I have a circuit where I am trying to detect when a systems resistance drops from ~5-20 mOhms to half or lower (~<1-10 mOhms) with a micro controller. my microcontroller can read between 0-3.3v with 10 bit precision, or about 3.3 mV changes. I was hoping to get much larger changes out to give me confidence that I'm actually measuring the drop in resistance. My MC can also source 15-20mA of current at a pin.
My plan is to make a voltage divider with R1 much higher than R2, since making them even sort of equal will draw too much current from my pin. With V=IR being 3.3/.015= 220 ohms for the lowest resistor I can use for R1. At best this gives me ~ .3 mV of max voltage drop if my test circuit starts with 20 mOhms of resistance. I would like to build a voltage amplifier circuit to boost the voltage signal from the divider. I've drawn up a preliminary circuit ( you can replace "mosfet" with any transistor in your mind), but looking at diagrams of BJT and MOSFET transistor circuits that are used to amplify voltage, I am guessing I am missing something. I need around 1000x amplification.
I'm not sure the voltage that would be at the Gate of a MOSFET would be sufficient to turn it on at all, or that the voltage would be amplified. Maybe a darlington pair of BJTs? I'm worried that the power through the base of a BJT would interfere with my measurements.