oscillator

[dwuihf]

what is the difference in oscillation wave at point X and at point Y? for my knowledge, the diodes block the distortion that caused by OP amp and if i show the waves at node X, that means i am showing distorted waves,but i did not see this in both circuit. both circuit showing exactly same waves with same amplitude and non distorted even if i bypass the diodes!!!! kindly, i need advice and explanation.

[supsup]

Comparison of signals in points X and Y will not give the information on work of elements. To understand appointment of these elements, it is necessary to compare work of the device to them and without them. Try to understand appointment of each of elements, having compared oscillograms of these schemes. Pay attention to sensitivity of an oscillograph (Scale A) in all three cases.
(I am sorry for my not so correct English).

[dwuihf]

thanks.. i saw the difference in the waveforms for all the three circuits, but now how to summarize the function of the diodes?

actually in these circuits,when the diodes installed the output waveform looks like sinewave and without diodes ;the waveform is square waves,which is opposite on the theory operation of the diode? because the diodes here used to conduct the distorted parts of the output voltage.

i was suspecting that,when there is diodes in the feedback side,the waveform wil be square .  

[supsup]

In this scheme diodes, basically, serve only for restriction of target pressure. Considering their nonlinearity, on an exit the sine semblance turns out. However, the good sine in the scheme with Wien bridge can be received only at use in a chain of feedback MOSFET or a vacuum lamp.