Dear Team,

I am studying about the stability of opamp circuits using LT-spice with the help of this video. But the explanation is not clear to me.

I have couple of questions regarding this.

1. For checking the gain peaking the loop is not broken.Why Phase margin is not available from that gain peaking plot.
2. For finding the PM the loop is broken and a source is placed with DC=0 and AC=1.Why?
3. what will happen instead of DC=0 if I provide some other value like DC=5V
4. Can I break the loop anywhere
5. Is there any method to test the stability without breaking the loop
6. In both cases (gain peaking and PM) the input source is made as zero,why

Regards
Hari

 

Hi Hari,
Please post the asc file.
E

 

Simulation file

 

Hi Hari,
That is not the asc file for the circuit you posted?
E

 

The circuit I posted (the simulation file)is the one which I need to test the stability.The image what you see in my question is taken from the video file.

 

Two supplementary comments to post #6:
* For a second-order allpole system we can find a direct relation between phase margin and closed-loop peaking (see, for example, Sergio Franco: "Design with Operational Amplifiers Analog Integrated Circuits")
* There is a relatively new method to test stability and derive the corresponding phase margin without breaking the loop.
The principle: Injection of additional phase shift values into the closed loop and analysis of the corresponding closed-loop phase function.

 

Dear Team,

I am studying about the stability of opamp circuits using LT-spice with the help of this video. But the explanation is not clear to me.
I have couple of questions regarding this.

Regards
Hari

Hari - I have answered all 6 questions in "Electrical Engineering" forum (in response to your question).

 

There is a relatively new method to test stability and derive the corresponding phase margin without breaking the loop.

May I know which is that method

 

May I know which is that method

May I give you the link to the paper?
https://www.researchgate.net/public...m9maWxlIiwicG9zaXRpb24iOiJwYWdlQ29udGVudCJ9fQ

 

To break or not to break, different methods have been proposed.
apparently, there are some discrepancies but are close approximations.
The math behind them must be very close.
Those who measured them in the past must have been fairly close.

Overcoming the obstacle of not breaking the loop in LTspice.
Frank Wiedmann - Loop Gain Simulation (google.com)
zip file "loop gain probe"

 

Overcoming the obstacle of not breaking the loop in LTspice.
Frank Wiedmann - Loop Gain Simulation (google.com)
zip file "loop gain probe"

I think, for all the methods as mentioned in Frank Wiedmanns paper the result of the analyses is a plot of the loop gain - right?
That means: The loop has been opened - even if this is not readily apparent in the simulation setup .