LTspice – Fixing Nexperia Spice MOSFET Model Not Working

Sorry about the MOS symbol confusion... Here's a better answer:

I think there are three reasons why you get a very similar voltage gain.

First off, your output resistance \$R_3\$ is quite low. The cascode transistor improves output impedance to allow a higher voltage gain, but there is no overall improvement since in the first circuit:

$$ r_{out} = R_3\parallel (r_{ds1}+10k) \approx R_3 $$

Adding a cascode transistor won't be able to improve this.

Secondly, the cascode bias voltage is probably too high. You want it to represent a large resistance. You can either alter the bias resistor network or use a diode coupled NMOS to ground (increasing it's length and adjust the current through it until you get a suitable setting).

Thirds, you should know you are limiting gain also with \$R_1\$. Effectively \$M_1\$ is acting like a source follower and the change in current through M1 is set by the voltage over \$R_1\$. Effectively you get the transconductance as \$g_m=1/R_1\$.

Summing this up voltage gain in both circuits should be approximately

$$ A_V=g_m r_{out} \approx R_3/R_1 $$

This is defined as a subcircuit:

.SUBCKT PSMN2R0_30PL DRAIN GATE SOURCE

LTSpice needs this to have somewhat special treatment, so you will need to do the following:

CTRL+Right click on the device and you will get this window:

Now edit the Prefix and Value lines: The prefix for a subckt is 'X'. The model name is precisely as defined in the lib file.

Now click OK. You will need to add a spice directive on your schematic:

.include PSMN2R0_30PL.lib This assumes it is in the same directory as the simulation circuit.

LTSpice should now be happy with the part.

Here is what you should see on the schematic:

You can, of course, add it to the LTSpice model tree, but I find it easier to use this method.