How to configure an opamp for differential mode to trip an NPN?
You probably don't need any feedback but you will need a reference to compare the voltage you get from the current sense resistor to. A resistor divider from a stable voltage rail is the most basic, a divider across a zener diode is recommended or there are cheap little voltage reference ICs. I would recommend a comparator rather than an op-amp (they work in much the same way but comparators are better suited to this since they are not trying to be nice and linear).

I'm going to assume that the resistor isn't at a high voltage relative to ground, you don't need to connect one side to ground ('tis preferred though) but it can't be outside of the voltage rating for the comparator unless you have it on an isolated supply (which is kind of what I drew).

If you have a voltage rail that is stable enough you can get rid of the zener and the resistor that feeds it and just use the voltage divider. These values here will be appropriate for a 5.1V zener (a very common value).







simulate this circuit &ndash; Schematic created using CircuitLab

Also just a technicality but you don't really 'trip' a bipolar transistor since they conduct proportionally to the gate current, even though the circuit as a whole results in a 'tripping' effect at the current limit.