# How to calculate op amp output voltage?

#### OldSantiago

Joined Oct 10, 2020
3
Hello everyone,

Thank you.

#### WBahn

Joined Mar 31, 2012
26,398
You need to show your best attempt to tackle your homework problems. We will not work it for you, but we will look over what you have done and help you see where you might be going wrong and help you get past the specific points that are blocking your progress.

#### OldSantiago

Joined Oct 10, 2020
3
You need to show your best attempt to tackle your homework problems. We will not work it for you, but we will look over what you have done and help you see where you might be going wrong and help you get past the specific points that are blocking your progress.
I know that R1 is useless, so does this mean that this is a non-inverting amplifier circuit?

#### Audioguru again

Joined Oct 21, 2019
3,187
I know that R1 is useless, so does this mean that this is a non-inverting amplifier circuit?
Yes, that is correct.

An opamp has such a high voltage gain that when negative feedback is added then the output of the opamp will do whatever it can to make its - input voltage the same as the + input voltage. Since both inputs have the same voltage then R1 is useless with no current in it.

#### MrChips

Joined Oct 2, 2009
23,527
Here is the essential behavior of an op-amp circuit with proper negative feedback.
+Vinput = -Vinput

Just like what ag says above.
Take it from there.

#### LvW

Joined Jun 13, 2013
1,247
I know that R1 is useless, so does this mean that this is a non-inverting amplifier circuit?
I know and realize that - as it seems - the question comes from a beginner.
And - as far as an idealized opamp is concerned - I agree that R1 is "useless".

However, for real opamps with a frequency-dependent gain the resistor R1 is NOT useless. It resembles a special kind of external frequency compensation - thereby improving the phase margin as well as the step response. More than that, using such a resistor across the opamps input nodes allows us to use opamps which are not unity-gain stable even with 100% feedback (unity gain).

#### MrAl

Joined Jun 17, 2014
8,248
Hello everyone,
View attachment 219235

Thank you.
Hello there,

Would you like to know a very general method for handling these op amp circuits?
Once you get the answer to this i would be happy to post if you like.

#### OldSantiago

Joined Oct 10, 2020
3
Hello there,

Would you like to know a very general method for handling these op amp circuits?
Once you get the answer to this i would be happy to post if you like.
Sure, why not?

#### The Electrician

Joined Oct 9, 2007
2,834
Hello there,

Would you like to know a very general method for handling these op amp circuits?
Once you get the answer to this i would be happy to post if you like.
Hello there,

Would you like to know a very general method for handling these op amp circuits?
Once you get the answer to this i would be happy to post if you like.
Hi, MrAl,

Don't wait for the TS to solve his problem. I'm interested in your method, and others are probably also. Start a new thread in the general forum, Use some other circuits as examples.

#### MrAl

Joined Jun 17, 2014
8,248
Hi, MrAl,

Don't wait for the TS to solve his problem. I'm interested in your method, and others are probably also. Start a new thread in the general forum, Use some other circuits as examples.
Yes that is an interesting idea, thanks for the suggestion.
I wonder if it might be better as an article, or better yet, a new thread that might be able to be promoted as an article. I'll look into this.

#### dl324

Joined Mar 30, 2015
12,871
Don't wait for the TS to solve his problem. I'm interested in your method, and others are probably also.
The usual method is to use the zero differential input theorem.

#### WBahn

Joined Mar 31, 2012
26,398
I know that R1 is useless, so does this mean that this is a non-inverting amplifier circuit?
For an ideal opamp (which I'm pretty sure is what you are assuming) R1 has no effect. For real opamps it can serve a purpose, but that's not where you are yet.

So, yes, it is just a classic non-inverting opamp-based amplifier.

But what is important is not that you realize that R1 can be removed, leaving you with a circuit that you happened to have seen before and can come with with a memorized formula for, but that you understand how to apply ideal opamp circuit analysis principles to an opamp-based circuit even if it is completely new to you.