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I have a question about op-amp Colpitts oscillators. I am a hobbyist building a sine wave oscillator circuit for an experiment.

The frequency of the output is one of the variables in the experiment so the actual value doesn't matter too much, but will be in the range of 10-100 kHz. I have tried the circuit below without success in getting it to oscillate.

The single-supply voltage is 6 V. The problem is that the output produces a steady 5.7 V. It measures 5.7 V at the negative input pin as well.

There is no detectable voltage drop with my DMM across any of the resistors so the current flowing must be very small.

I have tried JRC4558D and NE5532 op-amps with the same result.I can get it to oscillate at the expected frequency with a 741 op-amp but the output is distorted at this frequency range, which won't do.

I have tried adjusting the R3/R2 ratio as well as the various values of R1 from 0 to 1 kΩ without any change in the output whatsoever. I have also tried removing R4 without any effect. I'm having trouble understanding where I am going wrong.

enter image description here

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  • \$\begingroup\$ After correction of input + at 3 V, tested with TL072C. Ok. Frequency of about ... 19 kHz, Duty ~ 50 %, quasi "rectangular" wave. \$\endgroup\$
    – Antonio51
    Commented Nov 28, 2021 at 16:54
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    \$\begingroup\$ That is designed for a bipolar supply, say +/-5V. \$\endgroup\$
    – user16324
    Commented Nov 28, 2021 at 17:17
  • \$\begingroup\$ Stackexchange is not the right site for long discussions -- but a Colpitts oscillator is just not a good circuit to use at these frequencies. If you just start with "I want it adjustable and low distortion" then the size of the capacitors and the coil get huge (like, several Raspberry Pi, if not dishwasher huge). This is why the go-to analog circuit for this is a Wein bridge oscillator, and why the serious professional way to do this involves digital circuitry to synthesize the sine wave. \$\endgroup\$
    – TimWescott
    Commented Nov 28, 2021 at 18:11

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You are using a single supply hence, the DC voltage on the non inverting input needs to be mid rail or about 3 volts. You can't expect most opamps to work like this withput correctly getting both inputs to mid rail.

You must also choose an opamp that is capable of working on a single supply as low as 6 volts. Some 741 models might but, they really are poor devices to use in this type of circuit due to their many weaknesses of performance.

You will also find that an opamp circuit needs gain stabilization else the output crashes peak to peak into clipping points near the supply rail voltages.

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