tuned circuit

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Re: tuned circuit

Post by Jalex2 on Sun Mar 26, 2017 9:12 pm

Thank you 
From your links it looks like I didn't need to isolate the tank circuit.
I could connect it straight to ground.
I searched for pages once but found very little on Fet amps.
I like to experiment with very low power RF circuits as 
years ago I got in trouble with the FCC and almost got fined. LOL

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Re: tuned circuit

Post by Glenndk on Sun Mar 26, 2017 6:51 am

Jalex2 wrote:Hi all
Lets say I have an rf fet like the 310 in a source follower circuit.
a 1k in the source
I put a tank circuit at the gate with a cap to ground and a 1 meg for bios.
the tank circuit tuned somewhere between 50 and 100 mhz.
with a short antenna to the gate.
Would that make a fairly narrow band width at the source?

Hi Jalex2

To some extend.

The 310 do not have infinite input impedance - and it has an input capacitance:
http://www.g1sle.com/files/downloads/j310.pdf
input conductance@100MHz: 0.5 mmhos = 2kohm
input capacitance: ca. 4.3 pF

You have to make a compromise between loaded resonator/LC-circuit Q and that the 310's input capacitance drift with temperature (and voltage).

If LC-circuit has Q=100 unloaded, you could ideally have between 0,5-1 MHz -3dB bandwidth. But the whip also loads the LC-circuit (1 kOhm?), so the Q will be lower.

Many use J-FET at VHF with common-gate circuit, because it has lower distortion and tend to be more stable. A dual-gate MOSFET might rival that.

Jfet cookbook:
http://www.wa5vjb.com/references/preamp-Cookbook.pdf
Quote: "...
Input:
...
Output circuits:
...
Tuning tips:
..."

U310 FET preamplifier:
http://www.ham-radio.com/n6ca/50MHz/50appnotes/U310.html
http://www.qsl.net/yo5ofh/projects/preamplifiers/u301_fet_preamplifier.htm

Preamplifiers:
http://www.i1wqrlinkradio.com/antype/preamplifiers.html
eg:
Interesting:
Missing decoupling over 68kohm gate2:
144 MHz preamplifier with BF981:
http://home.iprimus.com.au/toddemslie/bf981-preamps.html
(look at the good link references at the bottom)

Look in the upper right-hand corner (70-MHz dual-gate MOSFET-preamplifier):
https://www.rudius.net/oz2m/70mhz/transverter_schematic.png
from:
https://www.rudius.net/oz2m/70mhz/transverter.htm


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tuned circuit

Post by Jalex2 on Sat Mar 25, 2017 6:52 pm

Hi all
Lets say I have an rf fet like the 310 in a source follower circuit.
a 1k in the source
I put a tank circuit at the gate with a cap to ground and a 1 meg for bios.
the tank circuit tuned somewhere between 50 and 100 mhz.
with a short antenna to the gate.
Would that make a fairly narrow band width at the source?

Jalex2

Posts : 107
Join date : 2012-12-06
Age : 76
Location : Long Beach , Washington, USA

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