hi,
many times people prefer transistor in separate well but mainly to isolate from the substrate noise due high digital stuff and this is so called deep N-well process.

Thanks,

ontheverge wrote on Sep 28^th, 2010, 2:31pm:


Depending on your process technology, this option may actually be very attractive in some cases, and sometimes even necessary. There are many reasons for tying bulk-source, some of which are:

1. Reduced body effect
2. As a result of 1, you may have better headroom and mismatch
3. Avoiding overvoltage condition in charge-pumps (voltage boosters).
4. Higher output impedance in cascoded mirrors or gain-boosted amps.

The most common situation where this is needed and usually supported by the foundry is for high-voltage design. Eliminating body effect may be desirable for a designer but body effect by itself is seldom a killer. Overvoltage on the other hand is a killer.

Vivek