loop gain calcaluation question

[yefj]

Hello , bellow there is open and closed loop transfer function plots.
given these two how can i get the loop gain diagram?
Thanks.

 

[LvW]

Thats not the best resp. direct way for the shown circuit.
That means: You should simulate the loop gain separately.

Explanation:
Closed-loop gain is Acl=Hfd*Ao/(1-Hfb*Ao)
with loop gain LG=-Hfb*Ao
(Hfd: Forward damping; Hfb: Feedback factor)

Solving for LG gives: LG=(Hfd*Ao/Acl) - 1

That means: You would have to simulate Hfd separately.
Hence, the most simple way is to open the loop at a suitable node (e.g. opamp output) and find the loop gain response directly (using one of the known methods).

 

[yefj]

Hello LvW , could you recommend a method , so I will read about it and try to implement it on my case ?
Thanks.

 

[FvM]

E.g. shown in this post https://www.edaboard.com/threads/op...ich-didnt-show-on-ltspice.409576/post-1766396

 

[LvW]

Hello LvW , could you recommend a method , so I will read about it and try to implement it on my case ?
Thanks.

Set V3=0, open the loop at Vout and place an ac source between Vout and the input Vin of the feedback path.
Thats all. Perform an ac analysis and display loop gain LG=Vout/Vin (Magnitude and phase).

 

[FvM]

It's simplified Middlebrook method, applicable if OP.Zout >> Zfeedback. If Zout can't be neglected, voltage injection can be also performed at inverting OP input.

 

[LvW]

It's simplified Middlebrook method, applicable if OP.Zout >> Zfeedback.

Typing error?
Rather, we require Zout<<Zfeedback for simplified Middlebrook method.

 

[FvM]

You are right of course.

 

[stenzer]

Hi,

that might help https://www.analog.com/en/resources/media-center/videos/5579254320001.html

BR