New JFET by TI - very tightly specced Vgs(off)

Looks like digikey has the dual now, at $4.61 I guess you're saving money if you need more than one haha.
Looking at the datasheets I noticed the transconductance is much higher than a j201 on the new jfets, my understanding is that this is the gain of the transistor, which would make this a very high gain device? the j201 number i see is 500 microsiemens minimum and the jfe2140 the lowest i see is 2.1 millisiemens, but up to 30mS. I don't understand jfets at all still so I'd love to learn if I'm not interpreting this correctly.
 
Looks like digikey has the dual now, at $4.61 I guess you're saving money if you need more than one haha.
Looking at the datasheets I noticed the transconductance is much higher than a j201 on the new jfets, my understanding is that this is the gain of the transistor, which would make this a very high gain device? the j201 number i see is 500 microsiemens minimum and the jfe2140 the lowest i see is 2.1 millisiemens, but up to 30mS. I don't understand jfets at all still so I'd love to learn if I'm not interpreting this correctly.
In his amazing thread about jfets, Chuck has lectured me briefly about transconductance information in datasheets (https://forum.pedalpcb.com/threads/jfet-biasing-part-1.5177/post-143159): what matters most are Vgs-off and Idss (that you use to calculate the gain of a fet stage). In the datasheet, transconductances are typically measured at extreme conditions
 
Where can one find adapter PCBs for these SOT23-5 (5-pin) SMD devices? I got some 6-pin adapters, but they are a bit large for these JFE150 JFETs.
 
Has it been determined if this is a viable J201 substitute?
These may or may not be a drop-in replacement for J201. Depends on the circuit. They will work in circuits that like J201's with their Vp at the high end of the range. I can tell you they will not work in the Calamity Fuzz (Fairfield Circuitry Unpleasant Surprise). The higher transconductance will increase the gain in circuits where the source resistor is bypassed (SOB 2nd & 3rd stages, for example).

I just perused the datasheet. Some interesting stuff there. Reading between the lines, I strongly suspect that this device is fabricated by putting many (20 or more) JFETs in parallel. Doing this will increase Idss and gm by N (N = number of parallels FETs) and decrease the voltage noise by the square-root of N. Power MOSFETs contain 100s of devices in parallel, so the technology is not new.
 
Surface mount J201 are still widely available though no?
(This one is also SMD so no benefit there)
I suspect the "benefit" would be that the JFE150 has a tighter Vgs(Off) range specification than the J201.

The J201 has a Vgs(Off) range of -0.3v to as low as -1.5v

The JFE150 has a Vgs(Off) range of -0.5v to as low as -1.1v with the Ids @ ~2mA - and from -0.7v to -1.3v with the Ids @ ~100µA

The tighter Vgs(Off) range makes it easier to find a proper/usable Vgs(Off) value when selecting a specific N-Ch JFET for a given circuit.
 
Vgs,off is usually measured with Ids at or below 1μA. This is from the JFE150 datasheet:

1673895823767.png

The 2nd line, and only the 2nd line, represents Vgs,off. The bottom two line represent Vgs at two specific Ids values.

The tighter Vgs,off range is helpful IF the circuit requires a Vgs,off in that range. Can also come in handy if matched JFETs are req'd, such as in a phase shifter.

NB: the Calamity requires a Vgs,off at close to -0.3V to function correctly.
 
Vgs,off is usually measured with Ids at or below 1μA. This is from the JFE150 datasheet:

View attachment 40349

The 2nd line, and only the 2nd line, represents Vgs,off. The bottom two line represent Vgs at two specific Ids values.
Thanks for that clarification. It was a bit unclear which line(s) were specifically pertinent to Vgs(Off). Now I see it's the line with "cutoff" in the description. (Missed that on the first go-'round.) That put the JFE150 at a Vgs(Off) range variance of only ~0.6v at max. The J201 has a Vgs(Off) range variance of ~1.2v
 
I finally got off my butt and tested a JFE150 in a Rockman-style compressor and... it's entirely unsuitable as a drop-in replacement, unfortunately. While Vgs(off) is in the right range, the ON resistance is way, way lower than J201, or the factory-selected JFET in the Rockman units.

JFE150 Rds(on) ~20 Ohms
Rockman JFET Rds(on) ~ 600 Ohms (ish)

Back to hand-selected J201 or J202, I'm afraid... And reading more about transconductance.
 
You can always put a resistor in series with the drain lead. That will effectively raise the on resistance to whatever you like.
 
But in this application it's used as a voltage controled resistor controlling the gain of a non-inverting op amp. It really has to (more or less) match the resistance-vs-Vgs curve of the original JFET, over a wide range of Vgs. 20 Ohms when fully on throws the gain waaaay out of whack, by more than an order of magnitude.
 
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