Why does a diode connected MOS should have an output resistance of 1/gm?

[anil555]

Hi all,

I was wondering why does a diode connected MOS should have an output resistance of 1/gm. I understand that a voltage controlled current source i.e Gm*Vgs, would give a resistance of 1/Gm if same voltage is applied across it but i am not able to figure out what makes it happen in MOS, if taken as a device rather than a model.


Any help would be appreciated as this one has been bugging me a lot.

Thanks
Anil

 

[safwatonline]

Diode connected MOS

consider a current source of current I=gm*Vgs and the drop on this source is Vgs then the equivalent resistance seen is R=V/I=Vgs/(gm*Vgs)=1/gm

 

[anil555]

Re: Diode connected MOS

Thanks for the reply.

I got the circuit part of it. But could u explain in physical terms. i.e in terms of working of the mos. I guess this is too much to ask but just curious.

Is it that when u connect drain to gate, which are 180 degrees out of phase for small signals results in lowering of output resistance from r0 to 1/Gm. I mean i dont understand why when the MOS is in saturation(though it is diode connected) should it drop its resistance from r0 to 1/gm.

Thanks

Anil

 

[vndjn]

Re: Diode connected MOS

mos has two control for its current, one is vgs and other is vds.
vgs control called transconductance "gm" and vds control called output impedance "ro"
for mos generally gm*ro>>1, so ro>>(1/gm)...
when you made diode conncted configuration gm and ro comes into parallel and effective ro becomes (1/gm).