This small piece of circuit is supposed to provide some extra control to an oscillator circuit we built.

Could anyone perhaps describe how it might operate please?

It was added in parallel with feedback resistor in a Wien Bridge Oscillator circuit and I am struggling to explain it properly

Thanks

 

Extra control in what sense?

The parallel branch with the anti-parallel diodes is a non-linear section. Current will not flow in this part of the overall feedback branch until the diodes are forward biased - typically one would see ~0.6-0.7V forward bias on the diodes. So when the diodes conduct as the oscillator output increases (in both the positive or negative excursions) the effective feedback resistance changes (decreases).

This might then provide a form of output limiting near the peak excursions which is probably intended to modify the oscillator wave shape - perhaps to make it more sinusoidal....?

 

Yes, it's an amplitude limiting scheme. The idea is that the loop gain should be a bit more than one at small amplitudes, with (mainly) the large resistor providing feedback. That allows the oscillator to start up, but if nothing was done about the loop gain the amplitude would build until the amplifier clipped.

The branch with the diodes is introduced to provide a rather softer kind of limiting, so that whenever the signal is big enough to turn on the diodes more feedback is applied. This is intended to stabilise the amplitude with softer clipping than would occur if the amplifier was limiting.

The result can be a fair semblance of a sine wave if the resistor values are carefully chosen, but don't expect miracles in terms of low distortion.

 

This might then provide a form of output limiting near the peak excursions which is probably intended to modify the oscillator wave shape - perhaps to make it more sinusoidal....?

The opposite happens. The peaks get squashed a little which adds odd-harmonics which is distortion.

 

I was thinking more like what is shown in the attachments.

The waveform changes from a "square wave" shape to a more "sinusoidal wave" shape depending on the feedback variable pot value 100% to 0%. The frequency of oscillation and output amplitude also change but that would be expected.

The harmonic distortion in the "square wave" will be higher than in the "sinusoidal wave".

Hope that clarifies my meaning.

 

Of course the opamp must not be allowed to have clipping.
Diodes are non-linear and squash the waveform to limit the amplitude. The squashing causes distortion.
A long time ago a light bulb was used as the amplitude limiter by using its resistance changed by changes in its current. Its very slow response produces low distortion but causes the amplitude to bounce before it settles down.
Then an LED shining on an LDR was used. Then a Jfet was used.