Why doesn't this Wien Bridge oscillator work?

Question

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

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.

Answer 1

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.

Answer 2

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.

Answer 3

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.

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

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