Q. Why positive feedback is used in oscillator?

The main application of positive feedback is in oscillators. An oscillator generates AC output signal without any input AC signal. A part of the output is fed back to the input and this feedback signal is the only input to the internal amplifier. This signal is amplified again and again to produce oscillations. In positive feedback the feedback voltage is in same polarity with the input signal. And this positive signal is amplified to produce oscillations. Thus positive feedback is used in an oscillator to increase the gain. The case of negative feedback the feedback voltage is in opposite polarity with the input signal. This is amplified to produce growing oscillation. Thus the gain will decrease.