Sharp transistor cut off outside active region

Question

I'm building a simple temperature controlled NMOS switch for regulating the speed of a fan.

It's a 12V fan and the controller works great (as long as the transistor is in it's active region.)

This is the circuit I'm using, where R2 is the temperature sensor:

However, the fan only turns on at a voltage of ~3V or higher, and makes squeaking noises on anything lower than that. The voltage output vs. temperature (C) looks something like this:

Ideally I'd like it to "cut off" completely when reaching an output below 3V with a result like this (in red):

I want to remove the exponential behaviour when exiting the active region.

Is there a way to "reverse clamp" the gate voltage, immediately bringing it to 0 if it's too low, or is there some other solution to this problem?

Answer 1

I've found the source for the squeaking noises. It was caused by power supply noise propagating through the voltage divider and making the mosfet 'flicker' when approaching it's cut off voltage.

Adding a capacitor to the gate pin of the transistor solved the issue. I hade a 10u cap laying around and that's what I used. Just leaving it here in case somebody else decides to build this.