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

PSRR is better if bulk is connected to source.