Introduction

Almost every electronic system that generates, times or transmits a signal begins with a source of alternating voltage. A radio set, a television transmitter, a radar unit, a wristwatch and the clock inside a computer all rely on a circuit that produces a steady alternating (AC) waveform of a chosen frequency without any external AC input to drive it. That circuit is called an oscillator — it takes energy from a DC supply and delivers it as a continuous AC signal of the required frequency.

In this chapter of Karnataka 2nd PUC Electronics you will build up the idea of an oscillator step by step: how positive feedback lets an amplifier sustain its own output, the Barkhausen conditions that decide whether the oscillations decay, grow or stay steady, and then the main practical families used in the II PUC board exam syllabus — LC oscillators (Hartley and Colpitts), RC oscillators (phase-shift and Wein bridge), the crystal oscillator, and finally a non-sinusoidal oscillator built around the 555 timer. For each type you will meet its circuit, its working, and the expression for its frequency of oscillation, so that both the theory questions and the numerical problems in the PUC Electronics question bank become routine.