1. The problem statement, all variables and given/known data So this problem is on Introduction to Electric Circuits 9th Edition (Richard C.Dorf) This asks me to express the gain of this circuit Vo/Vs by terms of the resistance of R. 2. Relevant equations KCL, KVL, some ideal OP AMP characteristics... 3. The attempt at a solution Look at the undermost (horizontal) wire. Leftmost (perpendicular) wire has escaping current Vs/20(A). Middle (perpendicular) wire has no current (connected to OP AMP). Rightmost (perpendicular) wire has entering current Vo/80(A). Apply supernode KCL. Vs/20=Vo/80. Vo=4Vs. So gain is 4. Huh, this is weird. There is no variable R in this gain. Actually, provided answer sheet says gain is -4R/(R+80). But I cannot understand this answer. This gain has minus sign, so Vo and Vs must have different sign. This means undermost wire can only have entering current or escaping current. What's the problem with my logic?