Cadence VCO how to find total capacitance at the source of the nmos cross couple pair

[bestvlsi]

Hi all,

I have to find out the total capacitance at the source node of the cross coupled nmos pair in an LC VCO.

I am using Cadence.I ve done the dc analysis and print--> operating point which is showing different types of capacitances like Cgs, Csg ,Cjs, Cssbi etc. and also Css. Now for finding out the total capacitance at the source node do I have to add these capacitances . I think that Css represents the total capacitance but I am not sure. Moreover would like a confirmation that after getting the Source Capacitance for an NMOS ... I've to multiply by 2 to get the total capacitance ( because there are 2 NMOS transistors cross coupled and connected at the source for an LC VCO )...am I right?

Any help will be appreciated.

Regards.

 

[erikl]

A very practical way is to inject a unit (1A) ac current (via a huge cap -- in order not to short circuit the node bias) into the node, run an ac analysis and plot the ac voltage at this node -- which directly represents the node impedance in Ohms -- vs. frequency. From this representation you can calculate the total node capacitance.

 

[bestvlsi]

A very practical way is to inject a unit (1A) ac current (via a huge cap -- in order not to short circuit the node bias) into the node, run an ac analysis and plot the ac voltage at this node -- which directly represents the node impedance in Ohms -- vs. frequency. From this representation you can calculate the total node capacitance.

Hi erikl, Thanks a lot. I've tried and found the value... Thanks again!

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