Hello I need help
This is not a homework but a training exercice and I need to know how to solve it please
In the figure, VIN is very small and we hve to find the minimum VG such that the circuits attenuates VIN by more than 5%.

We have the values for k'(W/L), RL and Vt..
can you help me?at least a hint
thank you

 

For small enough signals, that MOSFET's vds and Id characteristic is close to linear. This means that you've effectively got a resistor who's resistance is controlled by Vg - Vs. If you can derive or find a small signal equation that relates vds to Id linearly (and depends on Vg - Vs of course), then you can solve this like a voltage divider. It may get a little complicated though, since Vs depends on the output and therefore kind of provides feedback to your "resistor". It would have been easier if the MOSFET had been grounded instead. ;P