So, for some input circuit I'd like to use 1.8v transistor (better characteristics, higher freq, etc ...) but I'd still like the input to be resilient to have 3.3v applied to it. So I was thinking of putting some protection circuit to isolate the transistor gate if the voltage is too high. Something like : Is that something commonly done or is that a crazy idea that won't work ? 😅

Protection transistors are often used in level shifters. See this circuit here: https://electronics.stackexchange.com/questions/237133/level-shifting-in-hv-cmos-processes

Yeah, I saw that arrangement when I was looking at level shifter, but not exactly the same setup.

@tnt Is VAA is set at around 2+Vth(nfet_g5v0d10v5? I’ve seen voltage protection circuits like this without the M1 mos diode. I guess there’s enough of a bias leak in M4 to prevent vout from raising to above VAA.

@Mitch Bailey VAA is the 3.3V rail so yeah, it's around there.

it looks pretty good as long as you can tolerate the process and temperature variations... it would be interesting to resimulate that curve with those variations...