Soooo...we had a quiz last night in Electronics II and a rendition of the first problem is below:

We were asked to solve for the voltages out of the four opamps. My thinking is: U1 and U2 are voltage followers, so I just have 8V out of U1 and 5V out of U2. Since these are ideal opamps (as specified in the problem), no current flows into the inverting input of U3. So R's 4 and 5 form a simple voltage divider and I have 4V at both inputs of U3. Moving to R1, with 5V on the left and 4V on the right, I have .1 mA flowing through it. Since that cannot flow into U3, it must all flow through R6, giving me 3V at the output of U4. Since both inputs of U4 are at ground potential, I have 3V across R2 and .3 mA through it. Again, this cannot flow into U4, so it must all go through R3, giving me a voltage of -9V at the output of U3. That was how I answered the problem. I was curious, so I simulated it and got nothing anywhere near those values. Is this just because the simulation doesn't involve ideal opamps, or is it that my solution is wrong?

 

I wish I could say that you asked for an unreachable voltage (i.e. 20V) from an OpAmp output, but this is not the case.

I noticed that a significant current is flowing into U3, and that may hint towards non-idealities. But I wish someone more experienced explained the situation.

 

You assumed negative feedback on U3 and U4. Unless you drew the schematic incorrectly (inputs swapped on either U3 or U4), then U3 and U4 form a positive feedback loop, and their outputs will latch up at one of the rails.

 

You assumed negative feedback on U3 and U4. Unless you drew the schematic incorrectly (inputs swapped on either U3 or U4), then U3 and U4 form a positive feedback loop, and their outputs will latch up at one of the rails.

That's exactly what I was getting in the simulation. I drew the schematic from memory and I bet I did reverse the connections on U4. When I swap them I get what I had calculated. I have pretty much memorized the common configurations and the expected behavior, but I don't have a good understanding of what's really going on inside that little triangle. I guess that's the problem with glossing over the fundamentals. I still don't fully understand what was going on or how I would have predicted that response mathematically.