J310 and U310 FETS

By Steve Ratzlaff

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)