Gain Margin significance

[nannapaneni]

I had doubt regarding Gain Margin.
I know importance of Phase and Gain Margin those values make sure loop stable.
To have transient response smooth without ringing, we require phase margin of 60degrees.
What value of gain margin is required for smooth transient response/
I saw many literature that min. of 10db gain margin is required, why it is so?
what will happen if my gain margin is 3db.

 

[crutschow]

If you have a gain margin of only 3dB then the circuit is close to instability and will likely exhibit overshoot/ringing in it's transient response. It's related to the phase margin and, in a well mannered system, you normally will have a good phase margin when you have a good gain margin.

 

[nannapaneni]

What value of gain margin is required for smooth transient response? why?
there may be systems with good PM as 60degress, and bad GM as 3dB(because of RHZ after UGB.

 

[FvM]

there may be systems with good PM as 60degress, and bad GM as 3dB(because of RHZ after UGB.

The well-known design rules like 60 degree phase margin for no overshoot in transient response are valid for low-pass loop gain characteristic with a dominant pole. For more complex transfer functions with arbitrary magnitude and phase response, there's no simple rule of thumb. You better analyze the close loop response in frequency and time domain.

A gain margin criterion like 10 dB might work as a first estimation.

 

[LvW]

If you have a gain margin of only 3dB then the circuit is close to instability and will likely exhibit overshoot/ringing in it's transient response. It's related to the phase margin and, in a well mannered system, you normally will have a good phase margin when you have a good gain margin.

Hi Crutschow, have you any formula for the relationship between gain and phase margin?
I think, in general there is NO relationship between both margins. You can have a step response without any overshoot /ringing for a gain margin of only 3 dB.
The system response depends on pole and zero locations only.
The purpose of a sufficient gain margin is to have enough "room" for uncertainties and/or changes of the loop gain magnitude.