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Opamp calculations

Discussion in 'Electronic Design' started by bunny, Dec 1, 2005.

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  1. bunny

    bunny Guest

    I don't know if this is the right newsgroup but:
    I'm trying to figure out how to calculate what the Vout of an opamp will
    be based on Vin+ and Vin- and Gain. I've found websites that have this
    formula:

    Vout = G(Vin+ - Vin-)


    however my figures keep getting messed up...
    my circuit has:
    gain of 11
    Vin+ = 5.00V
    Vin- = .73V
    and the Vout from all of this is: 7.96V

    no matter what i do i can't seem to get the vout from the formula to
    match up with what i'm actually seeing as Vout. Is the formula wrong,
    or am i just going crazy?

    btw i'm going from a 9v supply to a 7805 Voltage Regulator to provide a
    clean +5V to the Vin+ of the opamp.
     
  2. Pooh Bear

    Pooh Bear Guest

    The voltage applied to the power pins of the op-amp ( Vin + - ) has *no*
    influence on gain at all.

    You need to learn some op-amp basics.

    Graham
     
  3. Except of course, if the op-amp is being run off 9v, then the maximum
    output it can generate might well be just under 8v, and would explain what
    he is seeing....
    The op-amp calculations only apply, if you are inside the input, and
    output range of the device used.

    Best Wishes
     
  4. bunny

    bunny Guest

    Any ideas where i can go to learn op-amp basics.. websites.. books.. etc?

    i'm interested in learning but finding sites that are tuned to beginners
    is kinda confusing :|
     
  5. You're far away. An OpAmp is an amplifier that
    amplifies the voltage between the + and - input
    almost infinitely. Since this is not really useable
    as it is, there are various possible feedback
    networks. Two of them are simply amplifying.
    Inverting and Noninverting. There, the voltage
    between the input terminals is always zero, at
    least in theory.

    Rene
     
  6. Rich Grise

    Rich Grise Guest

    Pooh Bear, quit trying to confuse the newbie. Vin is "input voltage"
    not the power rails, they're V+ and V-.

    What's wrong with bunny's circuit is that the amp is saturated at as
    close to the + rail as it can get.

    If Vin+ = 5.00V, and Vin- = .73V, then Vin+ - Vin- = 4.27V, and the
    opamp can't output 46.97V.

    Cheers!
    Rich
     
  7. Pooh Bear

    Pooh Bear Guest

    No.

    If there was 5.73V between the + and - inputs the op-amp would swing to rail.
    The closed loop feedback doesn't even come into the equation.

    Surely you know that feedback doesn't affect open loop gain ?

    Graham
     
  8. bunny said
    I use this book sometimes:

    IC Op-Amp Cookbook, Jung, Walter G.

    http://tinyurl.com/9jsja
    That price is ridiculous though.

    These are much better:
    http://tinyurl.com/arm3n

    hth
     
  9. Rich Grise

    Rich Grise Guest

    I guess when he said "gain = 11", I ass-u-me-d that he wasn't talking
    about open-loop gain.

    Thanks,
    Rich
     
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