Barkhausen criterion and various types of oscillators

The condition for sustained oscillations in a single valve self excited oscillator worked out by the German scientist Barkhausen is this that the voltage multiplication of the amplifier must exactly counterbalance the voltage dividing of the feedback circuit.

                                                β=1/A                                  

 

  

 Where, β=feedback factor, A= voltage gain

Various types of oscillators

Various types of oscillator have been designed to meet the requirement of different types of electronic equipments which are follows:

Tuned collector oscillator

Hartley oscillator

Colpitt’s oscillator

      Clapp oscillator

      Crystal oscillator

      Phase shift R-C oscillator

      Multi vibrator

      Wein bridge oscillator

      Neon lamp oscillator

      Master oscillator power amplifier

      Beat frequency oscillator    

TUNED COLLECTOR OSCILLATOR :

It is simplest type of oscillator circuit. It consists of two coils L1 and L2 wound like a transformer. Coil L1 is the connected in the collector circuit and coil L2 in the base circuit. The frequency of oscillations depends of oscillations depends on the values of L1 and C1 capacitor C provides a low reactance path to the oscillations.

When the switch S is put to ON, the collector current starts increasing and charges the capacitor C1. When this capacitor gets fully charged, it starts to discharge through L1 and the circuit begins to oscillate. These oscillations include some voltage in the coil L2 which is applied between the base and emitter of the transistor and the same appears in the amplified form in the collector circuit. The output can be obtained from the collector circuit trough a capacitor C2.

          The oscillatory frequency of the circuit is calculated by using the formula 

 There exists a ‘tuned base oscillator ‘circuit also which is based on a CB circuit.

Oscillators

Introduction:

The to and fro motion of electronics through a conductor produces electromagnetic waves. In the earliest type of type of transmitters, the electromagnetic or radio waves were produced by means of an electric spark. The to and fro motion of electrons through a conductor is known as oscillation and the devices used to produce them is called an oscillator.

 It is inconvenient and nearly impracticable to generate frequencies of the order of kilo hertz and megahertz by means of a generator. Transistor oscillators are suitable are suitable for the production of a defined and stable frequencies of said order.

ESSENTIAL REQUIREMENTS OF AN OSCILLATOR:

  

Following three items are essential for an oscillator 

Transistor amplifier circuit

Feedback circuit

Tank circuit

Amplifier:
An amplifier circuit is modified is such a way that can work as an oscillator.

Feedback circuit:
An oscillator requires positive feedback. A portion of the output is energy is received by the input circuit by means of feedback. If the amplifier circuit then the circuit will not oscillate at all.

Tank circuit:
A resonant circuit consisting of an inductor and a capacitor or crystal is known as tank circuit. This circuit produces oscillation and decides their frequency.

Working of Tank Circuit:

If a capacitor is charged and then connected across an inductor, it will start to discharge. The electrons will start to move from plate 2 to 1 through the coil. An alternating magnetic field will be produced around the coil due to the flow of electrons. A back E.M.F will be induced in due to alternating nature of the of magnetic field which oppose the flow of current and will thus will reduce the current.

When the number of electrons on the two plates becomes equal, the capacitor is fully discharged, the current will reduce further and the magnetic field will begin to contact. The flow of current will continue due to contracting. Magnetic field and the back e.m.f. will now oppose its contraction. The process will continue till the charge of the plate is not lost completely. In this way, the capacitor will be charged in opposite direction.

Now the former action will repeat. Hence, the electrostatic field of the capacitor will continue to transfer energy to the electromagnetic field of coil and vice-versa. As a result, an alternating current will start to flow in the circuit. If there occurs no loss in the transfer of energy, the oscillations will continue indefinitely, but an energy loss always exists in the circuit due to ohmic resistance of coil and on reducing continuously, oscillations goes on reducing continuously are called damped oscillations. The following formula is used for the calculation of oscillatory frequency of the tank circuit:

   

 Where,

ƒ= frequency, hertz

L= inductance, Henry

C= capacitance, farads

2π=2*3.142

If the loss of energy in each cycle of damped waves is compensated regularly, the oscillations will continue. Various types of oscillator circuit have been designed to meet the purpose.