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I've designed, simulated and build this simple JFET colpitts oscillator and I couldn't get it to work with real parts. I didn't have BF256B jfet model so I simulated it using some generic one. The circuit oscillates at correct frequency but amplitude varies with time and sometimes it doesn't oscillate at all. I tried to make it oscillate at lower frequencies to see if it would be stable there so I replaced L1 with 10µH inductor, C2 with 2.2nF cap and C1 with 10nF cap and I couldn't get it to oscillate at all (I also tried 2.2nF C1). This is my first time using JFET so I might be doing something fundamentally wrong. Does anyone see any possible issues with this design?

schematic

simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ I wonder how one actually finds the proper values of C1 and C2? of course the loop gain should be >1 and the capacitance C1*C2/(C1+C2) needs to be resonant with L1 at the desired frequency, but there is an infinite amount of combinations of C1 and C2 which satisfies these conditions. Also L1 may be chosen arbitrarily, doesn't it? \$\endgroup\$
    – T. Pluess
    Mar 16, 2019 at 16:52
  • \$\begingroup\$ @T.Pluess I just lock L1 and loop gain value so that this becomes solvable. L1, C2 and C1 need to be chosen so that parasitics won't interfere with oscillation. \$\endgroup\$
    – zakrent
    Mar 17, 2019 at 17:45

2 Answers 2

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It turns out I did my math wrong and my loop gain was less than unity, using wrong jfet in simulation didn't help to catch that issue.

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  • \$\begingroup\$ We've all been there. \$\endgroup\$
    – Hearth
    Jan 31, 2019 at 1:34
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    \$\begingroup\$ Also consider that JFET parameters vary so much from specimen to specimen, e.g. IDss can easily spread over 1:4 range. Do your math for the worst case. \$\endgroup\$
    – carloc
    May 11, 2019 at 17:27
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The cutoff voltage of the jfet should be less than the supply voltage. The capacitances are rather small, and the gate-source capacitance is nonlinear and of a similar magnitude. Increase the supply to 9 volts, capacitances to 100 pf, and the inductance to 10 uH, and you'll see some action!

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  • \$\begingroup\$ Also, increase the source resistance to 3.3k to improve waveform purity. \$\endgroup\$
    – M0RJG
    May 11, 2019 at 8:26

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