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[Moved]: finding overall transconductance of OTA

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shooterskull

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How do I measure the transconductance (gm) of the overall circuit? I need to find out how gm varies with common mode input voltage. I have attached a schematic of the circuit that I wish to measure. shcematic.JPG

From what I know, I am supposed to sweep Vin and find Iout. Then I should use dIout/dVin. However, I am unable to get a gm vs vcm curve that I think is correct. Am I supposed to sweep in AC voltage using the Vcm as a DC biasing point?

I am using Cadence and I am not very familiar with it. So if you guys are able to give me the calculator functions thats even better. Thanks!
 

Re: finding overall transconductance of OTA

Do a DC sweep by varying Vin and monitoring Iout. Than take dIout/dVin.

Doing AC sim will only give small signal gm around your operating point.
 
Re: finding overall transconductance of OTA

deba_fire said:
Do a DC sweep by varying Vin and monitoring Iout. Than take dIout/dVin.

Doing AC sim will only give small signal gm around your operating point.
Click to expand...

Hi deba. I understand that concept. However, since the load is a cap, there is no output current across the load if I run it on DC. My professor has given me two other options, one of which is to replace the capacitive load with a very high resistive load, say 1T ohm. Then I would measure the current across the resistor.

He also told me I could put a voltage source also Vin at the Vo. In this case, I would measure the current through the voltage source.

However, in both these cases, he told me to put the OTA as a voltage follower instead of floating the Vo. Is this correct? From my understanding, I thought Vp and Vn should be tied together and Vo floating. Thanks!
 

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