Small signal Model for series connected mosfets

Hi all,

I have this question about the small signal model for a CMOS or a series connected Mosfets.how does the small signal model look like. I think in the case of a CMOS there will be two gm values, but a friend of mine told me only one will be for both of them because they are set in series. Same question I have with an Ntype mosfet set in series with an Ntype mosfet, what will gm be? will I have to add the gm for NMos with the gm for the Pmos? I am just a bit confused about that, I can draw a small signal model for a mosfet amplifier easily when it is not in series with another Mos or BJT, but when in series I get confused. would any one please help on this.

Not sure I understand your question. You just use the model for each Mosfet or BJt and connect it in the circuit configuration you are analyzing. If you have two transistors in series, you connect the two models in series, etc.

I don't think he's talking about a simulator crutshow, he's talking about the mathematical models.

Simulators use mathematical models to do the simulation. It's a matter of writing the mathematics such that the output of the math model for one device feeds the math model for the next device. It's not generally trivial to do, of course. That's why we usually let the computers do the math in a simulator or use a program such as Mathcad to calculate the results.

Thank you guys for the reply, yes I am talking about the math model. I have a CMOS circuit with one PMOS on top and an Nmos on the bottom, and a circuit of 4 mosfets what would my value of gm be, I need to know my gm so I can calculate the Midband gain, Av, for both ciruits. thank you again