Oscillators are amplifiers with such a large amount of positive feedback that they produces an output signal with no signal applied to the input.
The output amplitude is determined by the gain of the amplifier and the feedback circuit.
Oscillators can produce sine waves, the frequency of which is determined by
Tuned circuits consist of a capacitor and inductance. Square wave oscillators use resistors and capacitors to determine the frequency of oscillation.
Ideally the frequency of an oscillator should be stable, but due to
temperature variations and mechanical vibration this may not be so. Precautions are taken against frequency
"Howl round", caused by placing a microphone too close to a loudspeaker, is
an audio oscillation caused by positive feedback.