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# How to measure the open-loop voltage gain of a OP??

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#### ZengLei

##### Full Member level 1
open loop voltage gain

I want to measure the open-loop voltage gain of OP37 with Pspice....

I found that the open-loop voltage gain of OP37 is about 50db,the result is absolutely wrong!!!!

But i don't know where i made mistake???

So,how to measure the open-loop gain of a op??
How to get the curve of Open-Loop VS Frequency correct with Pspice??
Thanks~~~~~~~

measuring open loop gain

the way you did is right. why do you think the answer is wrong?

pspice open loop gain

i'm wondering why your opamp hasnt gone to saturation with 1V at its input in open loop condition.... please post the output waveform....

measure open loop gain

Perhaps you should add a common mode voltage at the input of the OPAMP, and bias the OP at the right condition.

Ryan

closed loop voltage gain

A.Anand Srinivasan said:
i'm wondering why your opamp hasnt gone to saturation with 1V at its input in open loop condition.... please post the output waveform....

in AC analysis one usually applies 1V in input so that one can read the gain directly from the plot. the 1V will not affect any bias points. infact you can verify that through simulation.

open loop gain pspice

To amriths04:
Did u see the pic3,the open-loop gain of op37 is about 50db ,it's too small,so i think the result is wring~~~

To A.Anand Srinivasan:
I use the AC sweep,~~it will not saturate.

To RDRyan
Thanks for ur advice,i did according ur advice but the result is all the same~~~

measuring open loop gain

It looks like Pspice has some preoblems calculating db magnitude... I simulated this circuit myself and got same results in PSpice, but when I put AC=10 db magnitude rise for 20db, then I put AC=100 magnitude again raised for 20db...
Then I simulated it in Capture(should be the same as PSice) and got these results:

Adb≈125db

Your results are very wrong, Adb=50db, and your bandwidth is 100kHz, it is to big for open loop...

how to measure voltage gain

the_edge:

How to get the right result in Pspice??
Is it a bug of Pspice???

measure op amp open loop gain

I suggest U may make dc sweep for a dc source then differentiate the output with respect to the input

open loop gain op amp measure

Hi Ghitani:

If i make DC sweep ,i couldn't get the frequency response of the open-loop gain.

closed loop gain measurement

Why do U suspect the value of 50 dB ? If u see that it is not large enough , the OP-amp needs just enough gain to acheive the desired closed loop gain i.e. If this OP-amp is used as a buffer it just need an open loop gain of 20 dB !!!

To check that the open loop gain is correct or not , U can make a simple inverting amplifier with a closed loop gain of 40 dB then if the simulation results gives your expected gain ( 40 dB ) then the open loop gain is higher than 50 dB . If the simulation results gives gain less than expected then your open loop gain is correct .

measure op amp open-loop gain

Hi Ghitani:

Did u read the datasheet of OP37,if u did
u will found that the open-loop gain of op37 at very low frequency is about 130db,that's why i doubt the result of Pspice~~~~

how to measure the gain of a loop in spice

The try what I've suggested in my last post . There may be error in the model of the op-amp .......

open loop gain orcad

Hi Ghitani:

U think the model of OP37 is wrong in Pspice??

calculate open loop gain

Have u tried to obtain a closed loop gain of 50 dB ? If not then try !!!

### ZengLei

Points: 2
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op37 20db

Hi Ghitani:

I make a closed-loop ciruit
I can get the voltage gain >50db!!!!
It prove that the model of op37 can't works well in open-loop~~~

But i have a question:Why the Corporation who made the software did not revise the mistake??
Or maybe i should update the model of op37 ~~

Thanks!

opamp pspice simulation open loop gain

Hi, I think I got the answer... I done some model editing and I found what is responsible for our doubt... From your schematic I saw that you are using OP37G, not OP37, and I got that ≈50dB gain. Than I started comparing these two models, I will upload them... And I saw some small diferences. Than I started changing one by one difference, I take value from OP37 and put it in OP37G. Soon I got the pretty intresting result.
In this line from OP37: EOS 7 3 POLY(1) 30 43 10E-6 1
and this from OP37G : EOS 7 3 POLY(1) 30 43 100E-6 1

Than I typed in value from OP37 and got this results:

And it looks just as we think it should.
Now I dont think that it is their error, I think this is actual characteristic of OP37G

If you need help editing models just ask....

### ZengLei

Points: 2
Helpful Answer Positive Rating
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