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So I built the Wien bridge oscillator without automatic gain control and I am using a preset to tune the circuit instead.

My question is this, so when I reduce the gain of the op-amp to something smaller than 3, I can see the oscillations die out slowly on my oscilloscope. When I turn the preset and increase the gain to maybe 3.2 I still get stable oscillations, but larger Vpp and no clipping. Considering the Barkhausen criterion shouldn't the circuit render unstable, and the oscillations begin to clip?

This only happens when I increase the gain to larger values than 3.4. For a gain approximately equal to 3.3 I get a very weird sine wave almost about to clip. I'm curious to why this behaviour is incremental, I thought for a gain greater than 3, it should automatically clip.

For reference I am using the UA741 and my circuit is configured the same as the picture I included.

enter image description here

Values of capacitors are 10nF and resistors are 1060 ohm to obtain 15kHz oscillations. R4 is equivalent to my preset (20k) and R3 is composed of two resistors in serioes of 20k each.

Sredni Vashtar
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Bonavia
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1 Answers1

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When I turn the preset and increase the gain to maybe 3.2 I still get stable oscillations, but larger Vpp and no clipping. Considering the Barkhausen criterion shouldn't the circuit render unstable, and the oscillations begin to clip?

Slew rate limiting is something to consider: -

enter image description here

Picture source

If your peak voltage is 10 volts (at 15 kHz) then the maximum rate of change is 0.94 volts per micro second. Given that the 741 self-limits at 0.5 volts per microsecond this is the likely cause of the output amplitude appearing reasonably stable with some distortion. You might see something like this at the output (blue): -

enter image description here

Picture source

As can be seen (in this exaggerated scenario), if the input sine wave (red) increased in amplitude, there would be no increase in the output amplitude. This is a form of output amplitude limiting. In your particular scenario, I suspect that the limiting is only marginal but enough to act as a form of amplitude stabilization.

Reasons not to use a 741 - maybe you have found a good reason to use a 741 op-amp?

Andy aka
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    Very nice catch there, Andy. – analogsystemsrf Feb 27 '18 at 15:31
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    Sounds reasonable, easy check is to reduce frequency by a factor of ten or so. – George Herold Feb 27 '18 at 15:32
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    Yes - the large-signal bandwidth of the 741 type opamp is app. 10kHz only (due to the slewing effect). More than that, the induced slew rate distortions are connected with an additional phase shift, which somewhat detunes the whole circuit. Both effects are combined and result in the observed waveform. – LvW Feb 27 '18 at 15:39