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Help me! How to solve the phase margin problem for this op-amp

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2_1287749245.jpg

The picture shows the circuit.

Now I have a problem is that the phase margin is not enough, around 45 degree, the dc gain is 110dB, I think decrease the gain can increase the phase margin as well as the bandwidth. But I don't know how to keep the GBW unchanged and decrease the gain, increase the bandwidth at the same time. I try to increase the cap value for compensation before to increase the phase margin, but it does not work, if the cap value increases a lot, the bandwidth will sharply decrease. Please help me to solve this problem. Thanks!
 

You have to identify the additional poles in the circuit. If the topology and technology shouldn't be changed, increasing bias currents can possibly help to move the poles to higher frequencies.
 

Hello
If you had extra gain - you can remove the second stage consisting of M8-M9, this also eliminates poles second stage cre4ates.
Regards.
 
You are using 3 stages. The source follower gives you current gain.. Why don't you try then increasing the current capacity of the second stage rather than adding another one ? You can stick with you A-class output or try a B class. Also, You didn't compensate the right half plane zero. You need to put a resistor in series with the capacitor which resistance is equal to the equivalent output impedance of the second stage (kOhms). 110dB is a very good gain, which is the gain of the first stage ? If enough, you can decrease the gain of the second stage increasing gds8,9 and thus moving that pole to higher frequencies. Remember that Cc is around 0.1 CL, and you didn't give any info regarding your load.
 
hi, you said increase the gds8,9 how to increase it?
 

you can add a resistor to move the right half plane zero to left, it can counteract the nondominate pole, so the phase margin can be increased
you can read Phillip E.Allen 's "cmos analog circuit design ". it described this question in detail
 
actually I had already add a resistor in series with Cc, but it did't improve the PM so much, only a few degree. I find that the PM between second stage and first stage is enough, but after the second stage and before the source follower, it is not enough. The CL is around 33f. Do I need to add a Cc between the second stage and source follower? If so, it will affect the BW so much and how to solve the problem?
I am sorry that I can not remove the second stage, I don't know why, maybe for a large swing. This circuit is from a paper published by MIT. It is used for EEG signal acquisition
 

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