Closed loop amplifer stability question~~

[ljy4468]

https://obrazki.elektroda.pl/24_1185873737.jpg


In above picture, I modeled b=1/2(green line),b=1/100(blue line)
open loop(red line) with matlab. (b=beta=feedback factor)

Then, If I made closed loop amp with b=1/100(blue line),
PhaseMargin is at point A with red line (~90 degree)

Now I've a question,
why we see redline (open loop phase function) instead of blue line(closed loop phase function)??

 

[qutang]

phase margin is @0db.

 

[ljy4468]

thanks qutang!!
but I changed my question.
Could you explain that please??

 

[surianova]

https://obrazki.elektroda.pl/24_1185873737.jpg


In above picture, I modeled b=1/2(green line),b=1/100(blue line)
open loop(red line) with matlab. (b=beta=feedback factor)

Then, If I made closed loop amp with b=1/100(blue line),
PhaseMargin is at point A with red line (~90 degree)

Now I've a question,
why we see redline (open loop phase function) instead of blue line(closed loop phase function)??

hi!

a few thought i can give to you,

1. Phase margin is never measured in closed loop and it must measure in opened loop. You must break the loop to measure phase margin.

GH is the loop gain where G= open loop gain and H= feedback gain.

Loop gain is use to measure phase margin at 0 dB and find out the phase shift.

Why we chose GH or loop gain to measure phase margin?

bcoz in control system for closed loop:

closed loop gain= G/ (1+GH)

if GH = -1 , the close loop gain will go infinity bcoz G/0 which is untable and will oscillate and this is according to Barkhausen's criteria.

that 's why we use GH or loop gain at GH=1 or 0 db to measure the phase shift to determine the phase margin.


2. Closed loop gain is the gain you measure in the feedback loop. The system is in closed loop.

The closed loop gain is much lower than open loop gain (G) and loop gain (GH)

bcoz of formula as below:


closed loop gain= G/ (1+GH)

if G very high, basically the closed loop gain = 1/H

hope u clear about my explaination.

 

[iamxo]

generally, we measure the loop gain phase function, not closed loop or open loop.

and the worst loop gain case is when the feedback factor equals one, so it seems

we measure the open loop phase function.

anyone correct!

 

[surianova]

generally, we measure the loop gain phase function, not closed loop or open loop.

and the worst loop gain case is when the feedback factor equals one, so it seems

we measure the open loop phase function.

anyone correct!

yes, see loop gain GH in gain and phase to find the stability. But the closed loop is the real system. To test the response or transient analysis of the feedback, it is better to do in closed cloop. To check the stability, it must be in open loop, just put a inductor in the feedback loop, you will break the loop.

 

[li202]

Phase margin is improved at frequency not the phase value.

 

[surianova]

Phase margin is improved at frequency not the phase value.

sorry, phase margin is related to frequency and phase.

 

[analog_prodigy]

I have a question: How can the loop breaks when an inductor is placed in feedback?
Please give the explanation.

 

[surianova]

I have a question: How can the loop breaks when an inductor is placed in feedback?
Please give the explanation.

Bcoz inductor block AC signal.. in feedback, the AC singnal is the one that propogate and form the feedback. The inductor block AC but pass DC voltage.. that's why it work like open loop.

 

[analog_prodigy]

Thanks for giving the good explanation about loop gain role in stability. I understood well. even now i got the answer for my above question. inductor does not allow output ac signal to reach at -ve terminal and thus the loop breaks.

Thanks to all for the explanation especially to surianova

 

[didibabawu]

I have a question: How can the loop breaks when an inductor is placed in feedback?
Please give the explanation.

Bcoz inductor block AC signal.. in feedback, the AC singnal is the one that propogate and form the feedback. The inductor block AC but pass DC voltage.. that's why it work like open loop.

Hi surianova,
I also have something confused about use the inductor.Because inductor block AC signal,it also block some of the ac load. So I think the using of the inductor has some problem. Maybe we can add an equivalent load at the point we measure. How do you think of it?

Thank you.

 

[surianova]

Bcoz inductor block AC signal.. in feedback, the AC singnal is the one that propogate and form the feedback. The inductor block AC but pass DC voltage.. that's why it work like open loop.[/quote]

Hi surianova,
I also have something confused about use the inductor.Because inductor block AC signal,it also block some of the ac load. So I think the using of the inductor has some problem. Maybe we can add an equivalent load at the point we measure. How do you think of it?

Thank you.[/quote]

what is the meaning of ac laod? is it the load to the output?

if yes, below is the explaination:

the output is connected to 2 node in parallel, one go to load and the other go nagative terminal of the opamp, the inductor is put between the output and negative terminal. so, it is nothing to do with the output load.

 

[didibabawu]

what is the meaning of ac laod? is it the load to the output?

if yes, below is the explaination:

the output is connected to 2 node in parallel, one go to load and the other go nagative terminal of the opamp, the inductor is put between the output and negative terminal. so, it is nothing to do with the output load.

Hi surianova,

For the left circuit in the left,can i use the circuit in the right to check the stability?
I input signal at a and see the phase margin at a'.

3x.

 

[surianova]

what is the meaning of ac laod? is it the load to the output?

if yes, below is the explaination:

the output is connected to 2 node in parallel, one go to load and the other go nagative terminal of the opamp, the inductor is put between the output and negative terminal. so, it is nothing to do with the output load.

Hi surianova,

For the left circuit in the left,can i use the circuit in the right to check the stability?
I input signal at a and see the phase margin at a'.

3x.

can, no problem.. or u can replace the inductor with Iprobe in Cadence to do Stb analysis..

 

[didibabawu]

can, no problem.. or u can replace the inductor with Iprobe in Cadence to do Stb analysis..

Then when I see the phase margin at a',the inductor block some ofthe load of a', such as Zin, and the parasite capasite at the input of the amp, so I think some equivalent load need to be add at point a'.
And I have another question that how to add the input signal. Can I use an big capacitor like in the pic?

 

[surianova]

can, no problem.. or u can replace the inductor with Iprobe in Cadence to do Stb analysis..

Then when I see the phase margin at a',the inductor block some ofthe load of a', such as Zin, and the parasite capasite at the input of the amp, so I think some equivalent load need to be add at point a'.
And I have another question that how to add the input signal. Can I use an big capacitor like in the pic?

yes,can. You connect voltage source with ac=1 to the capacitor and also put a big resistor to gnd after the inductor.

you can put inductor, cap and resistor as 10G.

 

[didibabawu]

yes,can. You connect voltage source with ac=1 to the capacitor and also put a big resistor to gnd after the inductor.

you can put inductor, cap and resistor as 10G.

and what do you think the load at a'. Is my understand right, I'm always confused.