This document discusses the difference amplifier or op amp subtractor circuit. It begins by defining a difference amplifier as an op amp based circuit that amplifies the difference between two input signals and rejects any signals common to both inputs. It then presents equations derived from Kirchhoff's current law that show that for the difference amplifier to function as a perfect subtractor, the feedback resistors R1 and R2 must be equal and the input resistors R3 and R4 must also be equal. When these conditions are met, the output of the circuit will be the difference between the two input signals.