Simple Circuit Problem

[Insane1]

I have some problem for my circuit below. Suppose at point A, I will get a waveform as shown in the graph below. But, instead of getting the desire results, I got a sine wave ( Vp-p = 3.XX volts ). Actually I manage to get the desire results at the beginning of my test - but after a whille the waveform change into a sinewave with smaller amplitude and remain!. What's wrong with my circuit? Do I need to make ay changes?

View attachment 35589

 

[ZomBiE]

try changin diodes to superfast types. for example: UF4007, MUR1100 or similar.

 

[shekhar_dandya]

Rather than superfast diodes,diodes with low cutin like Ge diodes must be used.the waveform is merely showing 2 diode drops,0.7V peak.Its practically going to happen.
Actually,what's the aim of the circuit?