Introduction to Oscillators:

“An amplifier with positive feedback

acts as

an oscillator”.

.
Introduction to Oscillators:

The transfer function of a feed back

amplifier is as follows
A
Af 1 A


Where A is amplifier gain

 is feed back factor,

if (A  )= 1

The A  ,
which is condition for
f
n oscillations.
 Principle for Oscillations:

Sinusoidal electrical oscillations are of two

types:

• Damped oscillations

•Un-damped oscillations

Damped oscillations : The electrical

oscillations in which amplitude decreases
with time are known as damped oscillations.
it can be observed that the

amplitude of oscillations decrease with time,

though

the frequency remains same.

Why is so
it ?

It is due to the loss of energy in the

system

producing oscillations.
 Un-damped oscillations :

The electrical oscillations in which amplitude

does not change with time are known as un-
damped oscillations These are shown in the
figure 23.2

0
t

Fig 23.2 Un-damped

Oscillations
 •Here the amplitude of oscillations are constant
with time and there is no change in frequency.
These are the oscillations which are used in
various electronic equipment.

• These un damped oscillations are produced by

providing an energy compensation circuit
The circuit that produces these oscillations is known
as TANK

CIRCUIT

The frequency of oscillations depend upon the

parameters

used in this tank circuit i.e., L & C.

The frequency of oscillations is given by the following

formula. 1
f0
 2
LC
Oscillato
r:
Why oscillator is
necessary ?
• In many electronic application
electrical energy at a s, hig
specific
frequency ranging from few h
Hz
to several
MHz. is required.
This is obtained with the help of an
electronic device called
“OSCILLATOR”.
These are used in Radio, TV, Radar and
other communication/digital electronic
applications.
Definitio
n:
“An oscillator is just an electronic circuit
which converts dc energy into ac energy
of required frequency”.
Or

“An oscillator is an electronic circuit

which produces an ac output
without any input”.
Basic types of
oscillators :
Based on the waveform
produced at the output:

• Sinusoidal oscillators.

•Non sinusoidal oscillators

Sweep circuits
Relaxation oscillators.

. 1
6
Sinusoidal oscillators : A static electronic
device

that produces sinusoidal oscillations of

desired

frequency is called sinusoidal oscillator.

Eg. LC oscillators, RC phase shift oscillator

etc.,
 Amplifier as an oscillator

Amplifier with positive feedback which

works as an oscillator.

+ _
Amplifier
V
vi Gain (A)
n out
_ +

_
Feedback +
Circuit (ß)
+ _
Fig.
2.1(d)
We know that negative feedback is
employed in amplifiers for stability of
the output.

Here positive feedback is used to

produce oscillations.
From the circuit, it can be observed
that…
• The amplified signal available in the
ckt., is Avin.
• The fraction of this signal, i.e., Avin
is
fedback to the input.
• The feedback signal Avin must be in phase
with the input signal Vin, (i.e. positive feedback),
as shown in the
figure.

• Now the circuit starts acting like an oscillator.

•As defined earlier, the circuit (block diagram)

can be shown producing an o/p without any
i/p just by removing the input signal, as shown
in the next slide.
 Now assume that the above Circuit is modified
with the following conditions :
• Terminal ‘z’ is connected with ‘x
• input signal ‘v’ is removed.
Now the fig 2.1(d) can be modified
as follows as shown in fig 2.1 (e).

_
Amlifie V
r Gain out
(A) +

_ +
Feedbac
k Circuit
+ () _
Fig 2.1
(e)
Now three cases of feedback are
possible, let us discuss them
case by case.

Case 1 : If |A| < 1, then the

removal of input signal Vin will result in
ceasing of oscillations (damped
oscillations) as perAthe closed loop
Af
transfer function given below.
 1 A
• Here the output oscillations will
slowly reduce in amplitude and
finally die out.

• Such oscillator is ofno use

to any practical
applications. Its waveform is
shown in next slide.
The output oscillations for case:1 is as
follows:

damped
oscillations
Case 2:

If |A| > 1, then, for example, a 1 volt

signal

appearing initially at the input terminal

will, after a trip around the loop

reappear as a still larger voltage than 1

volt.
• This larger voltage then reappears
as a still larger voltage, and so on,
building theoscillations with
increase in its amplitude
without limit, as shown below:

Fig 2.1(g) grouping

oscillations
Case
3:

If |A| = 1, then Af 


The condition of unity loop gain |A| = 1 ,

is called the Barkhausen criterion.

