Q. What is the basic principle of oscillators?

 In an oscillating circuit, the amplitude of voltage or current oscillations decays with time owing to the dissipation of energy in the resistance contained in the circuit. If a negative resistance is incorporated in the circuit to generated energy that compensates for the loss of energy through the passive resistance, oscillations with undiminished amplitude can occur. Basically, therefore, a negative resistance must be provided in an oscillator. This is accomplished in a feedback oscillator by providing an external positive feedback to make the overall gain infinite. In a negative resistance oscillator, the desired negative resistance is supplied by internal positive feedback.

 No external signal is applied to an oscillator. When the power supply to the system is switched on, a noise voltage is produced. The frequency spectrum of noise being very wide, it always has a voltage component at the frequency required for the oscillator. Thus the initial signal to trigger the oscillations is obtained from the noise voltage. The ac power of the output signal is supplied by the dc supply source.