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how can i change cgd capcitance of nmos or pmos

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essam sheta

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Hi everyone,

how can i change cgd capcitance of nmos or pmos
im design PA and i need to study the effect of miller effect, how can i change cgd to be equal zero or any another value i want .



thank you for help
 

crutschow

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Do you mean in simulation or with a real device? In a real device it is fixed by the device geometry and can't be changed. In a simulation you should be able to change the model parameters for those capacitances.
 

essam sheta

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Do you mean in simulation or with a real device? In a real device it is fixed by the device geometry and can't be changed. In a simulation you should be able to change the model parameters for those capacitances.

yes i mean in simulation (ADE simulation) how can i change cgd in model parameter for those capacitance ,where the cgd parameter are not in the cdf of the nmos transistor
 

crutschow

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I'm using LTspice and the Cgd parameter is in the MOSFET model along with Cgs, and Cjo. What parameters are given in ADE?
 

erikl

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... where the cgd parameter are not in the cdf of the nmos transistor

In such case, cgd will be calculated from device-specific values like area (w, l), gate overlap length and tox, and from application-specific values like gain and feedback (if present).
 

essam sheta

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I'm using LTspice and the Cgd parameter is in the MOSFET model along with Cgs, and Cjo. What parameters are given in ADE?

thanks in advance crutschow
i setup the ltspice in my pc and build my circuit and work ok
can you help me how can change cgd parameter of the mosfet
 

mtwieg

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You can't simply change the Cgd of a mos model. Those values are derived directly from known process characteristics, and the defined device size. It's that way so that the models reflect reality closely.

If you want to lower Cgd (which is effectively impossible in a real implementation) then you need to make your own model. If you want to increase Cgd then just add in your own as a discrete element external to the model subcircuit.
 

dick_freebird

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Follow the Setup>Model Files chain and find your FET
model card as referenced in the netlist. Edit that to add
a "k factor" (like, cgdo=3.9E-15 -> cgdo=3.9E-15*kncgdo)
and then add that same variable to your ADE variables
list, so you can mess it around with parametric analysis
or whatever.

If your PDK models are locked down then copy the whole
tree to someplace you own, edit that copy and redirect the
Setup > Model Files to it.
 

essam sheta

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thanks dick_freebird

dear dick_freebird

im using cadence virtuoso ,
i worked the following steps
from ADE>setup>model library.... then there are path added to the menu usr/local/..../.../tsmc13rf.scs then i opened the file tsmc13rf.scs from its location directory to change the value of cgdo , the file did not contain cgd0
 

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