The Designer's Guide Community Forum
https://designers-guide.org/forum/YaBB.pl Design >> Analog Design >> substrate connection of cascoding device https://designers-guide.org/forum/YaBB.pl?num=1206970536 Message started by Julian18 on Mar 31st, 2008, 6:35am |
Title: substrate connection of cascoding device Post by Julian18 on Mar 31st, 2008, 6:35am Hi, there Seems this is too trivial, but ,I just want to know if you have triple-well process and free to connect the substrate to source, what would you do to connect the substrate of cascoding device, to the source or to the GND? what is the pros and cons if I just connect the substrate to the source. Thanks. |
Title: Re: substrate connection of cascoding device Post by vivkr on Mar 31st, 2008, 7:07am Hi Julian, Excellent question. But you are talking of bulk node and not substrate. There is only 1 substrate on your chip. Connecting bulk-source together PROS: 1. More signal swing available. 2. Impact ionization reduced. This might be critical if you are using a process with L <1 um and trying to achieve very high output imepdance through the cascode, particularly if you are gain boosting the cascodes. The additional current due to impact ionization will flow out of the bulk terminal, acting as an additional impedance in parallel to your cascode output impedance. Shorting source-bulk eliminates the problem. Note that impact ionization is almost never modelled in most cases. A simple test is to sweep VDS for a large VGS value and look at the bulk current. If you see zero current or only a very small fixed current regardless of the drain potential, then you know you are up for it. CONS: 1. Slightly reduced gain. In a cascode, the body effect increases output impedance (one of the few +ve benefits of the body effect). 2. More parasitics at the source node, specifically those related to the well in which the cascode device now sits. This can be important. In any case, do not forget to properly bias the outer well with a clean supply. Regards Vivek |
Title: Re: substrate connection of cascoding device Post by solidstate on Apr 1st, 2008, 1:01pm If you have more than one cascode (e.g. in a differential architecture), you might also consider that matching decreases if you connect bulk and source. Well-to-well spacing is much larger than device-to-device spacing, so more area is required and the wells will be at slightly different potential. |
Title: Re: substrate connection of cascoding device Post by gsheng on Apr 2nd, 2008, 12:14am PSRR is better if bulk is connected to source. |
Title: Re: substrate connection of cascoding device Post by loose-electron on Apr 17th, 2008, 3:42pm Vivek had some very good points - with one other issue -- all of these seperate wells will grow the size quickly in the layout stages due to wide seperation of wells needed. That becomes both an area penalty and often a BW penalty due to the extra metal parasitics. |
The Designer's Guide Community Forum » Powered by YaBB 2.2.2! YaBB © 2000-2008. All Rights Reserved. |