I stumpled over the following article comparing the following difference amplifier circuits in terms of errors due to input offset voltage and resistor tolerances.
To my understanding the right-most circuits gets away with fewer and possibly also less accurate, hence cheaper resistors. This made me think wether I could adapt the circuit to measure much larger differential voltages such as the the cell voltage in a stack of batteries.
I played around in LtSpice and it seems that it might work, however, I really don't understand the rightmost circuit. Where is the feedback path? What influence does the transistor have? Spice indicates that a PMOS also works, but how and why? Are there device that alreay include the transistor and what are they called?
I found some shunt current devices which seem to implement the rightmost circuit, but those are only specified for small differential voltages. Why is that?