I'm studying electrical engineering and I've came to a problem, where I cannot proceed. It's a "simple" task, where we're talking about an inverting op.amp with 2 voltage sources, each 1V, while all resistors are 10k, except the one in the feedback, which is 20k.
I I know, that the solution is Uout = -7V, but how to get to this result? By equations I've calculated, I've got result either -0.5 or -3V, but never -7. And also, I've simulated in LTSpice and got -7V.
Will appriciate all the help!
Tidied up schematic for OP review. [Transistor]
I'll let someone else do the sums formally/generally, but there is an easy approach here that will allow you to solve it by inspection.
As you know, the op-amp inputs are both going to be 0V at balance (if there is negative feedback and the op-amp can balance). So let's assume that.
That means that 100uA is flowing through R3, and since our ideal op-amp input contributes nothing, 100uA must be flowing out of R2, by KCL.
So now we know the voltage at the junction of R1/R2/R5- it must be -1V.
So the current through R1 is 200uA, and the current flowing out through R5 must therefore be 300uA (100uA + 200uA). So the output is 6V below -1V or -7V.
(scrawl arrows on the schematic to keep the current directions straight, if it helps).