Fet J310

Posted : admin On 1/29/2022
Fet J310
  1. Transistor Jfet J310
Fet J310
U310 FETpreamplifier

The J310 has a noise figure of around 1.5 to 2 db in the 6 meter band depending on the device and IF it matched (mismatched may be moer like it) to the antenna. Don't recall the gain in anything but one fet in. Adaptation to the circuit: the circuit also works properly when you disconnect the backcoupling coil (L1) and connect the 1K5 source resistor directy to grou. JFET VHF/UHF Amplifiers, J310 datasheet, J310 circuit, J310 data sheet: ONSEMI, alldatasheet, datasheet, Datasheet search site for Electronic Components.

This FET preamplifier has been around for almost 25 years. It isextremely reliable and can handle several watts directly into eitherthe input or the output without failure. Noise figure is typicallyaround 1 db and gain is about 13 db. It is unconditionally stable. Itis also very re-produceable. Layout is not critical but it shouldfollow the schematic in layout fashion. It is also wise to put it inan enclosure.

Noise figure bottoms out in the U310 from 12 to 18 mA which isbetween 1.0 and 1.5 db typically up to 144 MHz, and about 1.8 db a222 MHz, which are all you really need for virtually all terrestrialapplications. EME is a different story. Output compression point isabout +12 dbm. The U310 is the top of the line in it's family. It'sin a metal can where the gate is the case. The E310,309 & 308will give a much worse noise figure. I usually mount them upside downfor a shorter gate lead but you don't have to do that. Alignment issimple: set input capacitor mid range for 28 & 50 MHz and nearmaximum for 144 & 222 MHz and tune the output for max gain. Thatwill put you very close to optimal alignment. A noise figure meterwill help but it isn't really needed in most cases as you will beclose. In cases where you want to put a great little preamplifier inyour favorite radio merely build this critter on a very small boardand use hard soldered coaxial cables for the interfaces. I havebuilt them as small as 3/4 inch square.

Fet J310Fet j310 equivalent

For your info, I only use U310 preamplifiers on all bands from 50thru 222 MHz for terrestrial weak signal operating. When used inconjunction with good band pass filters you can bring any good radioup to its best sensitivity withour blasting it with 24 db gain from aGaAs FET preamplifier. Your radio won't handle that much gain anyway.Stick with the 12 db gain and you will be much happier with yoursystems performance. These FET preamplifiers are very forgiving toaccidental over-drive. They will handle at least 3 watts CW to theinput and 25 watts directly into the output without damage (amazing).


C in

Input Inductor

Output Inductor

C out


Bypass cap


28 MHz


20 turns T25-6 #30

24 turns T25-6 #30



.01 micro F

10 micro H

50 MHz


13 turns T25-6 #30

16 turns T25-6 #30




5 micro H

144 MHz


5 turns #18, 1/4' dia

5 turns #18, 1/4' dia



200p - 300p

2 micro H

222 MHz


3 turns #18, 1/4' dia

3 turns #18, 1/4' dia




1 micro H

performance plots to follow soon.......

Transistor Jfet J310

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