substrate connection of cascoding device

[Julian18]

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.

 

[Hughes]

I would connect the bulk terminal to GND because the layout size would be smaller and performance penalty is minor.

 

[kola]

I think it depend on you design. If you make voltage divisor - connect well of each MOSFET to source, if you make simple resistor (transistor) - connect well to GND (one well) to reduce area...

 

[spring1860]

i always connect to GND for further nondominant pole.
plz correct me if i'm wrong.

 

[yxo]

If you connect substrate to ground, it will cause degradation of gain because impact ionization effect will apear. All depend on what you want. Small size or high linearity.

 

[rfsystem]

Triple WELL have free WELL connection for both NMOS and PMOS:

Pro:

noise isolation
lower Vth (no substrate lift)

Cons:

additional WELL to substrate cap

I think in questions is the bulk connection of the second level cascode devices. I found that the best connection is to use isolated NMOS and PMOS also in the bias definition and connect the bulks of the active cascode devices to the bulk-source connection of the bias network.

Then you get isolation, lower Vth and low capacitance at the same time.

 

[lovexxnu]

it depends on the application.
sometimes it is more convenient to connect to gnd for layout.