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

enter image description here

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:

enter image description here

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:

S

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  • \$\begingroup\$ Isn't your NMOS load a source follower, and thus a very low impedance load (just about the opposite of what you want)? \$\endgroup\$ – user_1818839 Apr 27 '20 at 22:45
  • \$\begingroup\$ 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. \$\endgroup\$ – Leoman12 Apr 28 '20 at 4:21
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diode-connected FETS unless very long channel will be high-GM, low-incremental resistance, causing low gain and high distortion.

Using the Pchannel as active-load may result in very high gain (depends on the channel length, so lambda-coefficient is not a bother) and low distortion.

In any case, long-channel FETs have higher channel-bulk capacity that has to be charged and discharged.

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  • \$\begingroup\$ So you are saying the P-channel provides higher resistance with a shorter channel and lower capacitance than N-channel? \$\endgroup\$ – Bruce Abbott Apr 28 '20 at 5:57

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