Please help with OpAmp simulation problem!

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:

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(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.

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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

Reply to
Steven O.
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Swap the inverting and non-inverting inputs (pins 2 and 3).

Reply to
Matthew Kendall

"Steven O." schreef in bericht news: snipped-for-privacy@4ax.com...

Try swapping pin 2 & 3.

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Thanks, Frank.
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Reply to
Frank Bemelman

Steven O. wrote: (snip)

You need to swap the connections to the + and - inputs.

Reply to
John Popelish

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

Reply to
Fred Bloggs

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.

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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

Reply to
Steven O.

[snip]

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

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

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

Reply to
pmlonline

Not gonna help w/Vos.

Reply to
Fred Bloggs

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

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

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.

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Tim Wescott
Wescott Design Services
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Reply to
Tim Wescott

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.

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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

Reply to
Steven O.

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