Electronics > Beginners
Anyone knows if FET gm tracks Id over temperature?
magic:
Question as in the title: can I expect a lightly/not degenerated MOSFET or JFET common source amplifier to maintain roughly constant transconductance over temperature provided that drain current is regulated?
I suppose it would work for BJT if one can live with the minor variation resulting from thermal voltage, but I have little idea how FETs behave. I think the subthreshold region looks particularly promising because supposedly Id is exponential with Vgs so gm ought to be proportional to Id, but how does temperature factor into it?
I know that there are "feedforward" biasing tricks which stabilize gm itself but that looks like no fun given all the thermally coupled parts required.
T3sl4co1l:
Offhand, Vgs(th) and gm fall with rising temp, so that Rds(on) and Id tend to rise. Not sure offhand if Id falls, or rises relatively faster, in subthreshold.
Tim
Wimberleytech:
--- Quote from: magic on January 09, 2020, 02:31:26 pm ---Question as in the title: can I expect a lightly/not degenerated MOSFET or JFET common source amplifier to maintain roughly constant transconductance over temperature provided that drain current is regulated?
--- End quote ---
Not sure what you mean by "degenerated."
For a MOSFET configured as a common-source amplifier with a current source bias and operating in strong-inversion, saturated region, the gm is given by (2 K' W/L I)1/2
K' = u0 COX
We can say that the COX and W/L terms are not temperature dependent to the first order at least.
The mobility term is very dependent on temperature: T -1.5
So if you design a temperature independent bias current (easy to do), the gm will vary according to the mobility term. Plug it in and do the derivative.
The above analysis is based on the simple Schichman-Hodges model.
--update--
Pondering this further, the answer above is correct, but may not really be useful in your circuit configuration. For example, if you are making a single stage ac-coupled amplifier with a MOSFET biased with resistors and a resistor load, the analysis has to be done differently. However, if you are considering a common-source amplifier with a current-source load as part of an opamp design (integrated) then the above analysis applies.
So, to zoom in on your answer, the circuit topology would be useful.
magic:
Basically, what happened is that I found some pretty low capacitance / highish transconductance jellybean MOSFET (or so the datasheet says) and I'm wondering how fast of an amp it could do.
So, discrete and single stage ;)
Degeneration = source degeneration = resistor in the source circuit to set bias current and control effective transconductance.
But it's a gain killer, so the idea is to keep constant current by shifting gate bias (has to be done anyway to prevent thermal runaway) and pray that gm won't go off the charts. I guess I should pull out a breadboard and try it with some 2n7000 but I have a mess now, don't ask :P, so I wondered if perhaps there is any known theory of that.
Wimberleytech:
--- Quote from: magic on January 09, 2020, 09:09:08 pm ---Basically, what happened is that I found some pretty low capacitance / highish transconductance jellybean MOSFET (or so the datasheet says) and I'm wondering how fast of an amp it could do.
So, discrete and single stage ;)
Degeneration = source degeneration = resistor in the source circuit to set bias current and control effective transconductance.
But it's a gain killer, so the idea is to keep constant current by shifting gate bias (has to be done anyway to prevent thermal runaway) and pray that gm won't go off the charts. I guess I should pull out a breadboard and try it with some 2n7000 but I have a mess now, don't ask :P, so I wondered if perhaps there is any known theory of that.
--- End quote ---
Ahhh...source degeneration--got it.
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