On Sat, 29 Jul 2006 00:00:12 -0700, Roy Lewallen
wrote:

Sorry, I have almost no experience in using dual-gate MOSFETS, for the
reasons I mentioned. Beginning somewhere around the mid-80s, when
manufacturing became highly automated, my employers strongly discouraged
designs which included any adjustments. Besides the labor required to
make the adjustment, the variable component lowered the product's
reliability, so we'd often design in a lot of parts to get around having
any tweaks. But individual adjustment is still a viable option for some
products. What I don't know is whether you'd get satisfactory
performance with widely differing devices all running at the same drain
current. That would depend on your design and application.

If you're considering making a product, I'd certainly do some modeling
with extreme component parameters to see what happens. And you might
consider some sort of device selection and/or incoming inspection or
sorting to make sure you don't get any truly extreme parts. One thing to
be careful about is that when a part is so poorly specified, other
companies might be buying large numbers of selected parts. That leaves
you with the leftovers. I've seen some really strange distributions
resulting from this -- parts with extreme characteristics on both ends,
but nothing anywhere near the "typical" values. It used to be common
with zeners, until they got better at making -- 10% tolerance zeners
would all be between 5 and 10% from nominal, in both directions, with
none closer than 5%. Those had been selected out and sold as 5%
tolerance parts.

Just a few of the things I've picked up in 30 or so years as a design
engineer.

Roy Lewallen, W7EL

Roy,

There are a large number of VHF and HF radios both past and present
that use the DGmosfets in place like the RF amp sometimes a balanced
mixer. A recent example is the TenTec 526 (6n2) 6 and 2 meter radio
as the RF amp. Another current example is the TT 1208 transverter
also as RF amp. If your not pushing the device for max gain or
ultimate IP3 possible they perform well and offer low noise for their
power needs. Neither require pots or other tweaking (other than
tuneable circuits). Can other deivces be used to do better, yes.
But, engineering is always about understanding and compromize.

I'd never use DGfets for something like a scope amp or other
instrumentation where DC operating point or balance is a
requirement. I have use them as a high impedence (1mohm)
AGC'd input for high input senstivity frequency counters.
Another place where I've used them is IF amps, they are just far
easier to AGC than CA3020 or MC1350 and quieter.

My favorite line from years back.

Good, Fast, Cheap, Pick any two.


Allison