Op-Amp positive feedback design

The following circuit has a voltage source that operates between 3V and 5V and is connected to the Op Amp’s positive input, across a resistor divider R2 and R1 at the node Va. The simulation is run for 200m secs.

opamp_gnd

The design is as following: applying super position

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Following is the Spice file :  opamp_gnd

Video :Positive Feedback