RSA
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Hi, I designed a 2 stage Miller compensated OTA for a GBW of 10MHz and PM of 63 degrees (worst case) in Cadence Spectre. In order to measure the GBW and PM, I performed an AC analysis and got them to be right. Now, I created a symbol for the OTA, and enclosed it in a unity feedback loop. And I tried to measure the stability by using cmdmprobe in spectre (I am trying the stb analysis for the first time). And when i tried to plot the loop gain from the stb analysis, i get a loop gain only 30dB against 60dB I got from the earlier measurement. The GBW also is shown to be some 3MHz and i get 88 degree phase margin. Is this discrepancy to be expected or have I done something wrong? I have attached a png file depicting the way I set the simulation for stb analysis. Please help me out.. (In the setup I have uploaded I missed to include the load capacitance which i have in my schematic)
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