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I use the following Colpitts oscillator schematic and I always need to put the scope probe on the inverting input of the opamp to make it start. Otherwise I get a DC voltage and no oscillations.

Do you have any idea about what I can do to to have a reliable startup at powerup ?

Thanks.

colpitts oscillator

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Try reading this tutorial: -

enter image description here

Note the use of R3 to prevent the op-amp output being directly loaded by one of the capacitors. Theoretically you have to have some phase shift between output and first capacitor and therefore you need a resistor.

Note also that the gain in your circuit is far too high.

I'm not ruling out anything else too.

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  • \$\begingroup\$ Yes - I agree to Andy`s recommendation regarding R3. The BJT acts as a current source - in contrast to an opamp which is a voltage source. For this reason an opamp based Colpitt oscullator needs such a resistor R3. \$\endgroup\$ – LvW Oct 20 '15 at 13:59
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Given that this is an old question, I will provide another answer just for reference of future generations. As a fellow electronics student, I found the following simple Falstad circuit to be helpful in understanding Colpitts Oscillator with an op-amp:

enter image description here

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  • \$\begingroup\$ So your answer is that s/he should add a 1kΩ resistor at the output of the op-amp? \$\endgroup\$ – Harry Svensson Jan 1 '18 at 8:13

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