polyam
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Ken,
I attached all screenshots of my op-amp in a single ZIP file.
"3Stage_opamp.png" shows a 3-stage no capacitor feedforward compensation op-amp. gm1, gm2, and gm3 all together make a 3rd order path. gmp2 is located in a 2nd order path and gmp3 is located in a first order path. It shows the topology of the designed op-amp without placing it in any feedback configuration (The transistor level schematics of all above gms are attached).
I also have three CMFBs (please see CMFB.png):
CMFB1 controls the CM level of gm1.
CMFB2 controls the CM level of gm2 and gmp2. (I also attached the schematic of gm2 and gmp2).
CMFB3 controls the CM level of gm3 and gmp3. (Please see the schematic of the gm3 and gmp3 and how they are related together)
Let me review some of my observations:
1- I performed an ac analysis on my op-amp. Please see "ac_response.png". It shows 82 dB DC-gain, 1.8GHz UGBW, and 65-degree phase margin with 3pF load capacitor.
2- I also applied a sin wave to the input of the op-amp. Input amplitude=100uV, input frequency=4MHz.
"in_tran.png" shows the input differential signal.
"gm1_tran.png" shows the tran response at the output of gm1.
"gm2_tran.png" shows the tran response at the output of gm2 (and also gmp2).
"gm3_tran.png" shows the tran response at the output of gm3 (and also gmp3)
The tran response looks logical until 1us and then it seems that the op-amp gets unstable. ( I think there is something wrong with my CMFBs).
Now, I have the following questions:
1- Is what I am guessing correct? CMFBs are not working properly?
2- I want to check the stability of the CMFB (to see the phase margin) no matter what my previous question is right or wrong. To do so, obviously, I have to put the op-amp in a feedback test bench (TB). Could active RC-integrator be a proper test bench since I am going to use this op amp in a CT delta sigma modulator with active RC integrators?
2-1 I think I have to use stb analysis to examine the stability of my CMFBs. So, I have to use three diffstbprobes to break the CM loops while op-amp is placed in a TB asked in question 2. Please put me right if I am wrong.
Many Thanks
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