Open-loop gain, why is inverting vs. non-inverting different?

Thread Starter

MikeA

Joined Jan 20, 2013
206
I'm experimenting with a real (imperfect) LM358P op-amp on a breadboard and configured it in inverting mode to see how it performs with open-loop gain.

While feeding it a 1hz 1.3mV Peak-to-Peak AC sine wave, it saturates to Vcc as much as an LM358P can, and the output almost looks like a square wave. At 1.2mV input, it doesn't output anything.

But when the same op-amp is configured in inverting mode, the same behavior happens at 0.5mV.

From what I understand, a perfect op-amp should behave the same way in both inverting and non-inverting configurations.

So my question is what is causing this difference? Offset voltage?

And why does the op-amp jump from no output to full saturation with nothing in between in open-loop?
 

LvW

Joined Jun 13, 2013
1,056
I'm experimenting with a real (imperfect) LM358P op-amp on a breadboard and configured it in inverting mode to see how it performs with open-loop gain.
..............
Open loop - without any feedback?
And what operating point do you expect?
 

Papabravo

Joined Feb 24, 2006
13,967
I'm experimenting with a real (imperfect) LM358P op-amp on a breadboard and configured it in inverting mode to see how it performs with open-loop gain.

While feeding it a 1hz 1.3mV Peak-to-Peak AC sine wave, it saturates to Vcc as much as an LM358P can, and the output almost looks like a square wave. At 1.2mV input, it doesn't output anything.

But when the same op-amp is configured in inverting mode, the same behavior happens at 0.5mV.

From what I understand, a perfect op-amp should behave the same way in both inverting and non-inverting configurations.

So my question is what is causing this difference? Offset voltage?

And why does the op-amp jump from no output to full saturation with nothing in between in open-loop?
Go back to opamps 101. You can realize any negative gain with the inverting configuration. In the non inverting configuration the gain must be positive and ≥ 1. these results are of course limited by the supply voltages. Sorry no infinite gain blocks.
 

Thread Starter

MikeA

Joined Jan 20, 2013
206
You can realize any negative gain with the inverting configuration. In the non inverting configuration the gain must be positive and ≥ 1.
Right, since I'm in open-loop the gain is most certainly ≥ 1 in both inverting and non-inverting configurations.

What I don't understand is why it takes a higher voltage to saturate in non-inverting vs inverting.
 

BobaMosfet

Joined Jul 1, 2009
1,121
I'm experimenting with a real (imperfect) LM358P op-amp on a breadboard and configured it in inverting mode to see how it performs with open-loop gain.

While feeding it a 1hz 1.3mV Peak-to-Peak AC sine wave, it saturates to Vcc as much as an LM358P can, and the output almost looks like a square wave. At 1.2mV input, it doesn't output anything.

But when the same op-amp is configured in inverting mode, the same behavior happens at 0.5mV.

From what I understand, a perfect op-amp should behave the same way in both inverting and non-inverting configurations.

So my question is what is causing this difference? Offset voltage?

And why does the op-amp jump from no output to full saturation with nothing in between in open-loop?
Welcome to theory .v. reality. As a very basic view to help you, an Opamp is nothing more than a comparator with constraints. the input and feedback loop determine which way the output is thrown. The only basic difference between inverting .v. non-inverting is that inverting is used to push the output in the opposite direction once a threshold is achieve based on the loop. If you were to zoom in on the output you would find it is not a straight line, in the inverting configuration it oscillates at the max frequency the OpAmp can adjust it.
 

Papabravo

Joined Feb 24, 2006
13,967
Right, since I'm in open-loop the gain is most certainly ≥ 1 in both inverting and non-inverting configurations.

What I don't understand is why it takes a higher voltage to saturate in non-inverting vs inverting.
What is the differential input voltage that gives zero output? This value referred to in the datasheet as Vos will tell you why things are unbalanced in one direction or the other. There are circuits for making this determination.
 

OBW0549

Joined Mar 2, 2015
3,457
I'm experimenting with a real (imperfect) LM358P op-amp on a breadboard and configured it in inverting mode to see how it performs with open-loop gain.

While feeding it a 1hz 1.3mV Peak-to-Peak AC sine wave, it saturates to Vcc as much as an LM358P can, and the output almost looks like a square wave. At 1.2mV input, it doesn't output anything.

But when the same op-amp is configured in inverting mode, the same behavior happens at 0.5mV.

From what I understand, a perfect op-amp should behave the same way in both inverting and non-inverting configurations.

So my question is what is causing this difference? Offset voltage?

And why does the op-amp jump from no output to full saturation with nothing in between in open-loop?
However you're trying to measure the open-loop gain, it's wrong. Analog Devices has a good note on techniques for measuring op amp parameters, here. See figures 4 and 5 to see how to measure DC gain and AC gain, respectively.
 
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