English: 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.