 Solution

ID: past-exam-of-the-physics-2-paper-of-the-university-of-oxford/2013/8/i/solution

Ground the non-inverting input and take the ideal operational amplifier in stable, unsaturated negative feedback. Its inverting input is a virtual ground and draws no current. Kirchhoff's current law there gives

\frac{V _{i}}{R _{i}} + \frac{V _{o}}{R _{f}} = 0, \frac{V _{o}}{V _{i}} = - \frac{R _{f}}{R _{i}}, ∣ A_{v} ∣ = \frac{R _{f}}{R _{i}} .

The minus sign is the phase inversion of the inverting amplifier.

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