For The fundamentals of electric circuits, fourth edition (spacewalk on the cove

Question

For The fundamentals of electric circuits, fourth edition (spacewalk on the cover),Chapter 5 problem number 9. There is already a solution posted but I dont understand the last part (noticed several comments asking as well). I get the first circuit, as there are two to analyze in this problem, but the second one is throwing me. When you know that [v sub a] equals [v sub b], how do you know that [ v sub b] equals 5V and not 2V. I know that for and ideal op omp the two v1 and v2 equal eachother, but there is a voltage source on each line. Then where do you go from there. what method was used in the solution on the last step?

Explanation / Answer

when you're dealing with ideal op amps, you always assume that thevoltage at the inverting terminal is the same as the voltage at theinverting terminal. you can actually thing of the 2volt source as a2 volt drop, so on the - side of the 2 volt source, (the Vo side)the voltage is Vb-2V=3V between Vo+ and Vo- (=ground) you lose therest of the 3 volts in the resisitor, so you have a total of 5volts dropped between Vb to ground and Va to ground, so you'vesatisfied KVL.

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