Understanding this circuit with op. amps

E.Wood 06/12/2018. 1 answers, 539 views
op-amp basic

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!

schematic

simulate this circuit – Schematic created using CircuitLab

Tidied up schematic for OP review. [Transistor]

1 Answers


Spehro Pefhany 06/12/2018.

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).

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