Hi,
I don't believe that there is a magic number for gain margin (GM) in any textbook which tells you how much GM you need to get a certain kind of settling pattern for a second order system. However, you may try to derive that and call this GM_min.
That being said, you need the same sort of rules for defining GM as for PM, i.e. GM should be large enough that despite all expected variations (process, supply voltage, temperature, signal level etc.), it is still sufficient to achieve GM_min.
Regards,
Vivek
raja.cedt wrote on Jun 7th, 2009, 1:36pm:hi all,
thanks for your reply,lastly i want to know what is the reasonable value of gain margin like 60 deg phase margin is good and above is good from stability point of view but slow.
As Sheldon said phase margin and gain margin are same indications means if PM is high means GM is also good but i think other way around is wrong because even though you have less PM there may be some poles beyond UGB which can give high GM, so now oscillation condition for magnitude is not satisfied and it may not oscillate.
thanks,
Rajasekhar.