I am struggling to find the Thevenin voltage and resistance across terminals A and B of the given circuit. enter image description here

I got as far as writing KCL at nodes 2 and 3, which gave this:

$$v_\text{AB} = - G_2/G_1 e + \frac{G_1 + G_2}{G_2} e_2$$ $$e_1 = e_2 = e_3$$

From here, I don't really know how to proceed to calculate \$v_\text{AB}\$. I don't even know how to start calculating \$R_\text{Th}\$.

  • \$\begingroup\$ Please explain why you think nodes 1 and 3 have anything to do with the solution. \$\endgroup\$
    – Andy aka
    Commented Jun 18 at 16:50
  • \$\begingroup\$ I'm sort of a beginner with op amps, why would'nt they have something to do with the solution? \$\endgroup\$
    – Diavo
    Commented Jun 18 at 16:54
  • \$\begingroup\$ Because it's a trick question. The circuit is a summing amplifier with the question concerning one of the two inputs. \$\endgroup\$
    – Fred
    Commented Jun 18 at 16:57
  • 1
    \$\begingroup\$ Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking. \$\endgroup\$
    – Community Bot
    Commented Jun 18 at 17:47
  • 1
    \$\begingroup\$ If you remove the op amp from the circuit, how does that change the circuit equivalence at nodes A & B? \$\endgroup\$
    – qrk
    Commented Jun 18 at 17:52

1 Answer 1


All the comments are trying to point out that the op amp is a distraction, i.e., it has no bearing on the question except to confuse you.
This assumes that the op amp is ideal which means infinite input impedance (the important parameter for this question), infinite open-loop gain, and infinite bandwidth. For school studies, this is a fair assumption for first level approximations.

Thus, the equivalent circuit is merely:


simulate this circuit – Schematic created using CircuitLab

By observation you should be able to figure out the Thevinin equivalent circuit from this.


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