# Nodal analysis (Node-voltage method) for op-amps?

Hi, I am just beginning to learn about op amps and I was wondering if there was a way to use the node voltage method (nodal analysis to some) to solve these circuit without replacing the op amp with its equivalent ideal amplifier. To me, it is much faster to use node voltage method than going through it "manually" If such a method exists and if anybody can provide a link or post the general rules, that would be greatly appreciated.

Here's what i wanted to use it on: http://img134.imageshack.us/img134/782/opampcircuit.jpg [Broken]

Thanks.

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## Answers and Replies

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dlgoff
Science Advisor
Gold Member
How about using this model:
http://en.wikipedia.org/wiki/File:Op-Amp_Internal.svg" [Broken]

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Thanks for the reply.
Actually, I wanted to work through the problem by directly using the op-amp and not replacing it with an amplifier model. That's what i meant by "solve these circuit without replacing the op amp with its equivalent ideal amplifier"

Born2bwire
Science Advisor
Gold Member
As I recall, I never replaced it with an amplifier model, at least because I always did ideal amplifiers by hand, non-ideal I reserved for SPICE or its proprietary cousins. What were the rules again....? I think you assume that the nodal voltages at the inputs are equal and no current flows into the inputs. However, I am not sure what the limitations of these approximations are.

ranger
Gold Member
Your aim when applying this to op amps is to end up with a transfer function. However, one must make the assumptions of an ideal amplifier and consider the currents flowing into and out of the nodes. The end result is very usable when compared to the real model. However, that is assuming that you do not violate the operating conditions of the component. For non ideal models, I'd much rather use SPICE or an equivalent simulator.

The Electrician
Gold Member
Have a look at this page:

http://www.swarthmore.edu/NatSci/echeeve1/Ref/mna/MNA1.html

There is a section on solving when opamps are involved.

The usual assumption is that an opamp has infinite gain and the inputs draw no current. Is that the assumption you want to make, or do you want to solve the case where the opamp gain is finite, and the opamp may have a rolloff characteristic in its open loop gain; perhaps a single pole rolloff?

Thanks all for your replies,
Yes, since we're just starting to learn op-amps, we're treating them as ideal, so i+=0 and i-=0, and when there's negative feedback v+=v-. I guess I should have said that in the beginning =)

That last link the electrician posted is exactly the type of site I am looking for.