sizing a switch using gm/Id

[MFayek]

Sizing a switched capacitor circuit using gm/Id

Hi Everyone,
I've used gm/Id for sizing a diff amp before. Now I'm sizing a switched capacitor. But I only have the required Ron of the Mos switch and I don't know how to use this only info "Ron" to use the curves of gm/Id methodology. Thus I cannot determine the size. Could anyone give me a hand in this issue

 

[ZekeR]

The gm/Id curves were generated for a MOSFET in Saturation, i.e., a MOSFET with substantial Vds. The Ron of a MOSFET is meaningful when the MOSFET is in Triode, i.e. there is very little Vds, so therefore the gm/Id curve is not helpful here.

See if you can find Ids/Vds curves. You will see a rising linear (Triode) region to the left, tapering off to a level, current-output (Saturation) region to the right. The slope of the linear region in Triode will be 1/Ron for that size of transistor. If you need less Ron, you can gang up MOSFETs in parallel (i.e., increase w and decrease l).

Even better, design rules will often have the "Ron" or the "specific resistance" published in a table somewhere.

If you can't find either of these, assuming that the models your foundry sent you are halfway decent (they usually are good enough), in simulation you can place a MOSFET and apply a current to its drain, then "measure" its voltage.

Hope this helps.

 

[MFayek]

Dear ZekeR,
Thanks for ur reply, You have mentioned 3 methods here

See if you can find Ids/Vds curves. You will see a rising linear (Triode) region to the left, tapering off to a level, current-output (Saturation) region to the right. The slope of the linear region in Triode will be 1/Ron for that size of transistor. If you need less Ron, you can gang up MOSFETs in parallel (i.e., increase w and decrease l).

the Ids/Vds curves u have mentioned, you mean to plot these curves based on the result saved from sweeping a current source(attached to the drain of a Mosfet).. then getting Vds ..and plot it vs Ids for different channel lengths?? am I getting this point right.If so ,i think that third method you have mentioned is the same as the first one! Or I have missed something ??

 

[varunkant2k]

Please go through this thread:
https://www.edaboard.com/threads/240003/

 

[MFayek]

I've used gm/Id efficiently sizing a diff amp , the problem now is to size a triode mosfets.gm/Id isn't applicable to design for required Ron for transistors in the triode region.

 

[varunkant2k]

Triode region resistance is VDS/ID. gm is proportional to Vds.

 

[micpro]

A switched capacitor circuit often characterized by transient measurements, with specs on voltage levels at certain times. I don't see how you want to tune this with gm/Id method looking on DC operating points only. A circuit optimization software may be helpful.