# Open loop closed loop logic of calculating OPAMP stability

#### LvW

Joined Jun 13, 2013
1,766
Hello LvW,i want to create a picture as close as possible to reality.I dont know how in the lab to know the parasitic capacitances i have in my OPAMP pcb.
Could you suggest a way to estimate parasitic capacitances?
So you really expect someone else to be able to assess your mechanical setup (opamp pcb) in terms of parasitic properties?

#### yef smith

Joined Aug 2, 2020
810
Hello LvW, sorry for the misunderstanding.
I will have a close loop responce of my PCB on scope.
i will have an overshoot which could help me estimate the phase margin.
what could i do with lab tools to inverstiget the loop gain? This is the heart of stability issue.

#### yef smith

Joined Aug 2, 2020
810
There is also some debate about the efficacy of this method close to the 0 dB crossover. Your question has piqued my interest in looking further.
ETA: It appears that there are multiple methods which go by the names Tian, Middlebrook, and Big Inductor. The Tian and Middlebrook methods are illustrated in the Examples folder of LTspiceXVII as LoopGain.asc and LoopGain2.asc. The big inductor method was described (I think) in Basso, Christophe, Switch Mode Power Supplies, 2014

Here is a thread that discusses the differences.
Calculating open loop gain using LTSpice | diyAudio

I'm not sure if those examples were in the LTspiceIV distribution.
Hello Papabravo,We are intresting in loop gain for stability.
Why the thread says open loop.Its not the same thing.

#### tonyStewart

Joined May 8, 2012
132
C depends on ratio of area to gap or on a wire it is width/gap per unit length. Naturally Dk increases this on the insulation.
So coplanar wires might be something like 1pF/cm if w/g=10 but on FR4 if w/g=0.1 with Dk=4.6 it is more 1 pF/ inch
Saturn Tools are free.

#### Papabravo

Joined Feb 24, 2006
21,261
Hello Papabravo,We are intresting in loop gain for stability.
Why the thread says open loop.Its not the same thing.
I'm not sure where in the thread I quoted it says that. Can you help me out here?

#### LvW

Joined Jun 13, 2013
1,766
Hello Papabravo,We are intresting in loop gain for stability.
Why the thread says open loop.Its not the same thing.
I must admit that I have problems to understand what your main problem is.

You are intersted in "loop gain for stability".
What does this mean?
Do you want to learn how to apply the stability theorem ?
What do you mean with "it`s not the same thing"?
The same thing as .... ?
Do you want us to explain the difference between open and closed loop?

I think - in your own interest - it would be helpful if you would spend some more words to explain and describe your problems.