Finding the Unity gain frequency

[aryajur]

unity gain frequency

Hello,
I am trying to find the unity current gain frequency of a NMOS through simulation in Cadence but I cannot do it. I biased it at a Vgs and put a AC current source in parallel with Vgs. Then I simulated it and tried to plot the drain current of the NMOS. But cadence says that it is not a waveform and will not be plotted!
What am I doing wrong? Or what would be the setup to see the unity current gain frequency, also what would be the way to find the unity power gain frequency.

 

[v_naren]

unit gain frequency

Hello,
I am trying to find the unity current gain frequency of a NMOS through simulation in Cadence but I cannot do it. I biased it at a Vgs and put a AC current source in parallel with Vgs. Then I simulated it and tried to plot the drain current of the NMOS. But cadence says that it is not a waveform and will not be plotted!
What am I doing wrong? Or what would be the setup to see the unity current gain frequency, also what would be the way to find the unity power gain frequency.

please make sure that the "save device currents" option is checked in your simulation's option box in spectre!

and then click on the source node or drain node or the VDD's + or - node

 

[aryajur]

unity current gain frequency

I cannot find the "save device currents" option. Where is it??

 

[TonyXu]

unity power gain frequency

"save deivce currents": open cadence analog design eviroment -->output-->to be saved,then in schematic, you can select the device terminal.
good luck!

 

[aryajur]

unity gain frequency calculation

Ya, I had already done that, thats why I get the error message!

 

[Vamsi Mocherla]

unity frequency

Dear aryajur,

Try and take the ac current through the VDD power supply(ideal voltage source). Then you can take the ratio of the output and the input currents to get the fT

 

[chinito]

unity gain frequency simulation

Bias the MOS with VGS and put the current source at the drain of the MOS. sweep the current and plot id/ig using calculator. When it hits unity, that's your unity gain freq. good luck!

 

[v_naren]

unity gain frequency ft

Hello,
I am trying to find the unity current gain frequency of a NMOS through simulation in Cadence but I cannot do it. I biased it at a Vgs and put a AC current source in parallel with Vgs. Then I simulated it and tried to plot the drain current of the NMOS. But cadence says that it is not a waveform and will not be plotted!
What am I doing wrong? Or what would be the setup to see the unity current gain frequency, also what would be the way to find the unity power gain frequency.

if I have helped you pleaseeeee click the helped me button and help me pleaseeee

 

[aryajur]

unity current gain

Thank You, I get it now.

 

[v_naren]

unity current-gain frequency

Thank You, I get it now.

Aryajur!

I would like to explain one small error however in this method of using the simulator to find the unity gain frequency!

I will explain with respect to my own circuit!

I have used a 0.18um MOSFET.....the problem is that the device has been characterized only uptil 20GHz by our foundry!!!!!

now what does this mean?

Its mean that the SPICE/Spectre model file which the simulator uses is valid only from DC to 20GHz!!!!!!

but however please kindly note that I have "courageously" plotted till 100GHz and very cleverly shown that the unity gain frequency is ~50GHz or so!

but if the model itself is valid only till 20GHz then the plot of the current gain above 20GHz is obviously erroneous!!!!!

hence even though a unity gain frequency of 50GHz "sounds" correct!

It may not be the case actually!!!!!


I dont know if I have conveyed this subtle...."subtle concept"...clearly or not....


please try to understand


PLEASE KEEP IN MIND THAT THE MOSFET I HAVE USED IN THE EXAMPLE HAS BEEN CHARACTERIZED ONLY TILL 20GHz

hope this helps!

Good Day!

 

[aryajur]

what is unity power gain frequency

Yes I get the catch. It was just that I wanted to confirm what cadence shows me from what I calculate theoretically. Even if it is a software extrapolation of the parameters, this is what the parameters will deduce, if we use that number in our calculations, within the frequency limits that are valid. So point taken, thanks.

 

[arunkumar446]

derivation ft unity current gain

is the ft which turned out to be 50GHz is correct?
if not how are we going to know the correct answer, please let me know asap.
Thanks in advance.

 

[dhaval4987]

Hi All,
I have biased a telescopic cascode but i dont know how to measure its gain and how to plot it... can any1 throw some light?