J310 noise figure?

[brandon7861]

I never have enough jfets so I ordered some SMD j310's. Without looking at datasheets, I went with 50 of the $0.50 model because the # in stock suggested they were the popular ones. I also grabbed a couple of the "low noise" versions for $5 a piece just to play with. The cheap ones do not list the noise figure on the datasheet, just the noise voltage (something I understrand to only matter at low frequencies because they were measured in the 1/f region, maybe I misunderstand how that works). The low noise version claims about 1dB NF at about 100MHz, but what can I expect from the $0.50 version? What kind of difference should I expect?

Do people use the cheap ones for receivers and mixers, or is that a bad idea?

Thanks!

 

[Justme]

The J310 is a high-Idss device, much higher than the general JFETs like your MPF102, 2N3819, 2N4416, etc. It's also the defacto device (along with the same die in a metal can, the U310 which is what I use) for almost all active whip-amps that use a JFET as the input device. (The RF System's DX-1 Pro and the Amrad whip-amp use an RF power VMOS FET with much higher input capacitance.)

The J308,309,310 are all in the same family, graded by Idss, with the 310 the highest minimum Idss; all 3 have about the same maximum Idss (Idss = "drain current with gate shorted to source"). I have determined in all my active whip-amp tests that best linearity occurs in the range of about 8-12mA drain current. Most seem to fall in the 9.5-11mA range.

With most JFETs you can measure Idss, but this family will heat up and destroy itself if you let unrestrained drain current go for more than several seconds; the factory determines Idss for these with a pulse test.


BASIC SPECIFICATIONS ARE:

25V max drain-gate voltage,

25 V max source-gate voltage

10mA max forward gate current

360mW max power dissipation.

Gate-source cutoff voltage 2.0-6.5V

Idss 24-60mA

Gate-source forward voltage 1.0V max

Common source forward transconductance (gain)
8000-18,000 micromhos

Gate-source capacitance (what the input sees in an
active whipamp circuit) 5pF max, 4.3pF typical

Noise Figure at 105MHz, 1.5dB typical; 2.7dB typical
at 450MHz

Wide dynamic range--greater than 100dB

Low-noise device, good for oscillators, mixers, amplifiers

J310 PINOUT:
Hold device with leads pointing toward you with
flat face up--left to right--drain,source,gate
(device is not symmetrical, i.e. source and drain cannot
be interchanged)​

Low noise as well, i don't see benefit of the other device