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MOSFET Datasheet Missing Transconductance

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LaserTazerPhaser:
Is there an easy way to linearly bias mosfets in common source arrangement without the transconductance value?

There are many parts which don't have that parameter specified.

T3sl4co1l:
I'm rather more concerned that you somehow intended to make use of it (or even, have in the past?) without any stabilization in the first place..!

But yeah, same thing as BJTs, you don't know the Vbe for a given Ic, and it's changing all the time.  So you degenerate the emitter with a series resistor (effectively reducing the Gm, at least at DC if the resistor is bypassed with a capacitor), or sense the current and feed back on that, or sense the collector voltage (with the collector supplied from a well defined bias current) and feed back on that.

Base current aside, MOSFETs are just BJTs with lower Gm, so, higher Vgs, both in terms of bias (Vgs(th) in the 1-5V range, depending on type; also, negative voltages for depletion) and gain (lower Gm).  And at high currents, quadratic rather than exponential gain (Id ~ Vgs^2, instead of e^Vgs), though a power transistor operated in the linear range may well do so in the "cutoff" (exponential) regime because you're running it at Id much less than Id(max).

Tim

Doctorandus_P:
Most modern mosFET's are designed for switching applications only nowaday's.
Using such mosFET's in lineair applications may create hotspots on the chip and destroy them at power ratings far below what is stated in their datasheets.

T3sl4co1l:
Actually, power-limited DC SOA ratings seem to have made a return.  In which case, the hot-spot mechanism doesn't activate at any power dissipation the package can actually handle.

Tim

LaserTazerPhaser:
Most transistors on the market aside from RF power types are made for switching. But does this mean MOSFETs will incur damage from hotspots even if they are linearly driven within their DC safe operating area?

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