LTspice – Why Can’t MOSFETs Be Edited by ‘nmos’?

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

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

A single transistor will not provide an interface with your FET AND do it without inversion. You can do the job with two transistors, though.

^{simulate this circuit – Schematic created using CircuitLab}

This will provide a decent drive for your FET with about a 1 usec added to on-time. That is, if you put in a 10 usec pulse, the FET will be on for about 11 usec.

Also note that R5 needs to be a 1-watt resistor.

EDIT -

It turns out I was using the wrong FET model, and my value for gate capacitance was too low. For a "real" IRFZ44, the added pulse width is about 3 usec, not 1. The problem is that, when you go to turn off the FET, The charge stored in the gate can only discharge through R5, and the time constant is in the 3 usec range. You can, of course, decrease R5, but then the power it dissipates when the load is driven on goes up. You can get a 2 usec delay rather than 3 by decreasing R5 to 100 ohms, but then you need a 2-watt resistor. If you need very narrow PWM cycles (low effective motor current) you're probably better off with a more sophisticated driver. I personally tend to go with the Maxim MAX4426/4427 (less than $4 each, with 2 channels per IC), but that's just habit.