hi yef,
Have you read the d/s for LT1128 OPA.?

E

 

Hello Eric,The loop gain simulation looks a lot worse then the closed loop.
both shown below.
What given this is one opamp system what data can i extract from loop gain simulations shown below?
i can see barkausen condition?
Thanks.

 

hi yef,
You are trying to run a LT1028 at 115MHz which has a BW limit of 75MHz, also we cannot be sure of the LT1028 model parameters,

Try using a much higher BW OPA.
E

 

Hello,i have made two types of opamps connected in series as shown below.
In ac sweep its all good, but in step responce i get nothing related to the square shape in the output.
LTSPICE files are attached.
Where did i go wrong in stabilising the situation when connecting these two?
Thanks.

 

hi yef,
Check the BW of the first stage, there is a peak which will cause distortion of the square wave pulse,
E

 

Hello Eric , I got the step response stable, however this is only simulation
what mechanisms you recommend in Layout to control improve stability in case in reality I will have oscillations.
for example there could be a problem where although we dont have feedback for input to output in reality there will be parasitic feedback from GND layer.
how this phenomena could be avoided.
Thanks.

 

Hi yef,
Have you noted the poor Slew Rate of the LT1128?
E

 

Hello Eric,i can go with AD8033 it has a better slew rate response shown below.
in the datasheet i cant see if AD8033 is stable 1 or -1.
the first stage is inverting and the other is non inverting but i am not sure if i put the correct op-amp to the inverting and non inverting configurations.
https://www.analog.com/en/products/ad8033.html

 

Hello, I want to investigate the stability of non inverting configuration, In the closed loop I
got this spike on the end .
In the loop gain simulation i get something very odd .
did i do the loop gain correctly?
how do i see if i can fix the spike on the end using the loop gain?
Thanks .

 

hi yef,
The open loop plot loks OK to me.
Look at this video and you will see your plot result compares OK.
E

 

For stability we need gain crossover (0dB) frequency be smaller then phase crossover frequency.
but in one opamp we will have only one pole.
two opamps is two poles thus we will have -180 phase shift and the ability to be unstable.
I have a system shown below. its two opamps system.
each opamp has its own loop.
How do i "open the loop " for such situation to see the loop gain and see the situation phase and gain margin with stability?
Thanks.

 

Hi yef,
It helps if you post your asc file when you ask a question.
E

 

Hello Eric in the attached file i have the system shown below.
each opamp has its own kind of loop but no single loop going from input to output.
So how can i do the LOOP GAIN trick being done in the video of opening the loop if there is no big loop betwenn them.
However there are 3 opamps and 3 poles minimal which could cause instability.
how do I investigate phase gain margin for such situation?
Thanks.

 

two opamps is two poles thus we will have -180 phase shift and the ability to be unstable.

Only if there is feedback from the output of the second op amp to the input of the first, which you don't have.

 

hi yef,
Not very elegant but my best attempt so far.
E

 

Hello , so in order for the shown system to be without oscillations I need to make each individual stage stable ?
Correct ?
Thanks .

 

Hello Eric, few question regarding the simulation:
1. could you please share the LTSPICE file?
2.What did you do to the input voltage source
3.Why the loop gain of the whole system is sum of loop gains? Va/Vb+Va1/Vb1+Va2/Vb2

 

Hi yef,
LTS asc file attached.
I did not change the voltage, it is as you set it. .ac dec 1000 1 1e8

The sum of the loop gains is just a very rough approximation as a demo.

As pointed out, you have no overall feedback path from the final output to the first input.

E

 

the loop gain of the whole system

There is no whole system loop gain since there is no whole system feedback loop.