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folded cascode opamp tail mos

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ljy4468

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hi!
i want to ask more about folded cascode opamp

If driver in nmos input pair,
there is tail mos under driver input pair

but i've seen cascode tail mos

what's the difference between that?
opamp applied by cascode tail mos produces more Av.
what's the drawback?

thank you for reading my text
have a good day!
good luck to you
 

The immediately effect will improve the CMRR and will reduce the common-mode range.
 

Hi.
I do agree with nxing. cascode tail will degarde the Input CM boundary. Because there is another (at least) Veff between input and negative rail (for NMOS input pair) so the minimum allowable input CM level should be larger in comparison with a single NMOS tail. But if there is no limitation on min voltage supply, this kind of tail (cascode tail) will increase CMRR. Because this cascode current source have a larger Rout and hence a smaller Ac (CM voltage gain), so CMRR will be improved. But I'm not sure about improving Av (differential voltage gain)!!!

Regards,
EZT
 

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    ljy4468

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While it will increase CMRR,which is mandatory, the cascoding of the tail current source has nothing to do with the Av part of it. If you look into the differential half circuit, you will notice that the tail current source will never come into that part.

Moreover,in your case, the OPamp might need a very high PSRR- as well. Hence they will have a cascoded tail current source. Also, the next stage could also be cascoded. Hence, they will try to cascode the tail current source.

The biggest loss will be a loss of an extra ΔV(in case of high swing cascode). So, this is not definately recommended in low voltage designs

I hope that I am clear because I was mostly summarizing the earlier two posts.
 

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    ljy4468

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