Manipulating Amplitude and Axis of symmetry of BUBBA oscillator in Simulink
Your questions don't make a whole lot of sense.

What is the purpose of Vpp in the first place? It just creates a DC offset for the entire circuit. And clearly it is not 20V &mdash; from your waveforms, it is more like 0.5V.


  
  Make the sine output symmetric about the Y-Axis
  


Set Vpp to zero.


  
  Change the amplitude of the output to -1 &lt;--> +1
  


Use a separate gain stage to set the amplitude to whatever you want.

There's no reason to expect that the amplitude of the oscillator should be any particular value. Indeed, the output of each stage is approximately 0.707&times; the previous stage. This is why after 4 stages, you need a gain of -4 to close the loop and sustain oscillation. The only thing that stabilizes the amplitude is the fact that the output of the -4&times; amplifier saturates. (But I'm not sure how that works in your simulation if you're using "ideal" opamps.)



Note that the outputs labeled "sine" and "cosine" are actually only 45&deg; phase shifted from each other, so I'm not sure what those labels are actually supposed to denote. And I have no idea why the the "sine" output is taken from the input side of the third buffer instead of the output side. This configuration is found in multiple papers found online, so I suspect that people have simply been blindly copy-and-pasting without thinking about it at all. It's probably a typo &mdash; the "sine" output should be taken from the other side of the resistor, directly from the output of the previous opamp.