falstad schematic link


My setup:




I made many attempts at building a Colpitts oscillator, and this one by far supposed to be the most advanced and with high chance of success.

Yet, it's still not enough. I tested voltages on the amplifier and they all seem to be as I predicted in calculations, so the amplifier should be fine. I researched the air gap problem on cylinder rod cores and used this calculator to verify that this solenoid of around 300 turns, 0.1 mm wire, 10 mm diameter, 94 mm rod length, 100 initial permeability (for carbon steel) should have around 25 effective permeability and so inductance is from 5 to 7 mH, which should give around 9 to 11 kHz frequency. Yet all this huge effort that I put into this thing seems like a waste now.

Theory does not work in practice.

What could be the reason?

  • 1
    \$\begingroup\$ Inductor quality is suspect. Quality as in "Q" (perhaps too much internal equivalent resistance). Try some kind of ferrite or powdered iron core instead. That 33nf capacitor seems small. \$\endgroup\$
    – glen_geek
    May 15 '20 at 15:04
  • \$\begingroup\$ When you say "Colpitts Oscillator" I think of oscillators that operate at 50 MHz or higher., at least that's common for these kinds of oscillators. For a 10 kHz oscillator a Wien bridge is much more common. I also think that getting the quality of the resonating circuit high enough is the issue. In the simulator, add realistic series resistances for capacitors and the inductor. Does it then still oscillate? Measure the DC resistance of your home-made inductor. How does that compare to a 6 mH inductor you can buy at Digikey? \$\endgroup\$ May 15 '20 at 15:14
  • \$\begingroup\$ Double check your resistor values, I think some of them are off by an order of magnitude, but it's hard to tell if it's just the rendering. Also, the red (battery?) wire is going to where your ground should be, and the black one is going to only one of the places that power should be going--it should go to the bus bar on the lower right. Normally red would be plus and black would be ground, but you don't show enough of your setup to be sure...double check that too, and then worry about the magnetics. \$\endgroup\$ May 15 '20 at 15:23
  • \$\begingroup\$ Measure the DC resistance of your coil and put that into the simulator. \$\endgroup\$ May 15 '20 at 16:15
  • \$\begingroup\$ Your inductor is far too lossy \$\endgroup\$
    – Neil_UK
    May 15 '20 at 16:30

I think that the low resistor values which you have are reducing your loop gain below unity.

Colpitts Oscillator


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