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You forgot to mention the context of your question, I guess it's compensation of multistage feedback amplifiers.
To decide about compensation for a particular amplifier structure, you'll want to analyze the uncompensated frequency response and determine feedback stability.
You forgot to mention the context of your question, I guess it's compensation of multistage feedback amplifiers.
To decide about compensation for a particular amplifier structure, you'll want to analyze the uncompensated frequency response and determine feedback stability.
it depends on the poles of the system. if output pole and interstage pole are both dominant you will have bad phase margin, and you will need compensation. even if your output stage is source follower with output impedance of 1/gm, if Cout is big enough it can still be dominant pole". since the gain of second stage is 1, a miller cap won't give you extra A*Cc boost on cap at the interstage node, so you can simply stack a cap to supply at interstage node to stablize the system.
i forgot to mention that even though you won't get the A*Cc cap boost at interstage, if your output stage is some sort of common source, the comp cap still pushes your output pole to gm/Cout, helping your PM. just pay attention to the zero introduced by the cap
The phase margin as a measurement of stability for the op-amp when it connected in a resistive f.b circuit, it define you how much you are far from the oscillation which make the circuit unstable, the more you far the more stability. if we know that the oscillation condition phase is 180 then it mean how much you are from 180.
we usually measure this difference at the unity gain frequency point which represent the worst case, it is comparable for the buffer connection to assure its stability if we connect the op-amp like a buffer.
the value of the phase margin should be >= 45, optimally 60, in more also
you have to trade between the stability and the settling time, higher stable doesnt mean faster settling time, so from the range from 45 to 90 you have the optimal value of 60 which give you both good stability and fast response. at 45 give you the minimum stability and slower settling time, at P.M > 60 give you better stability but slower settling time.
see the attachement
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