You can increase the number of segments used to approximate a curve using the *"Segments*" parameter of the QGIS's [Buffer][1] tool:

150m buffer created using the default 5 segments on top of a buffer created with 100 segments:

[![enter image description here][2]][2]

  [1]: https://docs.qgis.org/latest/en/docs/user_manual/processing_algs/qgis/vectorgeometry.html#qgisbuffer
  [2]: https://i.sstatic.net/zd13I.png