Schematic diagram of a twin T oscillator, a type of RC oscillator.  It is a feedback oscillator that produces a sinusoidal output.  The circuit in the blue box is a twin T filter; it is a notch filter which passes signals at all frequenies except at its notch frequency.  Positive feedback required to make the circuit oscillate is applied to the noninverting (+) input of the op-amp A  through the voltage divider R3 and R4.  However at all frequencies besides the notch frequency, the twin T filter applies the same signal to the inverting (-) input, so the loop gain is far below unity at those frequencies.  Only at the notch frequency is the loop gain above unity, so the circuit oscillates at that frequency.
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Chetvorno