The circuit in the figure employs positive feedback and is intended to generate sinusoidal oscillation.
If at a frequency $${{\text{f}}_0},{\text{B}}\left( {\text{f}} \right) = \frac{{{{\text{V}}_{\text{f}}}\left( {\text{f}} \right)}}{{{{\text{V}}_0}\left( {\text{f}} \right)}} = \frac{1}{6}\angle {0^ \circ }$$     then to sustain oscillation at this frequency