Guardrings

Generally for RF circuits such as LNA's mixers etc, is it recommended to use guard rings around individual transistors used in these circuits vs. guardrings around the whole circuit?

Guard rings can be put around the whole circuit to protect from substrate noise and to connect the GND line to the substrate.When the substrate is polarized with GND, it will act as a RF wall between disturbing elements and the guarded circuit.

Hi Bigboss,

Thanks for your input. I was looking at the pdk kit from xfab and they have all their RF primitive devices (pmos, nmos, varactors etc) with a guard ring around them (vs. putting a guardring around the whole circuit). I am seeing big differences between pre and post extraction simulation and I suspect this is because of the parasitics introduced by the guardrings - do you have any thoughts on this?

Best regards..

keep this guard ring far enough that minimizes the para-cities.

it depends on the space available to you. generally, the more guardring you have, the better it is shielded from noise. if matching is critical, place a guarding for each transistor so that the environment is the same for all