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ALL ABOUT ELECTRONICS. In this video, we will talk about the electronic
oscillator and we will learn about the basic working principle of the oscillator. the electronic oscillators are used in a wide
range of applications. They are used in laptop and smartphone processors
for generating the clock signals. While they are used in a radio and mobile
receivers for generating the local carrier frequency.
and even they are used in the signal generators which is used in the lab to test the circuits. So, this oscillator accepts the DC voltage
and it generates the periodic AC signal of the desired frequency. Now, the oscillators can generate the frequencies
from few Hz to even GHz. Now, the output of the oscillator can be either
sinusoidal signal or non-sinusoidal signals like a square wave and the triangular wave. In simple terms, this oscillator circuit is
nothing but the amplifier which is given a positive feedback. So, let's understand the working principle
of this oscillator. So, let's say some input sinusoidal signal
is applied to this amplifier. So, at the output, the input signal will get
multiplied by the gain of this amplifier. And the output signal will be equal to A times
the input signal. Now, let's say, this output signal is given
as an input to this feedback circuit. Now, usually, the feedback circuit used to
be a frequency selective circuit or the resonant circuit. And let's say the output of this feedback
circuit is equal to Vf. So, Vf can be written as a β times output
voltage. That is equal to Aβ times the input voltage. where here this β is nothing but the feedback
fraction. And it defines what fraction of the output
voltage is given as a feedback to the input stage. Now, if the phase shift that is introduced
by this amplifier and the feedback circuit is zero, in that case, this feedback signal
will be in phase with the input signal. Now, let's say, this feedback signal is getting
added to the input signal and at the same time, the input signal is removed from the
circuit. So, now this feedback signal Vf will act as
an input for this amplifier. So, after removing the input voltage whether
we will get the sustained oscillations or not, that depends on the product of this A
and β. And it is known as the loop gain of the oscillator. so, if this loop gain Aβ is less than 1,
in that case over the period of time, the input signal will die out. So, now let's say in one particular circuit
Aβ is equal to 0.9. And in this circuit, the input voltage Vin
is equal to 2V of a sine wave. So, now whenever this input signal passes
through this amplifier and the feedback circuit, then the input signal Vin will become 2V,
multiplied by the 0.9. That is equal to 1.8V.
And once again whenever this input signal passes through this loop, then the input signal
will get reduced by the factor of 0.9. So, in this way, every time this input signal
passes through this loop, the amplitude of the input signal will reduce. And over the period of time, the oscillations
in the circuit will die out. Similarly, whenever this Aβ is greater than
1, at that time, the oscillations in the circuit will build up. So, as you can see, in both cases, we are
not getting the sustained oscillations. And that is only possible whenever this Aβ
is equal to 1. So, when Aβ is equal to 1, at that time the
feedback signal Vf will be same as the input signal, provided the input signal and the
feedback signal has the same phase. So, in that case, we will get the sustained
oscillations at the output. So, in oscillator to get the sustained oscillations,
two conditions should get satisfied. The first is, the product of this Aβ should
be equal to 1. And the second is the phase shift of this
loop gain should be equal to zero, meaning that whenever the input signal travels through
this amplifier and the feedback circuit, the overall phase shift that is introduced by
the circuit should be equal to zero. And these two criteria are known as the Barkhausen's
criteria for the oscillations. Now, so far we have assumed that whenever
this oscillator is switched ON, at that time some finite amount of starting voltage is
applied to this oscillator. But actually, if you see, no signal is applied
to this oscillator. And still, we are getting the oscillations
at the output. So, the question is how is it possible? How we can get the oscillations at the output
without giving any input to this oscillator. So, the answer is, the thermal noise present
in every circuit. And if you are aware, this thermal noise contains
all frequency components. Starting from few Hz to even hundreds of GHz. So, initially, whenever this oscillator is
turned on, all the frequency components of this thermal noise will get amplified by the
amplifier. And the amplified output of this thermal noise
will be given as an input to this feedback circuit. Now, as I said earlier, this feedback circuit
is the frequency selective circuit. So, out of all the frequency components only
for one particular frequency, the phase shift that is introduced by this amplifier and the
feedback circuit will be equal to zero. While all other frequencies will have a different
phase. so, from all other frequencies, only one particular
frequency will get added with the input noise. And in this oscillator circuit, initially,
the loop gain Aβ is slightly set more than 1. And because of that the noise signal of a
particular frequency will get build up over the period of time. And once this signal reaches a certain voltage
at that time, the loop gain of the circuit will become 1. And it is possible because of the non-linear
behavior of either amplifier or the feedback circuit. So, in this way, the noise signal of the desired
frequency will get build up over the period of time. And once this signal reaches finite voltage
then the loop gain of the circuit will become 1. And in this way, it is possible to get the
sustained oscillations at the output. So, this is the basic working principle of
the oscillator. Now, earlier we had seen the two criteria
for the sustained oscillations. And these two criteria can also be proved
mathematically. So, let's say, the output of the feedback
circuit is equal to Vf. And this signal Vf will get added with the
input signal. So, suppose if the input signal is present
at that time, the input to the amplifier will be equal to Vin +Vf. And at the output, we will get A times (Vin
+Vf) Now, here Vf is nothing but β times output
voltage. So, if we put the value of Vf, then Vout will
be equal to A times Vin, plus Aβ times Vout. And if we simplify it then we can say that
Vout/Vin is equal to A/ (1- Aβ) Now, here in the oscillator, we are not providing
any sort of input signal. And still, we are getting the oscillations. It means that Aβ in the circuit should be
equal to 1. So, that this condition will get fulfilled. So, from this, we can say that the magnitude
of this loop gain should be equal to 1 and the phase shift that is introduced by this
loop gain should be equal to 0. So, in this way, mathematically these two
criteria can also be proved. Now, like I said before, in oscillators the
feedback circuit used to be a frequency selective circuit. So, this feedback circuit can be made up of
either RL, RC or RLC components. And even the quartz crystal can be used for
the frequency selection. So, depending on the type of feedback circuit,
the oscillator can be classified as either RC, LC or crystal oscillator. And moreover that depending on the arrangement
of these components, these oscillators can be classified further. Now, the oscillators which are mentioned over
here are the sinusoidal oscillators. Or even it is known as the harmonic oscillators. Because the output of these oscillators used
to be a sine wave. While some other oscillators also provide
a different kind of shape. Like square wave and the triangular wave. And these oscillators are known as the relaxation
oscillator. And these type of relaxation oscillators can
be build up either using op-amp or the timer ICs like 555 timer. And we will see the design of the different
types of oscillators in the future videos. so, I hope in this video you understood the
basic working principle of the oscillator. So, if you have any question or suggestion,
do let me know in the comment section below. If you like this video, hit the like button
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