How this peaking can cause the stability of the system?

[surianova]

hi!

I have a question about the stability. Refer the attachment, the graph have the GBW at ard 103 MHz which will give 50 degree phase margin. But the problem is, if u refer to the loop gain graph, there is zero after GBW ( right of A), is it can cause stability to this system?

Thanks for ur reply and answer..

 

[safwatonline]

stability

this looks really strange could u post the schematic

 

[jerryzhao]

Re: stability

After check the phase margin.
U should check the gain when the phase margin is 0 degree.
If the loop gain ≤-10db will be ok.
But you result too strange. Why u loop gain ≥1 in high frequence. That maybe not stable I think.

 

[sixth]

Re: stability

Yes, this kind of gain peak in frequency response is very dangerous and can cause stability problem. Usually, the amplifier like folded cascode with cascade compensation and amplifier with gain booster has the gain peak in frequency response. It must be eliminated.
sixth

 

[surianova]

Re: stability

Yes, this kind of gain peak in frequency response is very dangerous and can cause stability problem. Usually, the amplifier like folded cascode with cascade compensation and amplifier with gain booster has the gain peak in frequency response. It must be eliminated.
sixth

ya, i am using folded cascode for the amplifier. How to eliminate this peaking or zero?

 

[sixth]

Re: stability

ya, i am using folded cascode for the amplifier. How to eliminate this peaking or zero?

It is very difficult to eliminate the peaking. Fow low frequency application, you can change to miller compensation tu push the peaking far beyond the GBW. Fow high frequency application, it is profer to choose PMOS as differential pairs and NMOS as output device. Since the cascode device is NMOS transistor which has larger gm than PMOS ones, it maybe helpful for reduce the peaking in frequency response.
The following attachment will help you. Good luck!
sixth

 

[surianova]

Re: stability

ya, i am using folded cascode for the amplifier. How to eliminate this peaking or zero?

It is very difficult to eliminate the peaking. Fow low frequency application, you can change to miller compensation tu push the peaking far beyond the GBW. Fow high frequency application, it is profer to choose PMOS as differential pairs and NMOS as output device. Since the cascode device is NMOS transistor which has larger gm than PMOS ones, it maybe helpful for reduce the peaking in frequency response.
The following attachment will help you. Good luck!
sixth

Tx. Actually how this peaking can cause the stability of the system?

Or it will cause ringing to the transient signal or osillation?

 

[sixth]

Re: stability

Tx. Actually how this peaking can cause the stability of the system?

Or it will cause ringing to the transient signal or osillation?

It can cause transietn ringing and if at some certain condition, it will oscillate.

sixth

 

[jerryzhao]

Re: stability

I think If gain≥1, when phase≥180. maybe oscillate.So it is dangerous. But I don't know why the circiut have the gain peak in frequency response.

 

[paladinzlp]

stability

hai
the circuit have gain peak because of the Q of transfer function of circuit is too high.You must
reduce the Q if you want to eliminate the gain
peak. Of course, At first you must calculate the
transfer function.

Added after 2 minutes:

hai surianova
Can you post the schematic of circuit.

 

[surianova]

Re: stability

hai
the circuit have gain peak because of the Q of transfer function of circuit is too high.You must
reduce the Q if you want to eliminate the gain
peak. Of course, At first you must calculate the
transfer function.

Added after 2 minutes:

hai surianova
Can you post the schematic of circuit.

Actually the feedback loop is very big and quite hard for me to send the schematic. But i use folded cascode as a feedback opamp.

 

[paladinzlp]

stability

hai surianova
Can you tell me what value is the Q of transfer function. Or , Can you post the main transfer function?

 

[surianova]

Re: stability

hai surianova
Can you tell me what value is the Q of transfer function. Or , Can you post the main transfer function?

How to relate Q to pole and zero in transfer function?