= 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
$$
\boxed{R_{\rm in}=R_i=1\,\mathrm{k\Omega},\qquad R_{\rm out}=0\quad\text{for an ideal amplifier}.}
$$
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.
Back to article page
