Find only the gain Vo$/$Vsig$.$

$6\mathrm{.}148$ In the circult of Fig. P$6\mathrm{.}148,v_{\text{s}\text{i}\text{g}}$ is a small sinewave signal. Find $R_{in}$ and the gain $v_o^{T{v}_{\text{s}\text{i}\text{g}}}.$ Assume $=100.$