I'm pretty new to Spectre, and was simulating some basic stuff. Here's a question about getting ro using sp analysis that puzzls me.
I'm under the impression that for the drain-source impdance, the large signal value is 'ron' which is Vds/Id, and the small-signal value is ro==1/gds, which is dVds/dId. So, for small-signal gain, I expect to use ro to match my hand calculation to simulation, and it did work.
I also want to do a sp analysis at the output port of a single-transistor common-source amplifier. I set the load to be an RF choke so that the output impedance should be the drain-source impedance. In the MOSEFT and bias condition I used, ron=40 Ohm, ro=1/gds=220 Ohm, so there's a big difference between the two. However, when I look at the Z11 (input impedance looking into the output port, i.e. drain-source port), the real(Z11) is close to ron (40Ohm) ! I varied bias voltage, MOSFET dimensions etc, this issue remains. It's supprising to me that as a small-signal simulation, sp analysis picked the large-signal impedance!
As a control experiment, I replaced the 'port' element with a 50 Ohm resistor and a 'vsin' element in series and watch the voltage before and after the 50 Ohm resistor, it indeed appears that the voltage ratio is about 0.8, so the impedance should indeed be ~250Ohm.
ro is a key parameter in circuit design and being able to get the correct value through sp analysis is very important in RF designs. I'd really appreciate your take on this!