# Transresistance amplifier and current measuring

I was studying some shunt voltage amplifiers, and I came out with this circuit. R2 is for input voltage offset compensation only, and is the same value as R1.

simulate this circuit – Schematic created using CircuitLab

Now, I know that from various datasheets of components integrating this topology, the final gain is:

G = R4/R1

Assuming from the classical analysis of the source degenerated transistor amplifier a R1 >> 1/gm, where gm is the small signal PMOS M1 transconductance.

Now, I'd like to build with discrete components this circuit (just for learn/fun), but I ended up with a blocking condition, at least on paper. How do I polarize the MOSFET in order to have gm >> 1/R1? I know that is taken care from the op amp, but let's say that I use an BSS84P and I have this graph from the datasheet:

How do I use this plot?

EDIT: I think that as long as the source resistor is big enough, the gm variation with the current will introduce just small non linearities.

• Isn't some of the measured current going into the FET, and thus the voltage across R3 is not perfectly proportional to that current? May 2, 2017 at 14:24
• A lot of current sense devices are using this configuration, so I suppose is because it is perfectly proportional to the current delivered to the load, rather than the current running in the wire before the sensor. And, I think that for a good precision, it shall measures currents way higher than the one used by the PMOS. May 2, 2017 at 15:40