What I've read in microelectronic texts like Prof's Razavi's Fundamentals of Microelectronics is that for calculating output impedance, one has to make all independent sources 0, i.e., short all independent voltage sources and open all independent current sources.
Sighting a simple example of output impedance calculation in common source amplifier given in the picture, if we go by the rules, we'd short Vin and Vdd as shown in the picture. Now, from the small signal model, the output impedance is said to be Rd||ro. But my question is that if gate and source both are shorted making Vgs = 0, i.e., lesser than Vth, this implies there is no channel at all for current to flow from drain to source. So, why do we take into account ro in output impedance calculations when no current can even flow through the MOSFET due to Vgs = 0?
P.S.:You may either refer to Fundamentals of Microelectronics by Dr.Behzad Razavi or even his lectures on youtube for the prescribed method my question is based on, the links given below.
The book Fundamentals of Microelectronics :
Razavi electronics lecture 37 - at 45:54