Linearising FETs: How to choose a FET?

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 sheet 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

Reply to
Joel Kolstad
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Look for JFETs with the highest Vgs(off).

Siliconix AN73-1 is *the* app note to read first. It includes an abstract on how to estimate and reduce thd (from 12% to 0.1% in the given example).

--
Tony Williams.
Reply to
Tony Williams

Will do. From what I've downloaded, it looks like AN105 is an updated version (and is a lot easier to find as a complete PDF).

---Joel

Reply to
Joel Kolstad

I think most MOSFET spice models are not meant to be used in the subthreshold region. Generally for audio frequencies JFETs seem to be preferred, with a high Vgs cutoff voltage. They are much lower in noise. I have here a couple of UHF-attenuators with GaAs-Fets by Ma/com. They work from DC up to the GHz range. Download the databook_final.pdf by Interfet Corporation.

--
ciao Ban
Apricale, Italy
Reply to
Ban

I took a look... it's the same technique that Win describes in AoE. Interestingly, AN73-1 (the older paper) has considerably more information than AN105, although AN105 is handy in that they give a table for modern replacements of the VCR2N/4N7N.

I really ought to get some sleep now...

---Joel

Reply to
Joel Kolstad

The Siliconix original papers, by authors such as J.S Sherwin or W.Gosling (Prof William Gosling afair), are always better than later re-drafts. They give more complete information and tend to be more scrupulous about providing source references.

--
Tony Williams.
Reply to
Tony Williams

Hello Joel,

Check out the BF998 dual gate. I did some experiments with it for a publication where I need a variable resistance. Still have to finish that publication but then I got a call from a client.

--
Regards, Joerg

http://www.analogconsultants.com
Reply to
Joerg

Hello Ban,

You can obtain some good models on sites like the Yahoo LTSpice group, in the files section. And from companies like Philips.

For the GHz range I prefer PIN diodes but they won't do much good under a few MHz or with large amplitudes.

--
Regards, Joerg

http://www.analogconsultants.com
Reply to
Joerg

I vaguely remember some method of adding part of the signal into the gate path that considerably improved linearity.

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

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

Reply to
Chris Jones

Hello Jim,

Half the signal is fed back into the gate.

--
Regards, Joerg

http://www.analogconsultants.com
Reply to
Joerg

Jim Williams discusses this trick in one of his books, where he develops a modern version of the lamp-stabilised wein bridge oscillator

Cheers Terry

Reply to
Terry Given

Hi Chris,

Thank you for your comments; they've been helpful.

I started looking at MMIC attenuator data sheets, and it seems like it's a common technique. IP3 usually isn't very impressive, however -- the method is a lot more applicable to receivers than transmitters.

Ah, so that's why the MMIC data sheets show gate resistors...

Hittite has something like that:

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... nice little device (the real information on it is in an accompanying app note).

I'll keep that in mind... I probably need about 10-12 bits of switching, but at HF I imagine I can still get away with the PCB area needed to do that.

---Joel

Reply to
Joel Kolstad

Hello Joel,

I can't see all posts in this thread because my ISP's server dropped some. If it's a controlled RF attenuator you are looikng for and cost isn't the first priority look at Mini Circuits. They make stuff like that.

formatting link

--
Regards, Joerg

http://www.analogconsultants.com
Reply to
Joerg

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