Inverter propagation delay using the RC delay model

[Toyona10]

Hi~

Actually I am fine with calculating the delay but I haven't got the concept clearly enough. Both nmos and pmos have their equivalent RC models. I understand how the RC model of nmos works when its on or off but I don't get how the pmos turns out this way. When its off, the upper part won't work because its open circuit but when its on, isn't the resistor supposed to be included? Why isn't it there in the inverter's equivalent RC model when the pmos is on?

 

[analogdesign]

The resistor in the RC delay model is either the PMOS or the NMOS. They are both connected to "ground" because in either case the output is connected to a small signal ground (VDD in the PMOS case or VSS in the NMOS case) so as far as the delay model is concerned they are equivalent.

So to answer your question, yes the delay model does include the on-resistance of the PMOS when it is on.