What is wrong with my opamp gain?

[Fred556]

Could someone tell me why my opamp gain is like that?


it's suposed to be like this...


thanks in advance, I'm a beginner and I don't know what is the problem...

 

[goldsmith]

Dear Fred
Hi
I think you forgot to attach your schematic , isn't it ? how i can help you without understanding your problem exactly ?
Best Wishes
Goldsmith

 

[Fred556]

Sorry.. I totaly forgot..

schematic opamp:


bench test from book "Baker, 3 editio chapter 24":

 

[goldsmith]

What did you expect from that ?

 

[Fred556]

My line is like in "Zigzag" but I would like only one simple line like the second image in my first post. Do you have any idea why does this "up's and down's"?

I don't know if I explain very well...

thanks for helping me..

 

[goldsmith]

I think a proper , compensation network can solve it .

 

[Fred556]

compensation network? could you give me some examples to search on the internet?

thanks for the help.

 

[goldsmith]

Google these keywords : frequency compensation , or loop stabilization with compensation network or .....

 

[Fred556]

Thanks. I will do it. Thanks again for helping me.

 

[godfreyl]

My line is like in "Zigzag".......
Do you have any idea why does this "up's and down's"?

A real opamp can not do that. It is impossible.
I think there is a problem with the simulator and how you used it.

I think a proper , compensation network can solve it .

Why do you say that? Did you even look at the circuit? It already has compensation! And he is testing it open-loop, so bad compensation can not cause a problem! In any case, it is impossible for bad compensation to cause a result like that.

compensation network?

The 2.5p capacitor in your circuit is for compensation.

 

[goldsmith]

Dear godfreyl
Hi
I think that value isn't enough . i told proper value not improper . i know that is compensation network .
Respect
Goldsmith

 

[godfreyl]

Improper compensation can not cause a zigzag frequency response.

 

[goldsmith]

Dear godfreyl
Hi
I'm not sure about it . let me ask from my professor about it . i'll find the reason and i'll ask about effect of compensation on zigzag frequency response . Thank you for your remark . because i'm not very expert in compensation effect on problems such as this .
( i'll put here the result )
Best Regards
Goldsmith

 

[godfreyl]

It will be nice if you can show him the picture so he can better understand the problem.

 

[goldsmith]

What picture ?

 

[findsriharsha]

I agree with godfreyl, It should be an issue with simulation setting... may be very few sampled points to plot...whats the number of points per decade (in cadence -> ac sim settings) you are using?

 

[godfreyl]

What picture ?

The picture in post 1 that shows the zigzag frequency response.

 

[goldsmith]

I saw that . see the black picture in post 1 . please

 

[wuxy]

I think this kind result is related with simulation precision.
You may increase the cap and res in your test circuit and try again.