-Known Member

    Joined:
    Oct 8, 2003
    Messages:
    2,812
    Likes:
    121
    Location:
    Buenos Aires - Argentina
    ONLINE
    You attached two lists of resistors.
     
  11. JoeJester

    JoeJester Active Member

    Joined:
    Nov 6, 2009
    Messages:
    699
    Likes:
    105
    Location:
    Grand Prairie, TX
    Are we to assume you are using the 1/4 W 5% (E24 series) resistors ?

    You calculated the 10 mV input improperly.

    Correlate your diagram to the problem statement.
     
    Last edited: Oct 23, 2015
  12. floflo

    floflo New Member

    Joined:
    Oct 20, 2015
    Messages:
    11
    Likes:
    0
    Do you see the same picture? maybe I did a mistake. One picture in the list of the resistors that I am allowed to use.
    And the second picture is my schematic with my chosen resistors.
    thank you
     
  13. floflo

    floflo New Member

    Joined:
    Oct 20, 2015
    Messages:
    11
    Likes:
    0
    Yes I use these resistors.
    ha really... I only see this formula in my book, I will search more if I can find out. thank you :)
     
  14. JoeJester

    JoeJester Active Member

    Joined:
    Nov 6, 2009
    Messages:
    699
    Likes:
    105
    Location:
    Grand Prairie, TX
    Look at the words on your problem statement. You should be able to point each of those, plus more, on your schematic.
     
  15. floflo

    floflo New Member

    Joined:
    Oct 20, 2015
    Messages:
    11
    Likes:
    0
    That's true... With this drawing I have 2 output comparator and none one output/one input. I think I see my mistake thank you. But can I keep these resistors value for my input voltage divider comparator?
    We were supposed to do this lesson last week, but because some student are striking the school is close, sorry to ask a lot of questions.
     
  16. JoeJester

    JoeJester Active Member

    Joined:
    Nov 6, 2009
    Messages:
    699
    Likes:
    105
    Location:
    Grand Prairie, TX
    If you have a 9 mV DC signal at the input to the amplifier, what is the output of the amplifier, and is the low LED energized?
     
  17. floflo

    floflo New Member

    Joined:
    Oct 20, 2015
    Messages:
    11
    Likes:
    0
    My output will be low, and my LED will be off. is that right?
     
  18. JoeJester

    JoeJester Active Member

    Joined:
    Nov 6, 2009
    Messages:
    699
    Likes:
    105
    Location:
    Grand Prairie, TX
    No. That is not how you designed it.

    With 9mV at the input to the op amp, what is the input voltage to the comparator?

    Re-read your requiements.
     
    Last edited: Oct 24, 2015
  19. floflo

    floflo New Member

    Joined:
    Oct 20, 2015
    Messages:
    11
    Likes:
    0
    I redraw my schematic. My book says that : when Vin exceeds the reference voltage, the output goes to his maximum voltage.

    To my understanding if Vin for my output comparator exceed my Vref (11V) my LED goes ON.

    And for my input comparator when Vin exceed my Vref (10mV) my LED goes ON.

    20151025_163002.jpg
     
  20. JoeJester

    JoeJester Active Member

    Joined:
    Nov 6, 2009
    Messages:
    699
    Likes:
    105
    Location:
    Grand Prairie, TX
    There was nothing wrong with the previous topology.

    What you had incorrect was Using 10 mV as the threshold.

    I wanted you to realize the 10 mV into the x11 amplifier produced 110 mV at the output and 110mV was the threshold you needed and not 10 mV.

    What input to the amplifier causes the amplifiers output to be the maximum voltage ... Assume the maximum voltage to be two volts lower than your selected Vdd.
     
    Last edited: Oct 25, 2015
    • Like Like x 1
  21. floflo

    floflo New Member

    Joined:
    Oct 20, 2015
    Messages:
    11
    Likes:
    0
    Ha yes of course it makes a lot of sense... sorry to didn't see this mistake.

    (for the 11V output comparator) 1v to my input causes to be the maximum voltage to the output because 1v * 11(AV) = 11V and the threshold is 11V.

    With 9 mV input, the output of the amplifier * 11 = 99mV, with my wrong threshold is 10mV<99 mV my LED is ON.

    Thank you very much :).
     

Share This Page