Equivalent capacitor for a CMOS circuit

[dnanar]

Hi,


First of all this is my first post here (and I'm glad such a forum exist!) please excuse me if it's not the right forum section. I'm trying to properly learn digital electrical engineering through SPICE and the book "CMOS circuit design, layout and simulation".

For now, I'm wondering about the capacitance we put after a CMOS circuit to make a transiant analysis. I guess it "simulates" any circuit we want to put after the circuit being tested. However i don't understand:

1) Why most of the time people use a capacitance (and not an RC circuit or something else?)
2) How to choose the value of the capacitance? I guess it should be the equivalant value of the circuit we want to put after. But how to get that equivalent value (let's say for a simple inverter)?

Actually, my problem is I want to know given a technology and a Vdd/Vgs voltage what is the maximum frequency a CMOS circuit can run? For a simple inverter circuit, if i use a capacitance with a lower value Vout is going to be at its "expected" value more or less quickly, so I don't really know which frequency to take.


Thank you for your help!

 

[Audioguru]

When testing for transient response on a datasheet, the capacitance connected from the output to ground is probably the most "stray capacitance" or the input capacitance of the following stage that will ever occur in a circuit. Then you can be sure that your circuit will work the same or better that what is shown on the datasheet.

The datasheet for the CD4069 Cmos hex inverter IC has it spec'd with a load capacitance of 50pF.

 

[dnanar]

Ok thank you.

So if i get it right, if i put an inverter after my CMOS circuit, the equivalent conductance will be C=Cp+Cn, with Cp=Eo*Er*Lp*Wp/Tox and the same for Cn.

 

[Audioguru]

Why do you need to put an inverter after your Cmos circuit? Instead, pick a Cmos circuit that inverts.

The datasheets of CD4xxx Cmos logic ICs show conductance with various supply voltages (which affects Vgs) and various Vds. There is a graph for sourcing current and another graph for sinking current.