What is Oscillator ?
A device used to produce sinusoidal or non sinusoidal ( e.g. square wave ) wave forms without the application of an external input signal is known as an oscillator. In an oscillator, D.C power supply is connected to the input. Thus, the oscillator converts D.C power into A.C power of desired frequency.
Types of Oscillators
The oscillator which produces a sinusoidal wave form of certain frequency at the output is known as sinusoidal oscillator.
The oscillator which produces non sinusoidal (e.g. square wave) wave form at the output is known as relaxation oscillator.
The current voltage characteristics of the transistor used in an oscillator has linear region and non linear region. Sinusoidal oscillators operate in the linear region and relaxation oscillators operate in non linear regions of the characteristics.
These two types of oscillator are further classified into two categories
- feedback type oscillator.
- negative resistance type oscillator.
Feedback type oscillator
If part of the output is fed back to the input in phase, then the oscillator is known as feedback oscillator.
Negative resistance type oscillator
The oscillator is said to be a negative resistance type oscillator if the transistor used in the circuit provides negative feedback resistance (because the current voltage characteristics has negative slope over some part of its operation) to cancel the effect of positive resistance of the circuit.
Barkhausen criterion for oscillations
We know, the overall gain of the feedback amplifier is given by
Where Aβ is known as loop gain or return ratio, A is the gain of the amplifier and β is the feedback ratio.
If Aβ = -1 or -Aβ = 1, then the feedback is positive or regenerative. Hence eqn. (1) becomes
It means, the amplifier provides output voltage without the application of external applied input voltage or signal. Thus, the amplifier acts as an oscillator.
Thus, the conditions for oscillation in a feedback amplifier are :
- The loop gain or return ratio must be unity.
- The feedback of the amplifier must be positive or regenerative.
Essential components of an oscillator
The basic block diagram of an oscillator is shown in the figure.
The essential components of an oscillator are:
- Tank circuit – This circuit is also known as L-C circuit and consists of an inductor of inductance L and a capacitor of capacitance C. Tank circuit is used to produce oscillations. The frequency of oscillation is determined by the value of L and C.
- Amplifier – It may be BJT or FET amplifier, D.C power of the voltage supply is converted in A.C power which is fed to the tank circuit. The oscillations produced by the tank circuit are fed as input to the amplifier for amplification.
- Feedback circuit- This circuit is used to provide positive feedback to the input.