Question

What is an oscillator ? With the help of a circuit diagram explain the working of transistor as an oscillator in common emitter configuration.
Plz give me answer......:-)

Answer 1

The electronic circuit that produces periodically oscillating electronic signal such as sine wave, square wave or any other wave is termed as Electronic Oscillator. Oscillators can be classified into different types generally based on their output frequency. Electronic oscillators can be termed as voltage controlled oscillators as their frequency of oscillations can be controlled by their input voltage. Foremost electronic voltage controlled oscillators can be considered as two types namely: Linear Oscillator and Nonlinear Oscillator.

Electronic Oscillator

Nonlinear oscillators are used to produce non-sinusoidal output waveforms. Linear oscillators are used to produce sinusoidal output waveforms and are further classified into many types, such as Feed back oscillator, Negative resistance oscillator, Colpitts oscillator, Hartley oscillator, Armstrong oscillator, Phase shift oscillator, Clapp oscillator, Delay line oscillator, Pierce oscillator, Wien bridge oscillator, Robinson oscillator, and so on. In this particular article, we are discussing about one of the many types of linear oscillator circuits namely Colpitts Oscillator.

Answer 2

Oscillators

An oscillator is a circuit which produces a continuous, repeated, alternating waveform without any input.

Transistor Oscillator

The circuit contains a tuned circuit made of variable capacitor C and an inductor L in the collector circuit and hence is named as tuned collector oscillator

The feed back coil L2 connected to base circuit is mutually coupled with coil L1 (due to phenomenon of mutual induction ); the mutual inductance of L and L2 being M. In practice L1 and L2 form the primary and secondary coil of the transformer

The biasing is provided by emitter resistance Re and potential divider arrangement consisting of resistances R1 and R2. The capacitor C2 connected in the base circuit provides low reactance path to the oscillations and the capacitor Ce is the emitter by-pass capacitor so that the resistor Re has no effect on the ac operation of the circuit.

Circuit Operation:

When the collector supply voltage is switched on by closing switch S, collector current starts increasing and the capacitor C1 is charged. When the capacitor attains maximum charge it discharges through coil L1, setting up oscillations of natural frequency  

f = $\frac{1}{\sqrt[]{2 \pi LC} }$

These oscillations induce a small voltage in coil L2 by mutual induction. This induced voltage is the feed back voltage, its frequency is same as that of resonant LC circuit but its magnitude depends on the number of turns in L2 and coupling between L1 and L2 The feedback voltage is applied between the base and emitter and appears in the amplified form in the collector circuit. A part of this amplifier energy is used to meet losses taking place in oscillatory circuit to maintain oscillations in tank circuit and the balance is radiated out in the form of electromagnetic waves.

Positive Feed Back-

The feed back applied in tuned collector oscillator circuit is positive. This may be seen as follows : A phase shift of 180º is created between the voltage of L and L' due to transformer action. A further phase shift of 180º arises between base emitter and collector circuit due to transistor action in CE configuration. Thus the net phase becomes 360º (or zero) which is the required condition for a positive feed back. Due to positive feed back the energy fed back to the tank circuit is in phase with the generated oscillations, thus maintaining oscillations.