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
Sinusoidal oscillators may be any
one of the following three types:
(i) LC oscillators
(ii) RC oscillators
(iii) Crystal oscillators
Barkhausen condition for
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 , Af → ∞.
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.