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Hi I am trying to build a colpitts oscillator with frequency of 10K Hertz. This circuit works fine with L2, C3 and C4 as they are in the picture. However, there is no way I achieve 10mH of inductance even with winding a toroid due to lack of material. So I had to decrease it to 100uH as this is the inductor with highest inductance that I can find in my school's lab. I re-found capacitor values as 2.5uF and 0.25uF using a online calculator with feedback fraction of 10%. However, the circuit does not work now. I tried and played with everything, even I tried unity gain amplifier to isolate tank circuit output before it is given to C5 for coupling, but it does not work. What can I do? Oscillation does not start. The bjt below has approximately 0.7V base-emitter turn on voltage and its current gain is about 300.

the circuit that works fine

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  • \$\begingroup\$ With a very low-impedance resonator (using 100uH), you'll need much more collector current to achieve oscillation. If you have a 300mH inductor, why not use that as the resonating inductor instead? \$\endgroup\$
    – glen_geek
    Apr 8 at 14:03
  • \$\begingroup\$ @glen_geek I dont have. Also I have to figure out with what I will replace that L1 inductor. This diagram ideal and not fully buildable by me \$\endgroup\$
    – OnurTR
    Apr 8 at 14:14
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You might try using the resonating inductor to feed DC voltage to the transistor collector in a different arrangement of so-called "Colpitts" oscillator:
Colpitts oscillator


Be aware that using a 100uH inductor at 10 kHz gives a low-impedance resonator. If you want decent output amplitude at the collector, a lot of collector current is required.
Don't forget to add a large capacitor from the top of the resonating inductor to the bottom of those two resonating capacitors (C4, above). Much of the resonating AC current will likely flow through this capacitor.

A larger-value resonating inductor would improve this oscillator.

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