Transistor oscillators - Barkhausen condition for oscillation

Transistor oscillators

An oscillator may be defined as an electronic circuit which converts energy from a d.c. source into a periodically varying output. Oscillators are classified according to the output voltage, into two types viz. sinusoidal and non-sinusoidal oscillators. If the output voltage is a sine wave function of time, the oscillator is said to be sinusoidal oscillator. If the oscillator generates non-sinusoidal waveform, such as square, rectangular waves, then it is called as non-sinusoidal oscillator (multivibrator). The oscillators can be classified according to the range of frequency as audio-frequency (AF) and radio-frequency (RF) oscillators.

Sinusoidal oscillators may be any one of the following three types:

(i) LC oscillators

(ii) RC oscillators

(iii) Crystal oscillators

Barkhausen condition for oscillation

The gain of the amplifier with positive feedback is given by Af = A/1-Aβ. where A is the voltage gain without feedback, β is the feedback ratio and Aβ is the loop gain. When Aβ = 1, then , A_f → ∞. This means that output voltage is obtained, even if input voltage is zero, (i.e) it becomes an oscillator. The essential condition for the maintenance of oscillation is Aβ = 1.

This condition means that (i) the loop gain Aβ = 1 and (ii) the net phase shift round the loop is 0o or integral multiples of 2π.

These are called the Barkhausen conditions for oscillations.