Among the many uses of the operational amplifier is in an oscillator circuit. A positive feedback loop is used as shown in the circuit below.
Suppose the output voltage is positive at some time. A fraction
is fed back as positive feedback. The voltage across the capacitor rises exponentially as it is charged. After some time related to
the voltage at the non – inverting input, Q exceeds that at the inverting input P. The op – amp switches to negative saturation 
and because of the positive feedback the + terminal becomes negative.
The capacitor now charges in the opposite direction and the voltage at P falls until
at which point the op – amp switches to positive saturation and the cycle repeats.
The result is that the output voltage is given in blue in the graph below, and the voltage across the capacitor in red.
The frequency of the circuit is given by