Electronics > Beginners
Measuring transconductance of any circuit
promach:
May I know if the following transconductance measurement test circuit is correct ?
I suspect that I need to use some derivative function because changing the mosfets width by a large factor does not change the gm result much at all.
However, my test circuit is simulated in AC, not DC, therefore I really confuse if I should use derivative function or not ?
Note: X1 is a CMOS inverter
Wimberleytech:
What is your end goal here? Is this a step in solving a larger problem?
In small signal,
Vout = Vin * gm * Rout.
If you have Vin, Vout, and Rout, you calculate gm. For your inverter, gm depends on the dc bias point.
You can simulate this using .ac but you want the frequency to be low enough so that there are no reactive effects.
promach:
--- Quote ---What is your end goal here? Is this a step in solving a larger problem?
--- End quote ---
See this inductor calculation especially the the small gm values
However, this forum thread is focused on checking whether my transconductance measurement circuit is correct or not.
--- Quote ---For your inverter, gm depends on the dc bias point
--- End quote ---
OK, I will use this netlist instead for biasing the CMOS inverter in saturation mode for both mosfets.
Wimberleytech:
--- Quote from: promach on December 14, 2018, 03:30:41 am ---
--- Quote ---What is your end goal here? Is this a step in solving a larger problem?
--- End quote ---
See this inductor calculation especially the the small gm values
However, this forum thread is focused on checking whether my transconductance measurement circuit is correct or not.
--- Quote ---For your inverter, gm depends on the dc bias point
--- End quote ---
OK, I will use this netlist instead for biasing the CMOS inverter in saturation mode for both mosfets.
--- End quote ---
This is helpful. You need to simulate the entire circuit using dc operating point analysis (.op). This will bias the inverter properly. Then, simply look at the spice operating point output file. It will give you the gm of all transistors. For the inverter, you will sum the gm of each transistor and that is the total gm for the inverter.
Wimberleytech:
--- Quote ---This is helpful. You need to simulate the entire circuit using dc operating point analysis (.op). This will bias the inverter properly. Then, simply look at the spice operating point output file. It will give you the gm of all transistors. For the inverter, you will sum the gm of each transistor and that is the total gm for the inverter.
--- End quote ---
For example...see attached.
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