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https://designers-guide.org/forum/YaBB.pl Design >> Analog Design >> Body Effect https://designers-guide.org/forum/YaBB.pl?num=1131605603 Message started by vijay on Nov 9th, 2005, 10:53pm |
Title: Body Effect Post by vijay on Nov 9th, 2005, 10:53pm Hi, I simulated a single NMOS in saturation.The saturation current is 2.13mA.For the same setup I introduced a volatage at the bulkand source at gnd in my NMOS.Ideally the current should decrease as Vth increases.But the simulation results speak:- 1.When there was no body effect Id = 2.13mA 2.The current increases to 4mA max, when the bulk volatge is = 700mV and after that it started decreasing.FInally at Vbulk=3V the current reduced to a very low value. What has caused that sudden increase in the saturation current?Is it that that Isubstrate is not considered in normal mos current equation? Why my device follwed the theory of Id-decrease with Vbulk after 700mV and not immediately. Thanks in advance. V |
Title: Re: Body Effect Post by uncle_ezra on Nov 11th, 2005, 12:14am I believe wat u r seeing is usage of bulk-driven MOS. Let me summarise. Normally NMOS bulk is connected to the most negative supply in order to avoid forward-biasing the PN-junction.But it is known that slightly foward-biasing Vbs decreases Vth, hence u see the increase the current. However, if Vbs > Vdiode ~ 0.7V you end up forward biasing the PN junction. For more information about bulk-driven MOS go to http://amesp02.tamu.edu/~sanchez/607-bulkdriven-2005.pdf Hope this helps ! Let me know if you need a clearer explanation |
Title: Re: Body Effect Post by vijay on Nov 13th, 2005, 9:20pm Thanks, The link is of good help.My actual doubt for the setup was i needed an explanation for the following setup:- the set up is :- single stage amplifier is before a differential amplifier. The single stage amplifier (CS) has its source and bulk connected to ground to have no body bias effect. The next stage differential amplifier would in that case have its bulk already connected to ground,as in a n-well process all nmos share the same substrate. My question is in that case there would be a body bias effect in the DA?If the source and bulk in the DA is connected together and then connected to a tail current source. If the single stage CS has its bulk grounded then in fabrication the bulk of the DA would also be at ground. So what is the preferred configuration:- 1.Source and bulk connected together to a tail current source? 2.Since the bulk would in the process of fabrication be connected to ground we consider the bulk at ground and then simulate? Thanks in advance. V |
Title: Re: Body Effect Post by uncle_ezra on Nov 13th, 2005, 10:54pm My usual practice is to connect NMOS bulk to gnd unless I'm using a triple well process. So even though in a DA the current source may be at AC gnd I dont connect the source and bulk together due to process, temperate and voltage variation. |
Title: Re: Body Effect Post by vijay on Nov 14th, 2005, 1:01am Thank you very much. Regrads, V. |
Title: Re: Body Effect Post by ice_wind on Nov 24th, 2005, 6:58pm uncle_ezra wrote on Nov 13th, 2005, 10:54pm:
That is right. By the way, if connect the source and bulk together, then a cap will exist between this node and ground, for high frequency it is not a good news. |
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