Barkhausen Criterion for Oscillation

Before starting with the Barkhausen Criterion for Oscillation, we will see what oscillations are, and What the oscillator principle is. So let’s start with the Oscillator Principle and then we will discover the Barkhausen Criterion for Oscillation.

Oscillator Principle

An oscillator is basically an “amplifier” which does not have any AC input but it operates on the principle of positive feedback to generate an AC signal at its output.

Thus it is clear that an amplifier can work as an oscillator if positive feedback is made to exist. However positive feedback does not always guarantee oscillations.

An amplifier will work as an oscillator if and only if it satisfies a set of conditions called the Barkhausen Criterion for Oscillation

So We will see the Barkhausen Criterion for Oscillation in detail.

Barkhausen Criterion for Oscillation

The Barkhausen criteria should be satisfied by an amplifier with positive feedback to ensure sustained oscillations.

For an oscillator circuit, there is no input signal Vs. Hence the feedback signal Vf itself should be sufficient to maintain the oscillations.

Refer to the below figure to understand the Barkhausen Criterion for Oscillation.

Block diagram of oscillator
  • From the figure, the expression for output voltage V0 is,

V0 = A Vi

  • But Vi is the sum of Vs and Vf.

∴ Vi = Vs + Vf

Note that we need to add Vs and Vf because in positive feedback, Vs and Vf will be in phase with each other and hence it will get added.

  • The expression for feedback voltage is,

Vf = β V0

  • Substitute the value of V0, we get,

Vf = β A Vi

  • From the equation of input voltage Vi, we get,

Vi = Vs + A β Vi

∴ (1 – A β) Vi = Vs

  • For an oscillator, the input voltage Vs is absent i.e. Vs = 0, and the feedback signal Vf is supposed to maintain oscillations. Therefore substitute Vs = 0 into the above equation to get,

(1 – A β) Vi = 0 or A β = 1

This condition must be satisfied in order to obtain sustained oscillations. Along with this condition, the condition for the positive feedback states that the phase shift between Vs and Vf must be zero, and should also be satisfied.

With an inverting amplifier introducing an 1800 phase shift between Vi and Vo, the feedback network must introduce another 1800 phase shift to ensure that Vi and Vf are in phase.

These two conditions which are required to be satisfied to operate the circuit as an oscillator are called as the “Barkhausen Criterion for Oscillation”.

Statement for Barkhausen Criterion for Oscillation

Barkhausen Criterion states that:

  1. An oscillator will operate at the frequency for which the total phase shift is introduced, as the signal proceeds from the input terminals, through the amplifier and feedback network, and back again to the input precisely 00 or 3600 or an integral multiple of 3600.
  2. At the oscillator frequency, the magnitude of the product of open loop gain of the amplifier A and the feedback factor β is equal to or greater than unity.

\mathbf{\therefore |A\beta| \geq 1}

The product Aβ is called the open loop gain.

These conditions are diagrammatically illustrated in the below figures.

Barkhausen Criterion

Hello friends, my name is Trupal Bhavsar, I am the Writer and Founder of this blog. I am Electronics Engineer(2014 pass out), Currently working as Junior Telecom Officer(B.S.N.L.) also I do Project Development, PCB designing and Teaching of Electronics Subjects.

Leave a Comment

This site uses Akismet to reduce spam. Learn how your comment data is processed.

telegram logo Join Our Telegram Group!