ParametricPlot3D[{x,y,x^2+y^2},{x,-2,2},{y,-2,2},PlotRange->{{-2,2},{-2,2},{0,8}}]

Manipulate[ParametricPlot3D[{x,y,a x^2+b y^2},{x,-2,2},{y,-2,2},PlotRange->{{-2,2},{-2,2},{-8,8}}],{a,-1,1},{b,-1,1}]