What is an Oscillator ?
An oscillator is an amplifier that acts as a generator by using positive feedback concept.
Oscillator does not require any input signal. It produces an output waveform of constant amplitude and constant frequency up to several hertz as required (Depends on Resistor & Capacitor Values). It produces output oscillations as long as it is connected to DC power source.
How an Oscillator starts oscillating ?
Every resistors used in the circuit has some free electrons in it that moves randomly around in various directions. Due to their movement electrons generates a voltage called noise voltage. Noise voltage contains all sinusoidal frequencies. The noise voltage has a low amplitude and it is amplified which appears across output. This amplified noise drives the feedback network. At a particular frequency, the feedback voltage is maximum, which is represented as Frequency of Oscillation.