This condition implies that both |A| = 1 and

that the phase of -A is zero (ie., positive
feedback).
In the above conditions, no change
occurs at the output of an
oscillator and we get output
oscillations with constant amplitude
as shown in the figure 2.1 (h).

Fig. 2.1 (h) un damped

oscillations
Thus, to obtain sustained
(undamped) oscillations, the loop
gain A of positive feedback must
be unity.

Therefore, now, we can say that “A

positive feedback amplifier with
unity gain acts as an oscillator”.
Difference between an amplifier and
an oscillator:
 Ref:06103104HKN

Application of Oscillators
• Oscillators are used to generate signals, e.g.
– Used as a local oscillator to transform the RF
signals to IF signals in a receiver;
– Used to generate RF carrier in a transmitter
– Used to generate clocks in digital systems;
– Used as sweep circuits in TV sets and CRO.
 APPLICATIONS OF OSCILLATORS:
� Oscillators are a common element of almost all electronic circuits. They are used in
� various applications, and their use makes it possible for circuits and subsystems to perform
numerous useful functions.
� In oscillator circuits, oscillation usually builds up from zero when power is
� first
� applied under linear circuit operation.
� The oscillator’s amplitude is kept from building up by limiting the amplifier
� saturation and various non-linear effects.
� Oscillator design and simulation is a complicated process. It is also extremely
� important and crucial to design a good and stable oscillator.
� Oscillators are commonly used in communication circuits. All the
communication circuits for different modulation techniques—AM, FM, PM—the use of an
�oscillator is must.
� Oscillators are used as stable frequency sources in a variety of electronic
� applications.
� Oscillator circuits are used in computer peripherals, counters,
timers, calculators,
phase-locked loops, digital multi-metres, oscilloscopes, and numerous
other applications.
• An oscillator is a circuit that produces a repetitive signal
from a dc voltage.
• The feedback oscillator relies on a positive feedback of
the output to maintain the oscillations.
• The relaxation oscillator makes use of an RC timing circuit
to
generate a nonsinusoidal signal such as square wave
Sine wave

Square
wave

Sawtooth
wave
• The classification of various oscillators is shown in Table 12-1.
 Linear Oscillators

Figure 9.68 A linear oscillator is formed by connecting an

amplifier and a feedback network in a loop.
 Ref:06103104HKN

Integrant of Linear Oscillators

V
Vs + Amplifier (A) Vo

+
Positive Vf Frequency-Selective
Feedback Network ()
Feedback

For sinusoidal input is connected

“Linear” because the output is approximately
sinusoidal

A linear oscillator contains:

- a frequency selection feedback network
- an amplifier to maintain the loop gain at unity

EE3110 Oscillator
 Ref:06103104HKN

Basic Linear Oscillator

V
Vs + A(f) Vo

+

Vf SelectiveNetwork
(f)
and

Vo  A
 V 1
s

A only way that Vo can be

If Vs = 0, the
nonzero is that loop gain A=1 which
implies that

(Barkhausen
Criterion)
 Basic principles for oscillation
ifier
• An osc illator is an aVmepl w
A ith
feedback.
V
s
poVsitoive +
Vf

Ve  V s
V (1)
V f f βV o (2)
V  AV 
o e s
V   A V  βV
AV  (3)
f s o
 Basic principles for oscillation
Vo  AVe
 AsV f V s
 βV
o

  A V o
Vo  AV s  A  V
1  A  V o 
AVs
A V
• Thef closed

 V loop1gain
o is:
 Aβ
s
A

 Basic principles for oscillation
• In general A and  are functions of frequency
V written as;
and thus may be
A f s o s
A
Vs s 1  A s β s
 
Asβ 
s is known as loop gain
 BasicTps rinci
A s βpls e s for
   
• Writing the loop
oscillation
f s
becomes;
gain
A
 As 1
 T s

jω  A
• Replacin s with
g Af  j
 T
1 jω
 jω
T jω  A jωβ
jω
• and
 Basic principles for oscillation
• At a specific frequency f
T  jω0 0 0  jω0  
 A jω 1

β frequency, the cl os e 0 d lo op
• At this
A f  jω0  A  ωj 
gain;  1  A jω0 β
jω0 
will be infinite, i.e. the circuit will have finite
output for zero input signal - oscillation