 Solution

ID: past-exam-of-the-physics-2-paper-of-the-university-of-oxford/2013-resit/3/ii/solution

An external source sees the input resistor between its terminal and the virtual ground. The input current is

I_{i} = V_{i} / R_{i}

, so

R_{in} = R_{i} = 1 kΩ, R_{out} = 0 for an ideal amplifier .

The infinite input resistance of the ideal operational amplifier itself is distinct from the finite input resistance of the complete inverting amplifier. Negative feedback makes the output approach an ideal voltage source; a real amplifier retains a small nonzero output resistance.

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