Re: Linearising FETs: How to choose a FET?

Joel Kolstad wrote:

I've been looking at the "linearization trick" for getting a FET to behave
as
an approximately linear resistor as described in AoE, 2nd ed., pg.
139-140. I can reproduce the graphs on pg. 141, although I've come to
realize that one of the reasons they look so good is because they only
took Vds out to half a volt (JFET) and 200mV (MOSFET) -- if you run the
circuits through SPICE and let Vds run higher, things aren't nearly as
pretty.

In any case, what parameters should I be looking for on a data *** to
suggest whether a given JFET or MOSFET is a good candidate for
linearization? I had SPICE run through a handful of different randomly
chosen FETs, and some are markedly better than others (the VN13 that AoE
uses is among the better MOSFETs... and while I don't have a VCR2N model,
I suspect it would be among
the better JFETs). For that matter, other than preference and gate
current, in general are JFETs going to be better than MOSFETs for this
application? I've been getting that impression...

What I'd like to do is to build some electronically controllable RF
attenuators good to the HF range (30MHz); non-linearities in the
resistance
will translate into distortion (the creation of spurs). I was also hoping
to be able to pass some 10dBm, which is 1V peak into 50 ohms, so I was
thinking
I'd be biasing the FETs at around 1.5V or so. Should this be pretty
doable?

Thanks in advance for advice.

I'll post a representative plot of a linearized FET over on ABSE...

---Joel Kolstad

I don't have a copy of AoE in front of me so I don't know what this
linearising trick is, but I have looked at using FETs in the triode region
(i.e. VGD > VTH) as attenuators in a PI configuration. I think the trick
is to find a small enough FET so that you can get the desired resistance
with a sufficiently large VGS. That way, the conductivity of the FET which
is roughly proportional to (VGS-VTH) will not vary by such a large
percentage during the RF cycle. Another thing which can help linearity is
to put a large resistor in series with the gate, so that the gate voltage
tracks the average of the drain and source voltage as far as RF is
concerned.

HP used to make III-V chips with FET pi-attenuators on them, I have a data
book about them. They have a nice op-amp feedback circuit which operates
on a dummy version of the RF attenuator and which adjusts the gate voltages
so that the attenuation and characteristic impedance are right.

By the way, if you could tolerate using switched stepped attenuators instead
of continuously variable attenuation, then it is much easier to achieve low
distortion, In that case you could switch in and out the attenuator stages
using FST3125 or similar, though the on-resistance will limit the minimum
attenuation that you can get to.

Chris

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