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Questions about bulk driven OTA design

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leonken

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Does anybody use such bulk driven OTA? How large is the DC gain?
And how to improve the DC gain of such OTA?
Does the operating state of M2, M3, M4 and M5 must work in saturature region?
thanks
 

hoonsk

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I think M2 and M3 should still be in saturation region because it is just a current source. I never implement such circuit on silicon before but just do some homework assignments. The main adv of bulk is that the gm is lower and we can manupulate the threshold voltage of transistor to make it low voltage. The rest should be about the same as normal opamp. Thus, I think M4 &5 should be in saturation region as well. With lower gm, the linear region would be wider. To increase DC gain, I think the standard technique should apply (suppose we are fixed at 2nd stage) : incr channel length for M4&5, reduce or increase bias current depends on how small the gm becomes.
 

leonken

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hoonsk said:
I think M2 and M3 should still be in saturation region because it is just a current source. I never implement such circuit on silicon before but just do some homework assignments. The main adv of bulk is that the gm is lower and we can manupulate the threshold voltage of transistor to make it low voltage. The rest should be about the same as normal opamp. Thus, I think M4 &5 should be in saturation region as well. With lower gm, the linear region would be wider. To increase DC gain, I think the standard technique should apply (suppose we are fixed at 2nd stage) : incr channel length for M4&5, reduce or increase bias current depends on how small the gm becomes.
Thank you for your advices.
And I wonder that how to avoid turnning on the forward biased pn junction of M4 and M5?
And some detailed documents about such bulk driven amplifers?
thanks
 

willyboy19

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It is definitely a potential danger to use this bulk-driven circuit, due to the possibility of forward-biasing of pn junction of M4/M5. I would assume to use this circuit, the input common-mode will be close to the supply to avoid this problem. It is definitely not a common-practice to use this type of amplifier due to its limited advantage and all the potential risks.
 

fendy

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I still want to know where is the CKT advantage ?
 

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