Mosfet operation wrt AC signals
In the linear region we can calculate the transconductance like in the saturation region. 
$$
\frac{\partial I_D}{\partial V_{gs}} = \frac{\partial}{\partial V_{gs}} K^\prime \frac WL \left[ (V_{gs} - V_T)V_{ds} - V_{ds^2}/2\right] = K^\prime \frac WL V_{ds}
$$
Unfortunately the expression contains \$V_{ds}\$ which tells us that the transistor behaves like a resistor and the model is not very useful.


Nevertheless, we can calculate the gain which results in
\$A_v = K^\prime \frac WL V_{ds} R_{out} \$ so we can calculate it, if we make an assumption about \$V_{ds}\$. E.g. we could bias the transistor at \$V_{ds} = 100mV\$ and continue from there.


Since the current is fixed by the current source and the resistors are fixed as well, there will be no small signal gain. We have a gain of about 1 from Vsig to Vs, since the transistor acts as a source follower. Vo will stay constant.