Electronic – calculate op amp outputs problem separately

operational-amplifier

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I was quite confused of this problem since I cannot find the Vo2 using KCL for Vo1 – Vo2 but in the problem, it just said dont use KCL for output voltage. Is it the way op amp operating or I am confused don't know anything about it.

Best Answer

In theory your calculations are absolutly right and your results would make sense - IF the OpAmps could acutally output these voltages.

I can only assume that the question is targeting some thinking about plausibility: The upper OpAmp is supplied only by +/- 12V - so it will never be able to pull the ouput as low as -65V. It will go into saturation around -12V (let's assume the OpAmp can drive the ouput right to the supply rails. Most real OpAmps would need some headroom and would not even be able to go that low).
The same is true for the lower OpAmp: For the output to go to 8V, the OpAmp would need a positive supply rail of at least 8V, but it only has 6V available. So the theoretically absolute maximum output voltage will be 6V (again, assuming the OpAmp is capable of driving the output right up to the supply rail).
This would give output voltages of -12V and 6V.

But even these values seem a little bit unreasonable, when we take the resistor values into account: With V01 at -12V and the negative input at +12V this results in 24V/44 Ohm = 545mA of current that the upper OpAmp has to sink just from the input current. With V02 at 6V, there will be an additional current of 6V-(-12V)/3 Ohm = 6A! So the upper OpAmp would have to sink 6.5A. The lower OpAmp would have to deliver this 6A + 6V/6 Ohm (for the lower current path through the 2x 3 Ohm).

Without any detailed question I don't know what exactly they want as an answer, but the calculations show, that KCL will fail in this case, because the OpAmps go totally into saturation.