Can a power MOSFET for switching application be used as a linear amplifier?

Power MOSFETs nowadays are ubiquitous and fairly cheap also at retail. In most datasheet I saw power MOSFETs are rated for switching, without mentioning any kind of linear applications.

I'd like to know whether these kinds of MOSFETs can be used also as linear amplifier (i.e. in their saturation region).

Please note that I know the basic principles on which MOSFETs work and their basic models (AC and DC), so I know that a "generic" MOSFET can be used both as a switch and as an amplifier (with "generic" I mean the sort of semi-ideal device one uses for didactic purposes).

Here I'm interested in actual possible caveats for practical devices which might be skipped over in basic EE university textbooks.

Of course I suspect that using such parts will be suboptimal (noisier? less gain? worse linearity?), since they are optimized for switching, but are there subtle problems that can arise by using them as linear amplifiers that can compromise simple amplifier circuits (at low frequency) from the start?

To give more context: as a teacher in a high school I'm tempted to use such cheap parts to design very simple didactic amplifier circuits (e.g. class A audio amps - a couple of watts max) which can be breadboarded (and possibly built on matrix PCB by the best students). Some parts I have (or I could have) available cheaply, for example, include BUK9535-55A and BS170, but I don't need specific advice for those two, just a general answer about possible problems wrt what I said before.

I just want to avoid some sort of "Hey! Didn't you know that switching power mos could do this and this thing when used as linear amps?!?" situation standing in front of a dead (fried, oscillating, latched,... or whatever) circuit!

Answer

I had a similar question. From reading application notes and presentation slides by companies like International Rectifier, Zetex, IXYS :

• The trick is in the heat transfer. In the linear region, a MOSFET will be dissipating more heat. The MOSFETs made for linear region are designed to have better heat transfer.


• MOSFET for a linear region could live with higher gate capacitance

IXYS app note IXAN0068 (magazine article version)

Fairchild app note AN-4161