[SOLVED] mosfet transconductance and output resistance

[preethi19]

Hi for a mosfet say nmos the transconductance is given as gm=Id/VGS and the output resistance (channel resistor) ro= Id/VDS.

Also while calculating voltage gain of nmos it was given to be
Av=VD/ VGS.

My questions are
1) taking gm=Id/VGS means the change in drain current as VGS changes. But then the drain current also changes here when VDS varies. Wont this change the transconductance???
2) Same for ro=Id/VDS. Wont change in VGS have impact on the channel resistance formed.
3) For gain why do we consider VGS and input. Why not VDS??? Becoz VGS is only meant to form the channel from drain to source. But only when VDS is applied we get any current flow in the transistor right???
Can anyone kindly explain this.

 

[LvW]

My questions are
1) taking gm=Id/VGS means the change in drain current as VGS changes. But then the drain current also changes here when VDS varies. Wont this change the transconductance???

No - Id does not change when VDS varies (only a little, but this can often be neglected).

2) Same for ro=Id/VDS. Wont change in VGS have impact on the channel resistance formed.

You must strictly discriminate between DC values (VDS) and small-signal AC values vds. Here we have ro=d(Id)/d(VDS)=id/vds

3) For gain why do we consider VGS and input. Why not VDS??? Becoz VGS is only meant to form the channel from drain to source. But only when VDS is applied we get any current flow in the transistor right???
Can anyone kindly explain this.

It is always the voltage Vgs which controls the current Ic (see answer to 1). VDS is a necessary condition for the FET to work at all, but it is not considered as an input signal.

 

[shanmei]

ro=d(VDS)/d(Id)=vds/id, right?

 

[LvW]

ro=d(VDS)/d(Id)=vds/id, right?

Yes - of course, you are right.
Thank you for correcting my post#2