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Transconductance of an OTA

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samiran_dam

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Dear All,

Suppose an OTA is driving a capacitive load (CL=5 pF). Now in the behavioral model of the OTA, I have considered an internal high frequency pole [Iout(s)/Vin(s)], non-zero output conductance (Gout). Now when I plot Gm(s) [i.e. Iout(s)/Vin(s)] of the trans-conductor, due to high output impedance and 5 pF load cap, the transfer function is having a low frequency pole as well as the high-frequency one defined internally.

It means for low-frequencies [lower than the first pole frequency (Gout/CL)], transconductance of the trans-conductor does not remain Gm which it is supposed to deliver.

Is this fine? I mean is the model of OTA reasonable enough to try this in different OTA-based circuit?



Please help me.

regards
Sam
 
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Hi,

I also encounter the similar problem.

when I plot gm-freq curve, I found the gm at low frequency is not the value given by 1/(rout*CL). instead, it increases from a very negative (in dB) value. What happened?

another problem is, because there is no gm-freq plot function in cadence, so I thus run ac simulation by assigning a 1V ac input magnitude, and then I think the Iout-freq plot is actually the gm-freq plot. Am I correct?

Anyone can help us?

Noah
 

Dear Noah,

I guess we both trying wrong approach. By definition, "Gm(s) should be plotted in output short-circuit condition". Please have a look at the following analysis:

481f354b729abe17a3dd7280e1d9ac20.jpg


Hope this will resolve the problem you are having

best wishes,
Sam
 
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