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The output voltage V0 for the circuit shown in the figure is:

Op-amp: V2 goes through 5R to the inverting input, with a feedback resistor 5R from the output V0 to that input; V1 goes through R to the non-inverting input, which is also connected to ground through R

  • A.\((V_{1} - V_{2})\)
  • B.\((V_{1} - 2V_{2})\)
  • C.\((V_{2} - V_{1})\)
  • D.\(2(V_{1} - V_{2})\)

Answer: A

The output is \(V_0 = V_1 - V_2\).

This is a difference amplifier. The non-inverting input sits on a divider of two equal resistors R, so \(V_+ = V_1/2\). By superposition, the inverting stage has gain \(-5R/5R = -1\) for \(V_2\) and the non-inverting stage has gain \(1 + 5R/5R = 2\) for \(V_+\):

\[ \begin{aligned} V_0 &= \left(1 + \frac{5R}{5R}\right)\frac{V_1}{2} - \frac{5R}{5R}V_2 \\ &= V_1 - V_2 \end{aligned} \]

\(2(V_1 - V_2)\) forgets that the divider halves \(V_1\). \(V_1 - 2V_2\) doubles the inverting gain, and \(V_2 - V_1\) has the sign of the inputs reversed.

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