Why a PMOS active load is used for a NMOS common source amplifier?

let's consider this NMOS common source voltage amplifier with active load:

For small signals the PMOS transistor M2 acts as a resistor of value $$\\frac1{g_{mP}}\$$ (P stands for PMOS) and so the voltage gain of this amplifier will be:

G = $$\-\frac{g_{mN}}{g_{mP}}\$$

I have been told that in some IC it is not a good amplifier because it has a big sensitivity to temperature variations, since $$\g_{mN}\$$ and $$\g_{mP}\$$ change differently with temperature and so their variations are not balanced. It would be a better option to get a gain which are expressed as ratios of similar physical quantities.

So my question is: why not use always a NMOS transistor as active load? Something like this:

I'd say it will work exactly in the same manner. But I have always (or almost always) seen the amplifier with a pmos active load. Also in differential pairs, I have always seen only pmos active load for nmos amplifiers:

• Isn't your NMOS load a source follower, and thus a very low impedance load (just about the opposite of what you want)? – user_1818839 Apr 27 '20 at 22:45
• You can use a NMOS as active load, look up MOS CASCODE. It uses a common gate NMOS to current buffer. This increases gain and output impedance. – Leoman12 Apr 28 '20 at 4:21