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Some gain issues on gain-boosted opa

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jktheone1987

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Hi
I’m designing an opamp with gain-boosted structure,but I encountered an problem.
C582A47A-357A-42FF-BE3B-8B6B35E8F225.jpeg


By some techniques,I made the CMFB ideal(using ideal Vcvs to sense output),so the Cmfb circuit will not affect the differential output impedance. By seriesing a large inductor, the bottom impedance of Z is larger than the upper branch, so I can investigate the upper branch.

But the question arises, when I enhance the gain of gain-boosted amplifier, the total differential gain enhanced,but to a upper limit no matter how much gain I put on gain boosted(all DC conditions and operating points satisfied ). I don’t know why, maybe some friends may help me, thank you.
 
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How your CMFB can control the common mode DC output voltage, if Z is an inductor? In DC the output common mode voltage is 0V, you cannot control that I think. I don't understand how you set the operating points. I think you should use current generators instead of inductor, if the target is to eliminate the load resistance.
 
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frankrose said:
How your CMFB can control the common mode DC output voltage, if Z is an inductor? In DC the output common mode voltage is 0V, you cannot control that I think. I don't understand how you set the operating points. I think you should use current generators instead of inductor, if the target is to eliminate the load resistance.
Click to expand...

I did not plot the detail Z which is a cascode NMOS structure using gain boosted structure just like the upper lanch. Z is an inductor seriesing the cascode structure.
 

Try with ideal current sources, easier. How you check the gain? How is it limited?
DC gain has to be limited I think, obviously becasue the inductor is shorted at lower frrequencies, the NMOS cascode will determine Rout, and at higher frequencies the parasitic capacitors limit the gain of the PMOS portion. Gain-boost is good to increase Rout, but if Rload is lower the gain can't increase, and the gain-bandwidth is limited by Gm and C.
 
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