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So according to a book example (See Digital Integrated Circuits by J. M. Rabaey example 3.9) I'm trying to determine the Gate capacitance using the following schematics scetch: enter image description here

This comes directly from the book, however, a simulation gives me the error of a floating point.

What am I doing wrong or should I do different? I tried replacing this source with a voltage source what gave me no errors and a way to plot the current floating into the gate. This remains zero however. This making it look like it is a ideal transistor. (what it shouldn't be?)

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SPICE needs to be able to find a dc solution for the circuit before it begins a transient analysis. However, your constant current source into the capacitance of the gate creates a voltage that increases linearly to infinity, so the simulation fails.

Change the dc current source to a current pulse source where the current is 0 at \$t=0\$ then quickly increases to a steady-state value (and something like a milliampere is probably more appropriate). Run a transient analysis for a short time and see how long it takes the gate to charge to some specific voltage, like your positive supply voltage. Then you can back out the average capacitance from the slope of the voltage.

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