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See the circuit. I am running this from a 24 V supply. I need it to oscillate at about 700 Hz.

enter image description here

Does anybody see anything wrong with the circuit? I have spent about 3 days trying to figure it out, but I cannot seem to do so.

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    \$\begingroup\$ Where are your power supply decoupling capacitors? What does your PCB layout look like? \$\endgroup\$
    – Andy aka
    Aug 24, 2022 at 20:55
  • \$\begingroup\$ I do not have a PCB, I have this wired up on a breadboard right now. I have added decoupling caps and that did not resolve the issue - They are just not shown in the model. \$\endgroup\$ Aug 24, 2022 at 21:11
  • \$\begingroup\$ Breadboards are horrible. Are your wires poking up in the air and hanging all over? You want this built with a small, neatly laid out circuit. No flying probes. Every wire is a potential transmitting and receiving antenna. \$\endgroup\$
    – Kyle B
    Aug 24, 2022 at 21:17
  • \$\begingroup\$ Hi Kyle - I agree, breadboards are horrible, but I cannot afford to make a PCB for a prototype every time (If you have other suggestions - please share). I understand noise is an issue, but I expect to see some response or oscillations - I am not seeing any. \$\endgroup\$ Aug 24, 2022 at 21:50
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    \$\begingroup\$ @MissMulan You absolutely can have RC oscillators. Hewlett-Packard started their company with a Wien Bridge oscillator. Phase shift oscillators exist. For the theoretical background that makes this possible see the Barkhausen stability criterion \$\endgroup\$
    – Graham Nye
    Aug 24, 2022 at 22:30

3 Answers 3

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Firstly, you are trying to produce a bipolar output from a single positive supply. You need to either provide a bipolar supply or create a decoupled virtual ground at 12 V (i.e. half the supply rail) and return R5 and the bottom of R8 to it. (h/t to Jens). Another technique is shown in this Analog Devices application note.

Secondly, to get a low distortion output you need the closed loop gain of the op-amp to be exactly 3. However to get the oscillator to start up reliably you want it to be intially somewhat higher.

It's normal to use some form of automatic gain control to adjust the gain appropriately. Traditionally you used a thermistor but finding these now is like looking for hen's teeth or an index-linked final salary pension. HP famously used a small incandescent pilot lamp which is great if you have a valve/tube oscillator running at a few watts (though your high-power op-amp might qualify). Modern (low-power) op-amp circuits can use a rectifier-controlled FET or a LED/light-dependent resistor to adjust gain. There's a diode option but this can lead to distortion. Feel free to Google for example circuits.

You have a nominal fixed gain of 3.06. Given resistor tolerances that might either give you insufficient gain to start or enough gain to cause distortion.

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  • \$\begingroup\$ Hi Graham, Thanks for your response. I tried the above (biasing pin 4 to 12 V) and that does not do it. I have also tried adding a potentiometer to fine control the impedance to try and get the gain right, but that does not do it either. The OP AMP does work because when I connect it as a simply inverting/non-inverting amplifier it behaves as expected). \$\endgroup\$ Aug 24, 2022 at 21:47
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    \$\begingroup\$ @SlowMovingAir You must bias R5 to 12 V as well \$\endgroup\$
    – Jens
    Aug 24, 2022 at 21:52
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This question has come up many times. I was curious so I built one two years ago.

You need a bi-polar supply, as already mentioned.

Why are you using a high-power opamp? Have you tried a normal opamp? If you need more driving capability then put the amp after the oscillator.

The gain must be exactly 3 to get a non-distorted sine wave. Too little gain and it won't oscillate. Too much gain and the output will be distorted. A common solution is to use a non-linear element like a light bulb. This bulb has about 500 ohms at 1 Vrms. The output will adjust itself until it the light bulb warms to 500 ohms. This will occur at about 3 Vrms output.

This is a good circuit for a demonstration. If you want to build a variable frequency generator, beware. This is a very finicky circuit, at some frequencies it is unstable, it amplitude modulates.

enter image description here

enter image description here

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Here is a "general" circuit. Just switch whatever you want.

A parameter is also used (Fact) for seeing when/how oscillation starts (condition).

Made with a FREE simulator microcap v12 with interactive capabilities.

enter image description here

And with Bipolar supplies, Note that the oscillator does not start in the same way.

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Added harmonic distortion of the SECOND oscillator (h1,h2,h3,h4,h5).

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  • \$\begingroup\$ I just built this, it works good, the harmonic distortion is lower than I expected, the 3rd harmonic is -40 dB, only a little more than the light bulb method. My frequency adjustment with a ganged pot isn't working too well, probably because my cheap pot isn't tracking well. \$\endgroup\$
    – Mattman944
    Aug 25, 2022 at 15:49
  • \$\begingroup\$ If the gain is too "high" then the harmonic distortion is higher. I add the calculated distortion for reference in the answer. \$\endgroup\$
    – Antonio51
    Aug 25, 2022 at 21:17
  • \$\begingroup\$ NB: the feedback with the diodes can be changed for minimal distortion, but it takes time to find the "best" network (probably the "parabolic" function) \$\endgroup\$
    – Antonio51
    Aug 26, 2022 at 5:39

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