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Please help with OpAmp simulation problem!

Discussion in 'Electronic Basics' started by Steven O., Apr 8, 2006.

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  1. Steven O.

    Steven O. Guest

    I am taking a distance learning "lab" class in EE, using Multisim to
    simulate circuits at home. Unfortunately, e-mail communications with
    the professor sometimes does, and sometimes does not, result in clear
    answers, so I thought I'd go straight to the pros.

    We are simulating a 741 OpAmp circuit. Another unfortunate matter --
    Multisim's documentation on the pin usage on the OpAmp is rather
    sparse. I have created the circuit the way I think it's supposed to
    be, although I'm not quite sure if pins 1 and 5 are supposed to be
    used at all (they seem to have something to do with "Balance", though
    I'm not quite sure what that is).

    In any event, the circuit is supposed to be a simple inverting
    amplifier. Only, there is no amplication -- the output is simply a
    steady -11 volts. The circuit and output are shown at:

    http://www.oppenheimercommunications.com/OpAmpProblem.htm

    (Be sure to scroll down to see the output plot.) The output plot
    shows both the input signal, and the flatline output signal.

    If anyone can post something indicating where my problem might be, I'd
    be grateful. Please also post if you need more information or screen
    images to trouble-shoot the problem, I'd be glad to post more info,
    but I'm pretty sure the two images shown on the Web page have
    everything you need. Thanks in advance for all replies.

    Steve O.


    www.OppenheimerCommunications.com
    Technical and Marketing Writing
    "From Complexity To Clarity" (SM)

    Steven AATT Domain DDOOTT com
    To send an e-mail, substitute @ for AATT, a . for DDOOTT, and OpComm for Domain
     
  2. Swap the inverting and non-inverting inputs (pins 2 and 3).
     
  3. Try swapping pin 2 & 3.
     
  4. Steven O. wrote:
    (snip)
    You need to swap the connections to the + and - inputs.
     
  5. Fred Bloggs

    Fred Bloggs Guest

    It's hard to say just where you're lacking, but the best educator can
    only reach down so far.
     
  6. Steven O.

    Steven O. Guest

    In the immortal words of Homer Simpson: DOH!!! (head slap...)

    Thank you. The notes from the lab class had the OpAmp drawn with the
    minus sign (-) on top, but I failed to notice that. Anyway, you and
    the other who replied saved me from countless hours of spinning my
    wheels.

    While we are here... Now that I have the (basically) correct output,
    I notice that the output is offset slightly, by about two-tenths of a
    volt in the positive direction. I assume the balance pins are meant
    to correct that. Can someone describe the basis trick there?

    Thanks!

    Steve O.


    www.OppenheimerCommunications.com
    Technical and Marketing Writing
    "From Complexity To Clarity" (SM)

    Steven AATT Domain DDOOTT com
    To send an e-mail, substitute @ for AATT, a . for DDOOTT, and OpComm for Domain
     
  7. Jim Thompson

    Jim Thompson Guest

    [snip]

    It's input bias current. Put 10K in the positive lead to ground
    (precisely, put 10K||1Meg there).

    ...Jim Thompson
     
  8. Guest

    This is probably not what you want, but the free online applet "Circuit
    Simulator" has the entire 741 internals built in. Just go to the
    following site, wait for the java applet to automatically pop up, then
    in the top menu click on "Circuits", then "Op-Amps", then "741
    Internals"

    It's nice in that it has the entire 741 internals built for you and you
    can simulate it.

    Hope that helps a little,
    Paul
     
  9. Fred Bloggs

    Fred Bloggs Guest

    Not gonna help w/Vos.
     
  10. Jim Thompson

    Jim Thompson Guest

    True, in real life. I'd bet the model includes Ib but not Vos.

    ...Jim Thompson
     
  11. Tim Wescott

    Tim Wescott Guest

    To unpack that statement a little bit: A bipolar op-amp needs to draw
    current from the input pins for the base current of the input
    transistors. This is small but real, and it causes a voltage drop in
    your resistors. You need to find the equivalent parallel resistance
    from ground to your - input, then put in that same resistance to ground
    in your + input.

    Op amps also have offsets -- both a voltage offset between the input
    pins, and a bias current offset. The output will be off by the total
    input voltage offset times the gain.

    FET input op amps don't have this bias current, but they have higher
    offset voltages. Choosing between a FET input or bipolar input becomes
    a judgment call depending on your circuit.

    If your class hasn't gotten into this stuff yet it should get around to
    it -- if it never does I'd question it's worth.

    --

    Tim Wescott
    Wescott Design Services
    http://www.wescottdesign.com

    Posting from Google? See http://cfaj.freeshell.org/google/
     
  12. Steven O.

    Steven O. Guest

    This course is a 1 credit lab course associated with classes on basic
    circuit theory at the freshman/sophomore level. The OpAmps are
    introduced by way of enabling us to apply what we've learned about
    resistors, capacitors, etc., to basic, real amplifier circuits, sum
    and difference circuits, etc. We're not at the level of covering the
    fine points of OpAmps, which I assume are covered in greater depth in
    more advanced classes. I won't even be taking a transistor course
    until next Fall. Still, all the feedback is much appreciated, gives
    me something to look forward to.

    Steve O.

    www.OppenheimerCommunications.com
    Technical and Marketing Writing
    "From Complexity To Clarity" (SM)

    Steven AATT Domain DDOOTT com
    To send an e-mail, substitute @ for AATT, a . for DDOOTT, and OpComm for Domain
     
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