VCR2N Voltage Controlled Resistor Fet (Siliconix)

[electrosonic]

I see electronics goldmine has these on sale. Anyone familiar with these? How are they different than regular jfets?

Here is the electronics goldmine link...

http://www.goldmine-elec-products.com/prodinfo.asp?number=G4257


Andrew.

[Earthscum]

I second your question... I've always wondered about these things. I never bought any to try because they are so expensive compared to a typical fet.

[R.G.]

They are in fact JFETs, perhaps with a couple of resistors inside the package - can't remember, it's been a long time. They were sorted by their sensitivity to control voltage as I recall, eliminating one of the problems with JFETs as variable resistors.

Other than that, they have all the advantages and disadvantages of JFETs used as variable resistors.

[Cliff Schecht]

No resistors inside the package. I think they are sorted by their transconductance in such a way that you get similar Rdson vs. Vgs characteristics from device to device. Also not all JFET's are symmetrical devices but IIRC these are meaning you can swap the drain and source all willy nilly and not run into issues.

[PRR]

Their own documentation on the VCR2N is inconsistent:



The plotted curves work out to VGS(off) of 3.16V, not the 4V given on the sheet.

Note that to get any resistance very different from about 40 ohms, you need to know the VGS(off); however this varies device-to-device by a factor of two (3.5V to 7.0V). Pick two parts from the bag, you may need 2:1 difference of gate voltage to get a desired resistance.

In any case: this points out a possible difficulty with any FET. There's a range of maybe 1:10 where large chage of voltage makes reasonable change of resistance. However the voltage from 690 ohms to infinity (max resistance is not specified but is surely over 100K) is in the small crack from 3.0V to 3.16V (or 6.0V to 6.32V for another JFET from the same bag). Since we often want to sweep our JFETs 100:1 or more, a lot of the desired range is twitchy and uncertain.

[Cliff Schecht]

I think they are designed specifically to be very linear when working in the triode region. Not all FETs (JFETs and MOS) are linear here. Many have a soft knee that extends well into the triode region or have a very small triode region which means a lower threshold voltage (usually in the interest of increasing available swing for low supply voltages). Really it looks like Siliconix mucked with the doping and/or geometries in such a way to get a decently linear set of curves when compared to other FETs that we typically use, at least for the VCR2N/4N. Looks like the VCR7N in much less linear than the 4N or 2N, at least according to the curves in the datasheet. I doubt this would be noticeable with the standard linearizing resistors though.

The datasheet for these parts is pretty poor. Very little information and what is there is unclear. Pretty annoying when you have to do a lot of guesswork to use a part..

Also I agree with PRR that FETs are horribly inconsistent. In a lot of applications this isn't a big deal but in some situations it can be a real headache and/or deal breaker. Siliconix is guaranteeing about a 30% spread in Rdson which really isn't bad but isn't spectacular either. Circuits requiring precision would likely require trimming which adds cost and complexity in multiple ways. In a voltage controlled filter, for example, the poles would not line up very well which can make resonance circuits do funny things (this is why I like the poles to be fairly close in my filters). I've designed trems that use JFETs as the modulated resistance element and found that these designs all invariably needed trimmers. Once they were tuned in they sounded good but not really any better than other trem circuits.

Also I've never actually seen these used in a commercial design FWIW.

[R.G.]

Also I've never actually seen these used in a commercial design FWIW.

Me neither.

When these first came out - late 70s? 80s? - I saw them and thought that they'd be the solution to a number of issues for me. In the next couple of weeks, I messed with them and finally decided - naah, not too useful - learn about selecting ordinary JFETs instead.