Hi Venkatesh,
With your darlington transistors swapped in the wrong places and labelled wrong, and the top diode backwards, I think your circuit might have produced smoke instead of a sine-wave.
I corrected your sketch. If the input resistor's value is high enough, it will produce the same sine-wave waveform as the mains, except with a very small "crossover distortion" notch at zero crossings.
The darlington transistors are "complimentary emitter-followers" and boost the current output of the opamp. :lol: