Hi all.
Could anyone tell me how to simulate the gm of amplifier ?

Thanks a lot.



Glacier

To measure the Gm of the amp, you need to see how the current changes when the output is AC shorted.  Think of the way an amplifier may be represented (a controlled current source in parallel with an output resistance).  AC shorting the output allows you to see the current the controlled source is generating, while eliminating the output resistance, Ro, from the picture.  The DC bias conditions must remain intact though.

Let's say you have a common-source amplifier.  At the output, put a really big cap to ground.  The big cap makes the output node shorted to ground at higher frequencies, but it's open at DC, so it doesn't change the DC operating point.

Run your AC analysis and Gm=iout/vin, where iout is the current through the big cap.

Op will work for the operating point gm of the MOSFET.  As discussed in another posting, if you want to plot gm vs. something you need to do your dc sweep and use a command like 'save m1:all'

The gm of a MOSFET is only going to be the same as the gm of the amplifier in certain cases.

save m1:all goes into the spectre netlist.