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Chapter: Electronic Circuits - Oscillators

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Introduction about Oscillators

An oscillator is a circuit that produces a repetitive signal from a dc voltage.

Introduction about Oscillators

 

An oscillator is a circuit that produces a repetitive signal from a dc voltage. The feedback type oscillator which rely on a positive feedback of the output to maintain the oscillations. The relaxation oscillator makes use of an RC timing circuit to generate a non-sinusoidal signal such as square wave.


The requirements for oscillation are described by the Baukhausen criterion:

 

ü   The magnitude of the loop gain Aβ must be 1

 

ü   The phase shift of the loop gain A β must be 0o or 360o or integer multiple of 2π

 




Amplitude stabilization:

 

ü   In both the oscillators above, the loop gain is set by component values

 

ü   In practice the gain of the active components is very variuable

 

ü   If the gain of the circuit is too high it will saturate

 

ü   If the gain of the circuit is too low the oscillation will die

 

Real circuits need some means of stabilizing the magnitude of the oscillation to cope with variability in the gain of the circuit

 

Barkhausan criterion

 

The conditions for oscillator to produce oscillation are given by Barkhausan criterion.  They are :

 

ü      The total phase shift produced by the circuit should be 360o or 0o

ü   The Magnitude of loop gain must be greater than or equal to 1 (ie)|Aβ|≥1

 

In practice loop gain is kept slightily greater than unity to ensure that oscillator work even if there is a slight change in the circuit parameters.




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