MOSFET Datasheet Missing Transconductance - Page 1
Forum section: electronics / beginners

author: LaserTazerPhaser
author_rank: Regular Contributor
timestamp: September 03, 2019, 08:43:24 pm
content: 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.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 03, 2019, 09:16:08 pm
content: 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

author: Doctorandus_P
author_rank: Super Contributor
timestamp: September 04, 2019, 07:37:12 pm
content: 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.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 04, 2019, 07:41:14 pm
content: 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

author: LaserTazerPhaser
author_rank: Regular Contributor
timestamp: September 04, 2019, 08:43:01 pm
content: 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?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 04, 2019, 10:59:10 pm
content: I would recommend testing a demonstration or prototype first, but yes, that's exactly what DC SOA means. If they've done their homework and plotted a correct curve -- if they aren't outrightlying, it's fine.Do mind, however, the SOA can be more restricted at higher temperatures, and they don't usually give two plots, just the one, so that can bite you.I would be more than happy to yell at $mfg_rep that their datasheet is lying, if a device fails despite being apparently rated for it. The worst that can happen is they ignore me, the best is they say "oh yeah, don't do that" and, maybe just maybe, the SOA gets fixed in the next rev some years down the road.Tim