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:

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

 

Swap the inverting and non-inverting inputs (pins 2 and 3).

 

Try swapping pin 2 & 3.

 

Steven O. wrote:
(snip)

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

 

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

 

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

 

[snip]

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

...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

 

Not gonna help w/Vos.

 

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

...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.

--

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

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

 

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