ECE 322

Electronic Circuits (B)

3rd year communication

Oscillators
Lecturer:
Dr. Aya Hossam
Introduction to Oscillator

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 Introduction
• An oscillator is a circuit that produces a periodic
waveform on its output with only the dc supply voltage as an
input.
• The output voltage can be either sinusoidal or non-
sinusoidal, depending on the type of oscillator.
• Two major classifications for oscillators are:
Feedback Oscillators and Relaxation Oscillators.
• Essentially, an oscillator converts electrical energy from
the dc power supply to AC periodic waveforms.
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 Oscillators convert dc to ac.

Oscillator ac out
dc in

Some possible output waveforms

Types of oscillators

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• Different types of oscillators produce various types of

outputs including sine waves, square waves, triangular

waves, and sawtooth waves.

• Sinusoidal oscillator operation is based on the principle

of positive feedback, where a portion of the output

signal is fed back to the input in a way that causes it to

reinforce itself and thus sustain a continuous output

signal.
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 Types of oscillators
1. Feedback Oscillators
◦ Hartley
◦ Colpitts
◦ Crystal
◦ Clapp
2. Relaxation oscillators
Types of oscillators

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Feedback Oscillators

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 Basic principles for Feedback Oscillator


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 Basic principles for oscillation
 An oscillator is an amplifier with positive feedback.

Ve Vo
Vs A
+
Vf
b
Ve = Vs + Vf (1)

Vf = βVo (2)

Vo = AVe = A Vs + Vf = A Vs + βVo (3)

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 Vo = AVe
= A Vs + Vf = A Vs + βVo

Vo = AVs + A𝛽Vo

1 − A𝛽 Vo = AVs

❑ The closed loop gain is:

Vo A
Af ≡ =
Vs 1 − Aβ

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 In general A and b are functions of frequency and
thus may be written as;
Vo A s
Af s = s =
Vs 1−A s β s

A s β s is known as loop gain

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 Writing T s = A 𝑠 β 𝑠 , the loop gain becomes;
A s
Af s =
1−T s

 Replacing s with j
A jω
Af jω =
1 − T jω

 and T jω = A jω β jω

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 At a specific frequency f0
T jω0 = A jω0 β jω0 = 1

 At this frequency, the closed loop gain;

A jω0
Af jω0 =
1 − A jω0 β jω0

will be infinite, i.e. the circuit will have output for

zero input signal - oscillation

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 Barkhausen Criterion
 Thus, the condition for sinusoidal oscillation of
frequency f0 is;

A( jω0 )β ( jω0 ) = 1
 This is known as Barkhausen criterion.
 The frequency of oscillation is solely determined by
the phase characteristic of the feedback loop – the
loop oscillates at the frequency for which the phase is
zero.

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Barkhausen Criterion

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 Start up Condition
• When oscillation starts at t0, the condition Acl>1 causes the
sinusoidal output voltage amplitude to build up to a desired
level. Then, Acl decreases to 1 and maintains the desired
amplitude.

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 Feedback Oscillators
 The feedback oscillator is widely used for generation
of sine wave signals.

 The positive (in phase) feedback arrangement

maintains the oscillations.

 The feedback gain must be kept to unity to keep the

output from distorting.

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 LC Feedback Oscillators
 Use transistors and LC tuned circuits or crystals in
their feedback network.

 For hundreds of kHz to hundreds of MHz frequency

range.

 Examine Colpitts, Hartley and crystal oscillator.

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 1. Colpitts oscillator.
 The Colpitts oscillator is a
type of oscillator that uses
an LC circuit in the feed-
back loop.
 The feedback network is
made up of a pair of
tapped capacitors (C1 and
C2) and an inductor L to
produce a feedback
necessary for oscillations.
 1. Colpitts oscillator.

• The amount of feedback developed by the Colpitts oscillator is based on the

capacitance ratio of C1 and C2 and is what governs the excitation of the oscillator. This
ratio is called the “feedback fraction” and is given simply as:

Feedback Fraction “ꞵ” = C1/ C2

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 Problems
Q1. A Colpitts Oscillator circuit having two capacitors of 24nF and
240nF respectively are connected in parallel with an inductor of 10mH.
Determine the frequency of oscillations of the circuit, the feedback
fraction and draw the circuit.

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 2. Hartley Oscillator
 The Hartley oscillator is almost identical to the Colpitts
oscillator.
 The primary difference is that the feedback network of the
Hartley oscillator uses tapped inductors (L1 and L2) and a
single capacitor C.
 The analysis of Hartley oscillator is identical to that Colpitts
oscillator.
 The frequency of oscillation:

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2. Hartley Oscillator

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 Problems
Q. A Hartley Oscillator circuit having two individual inductors of 0.5mH
each, are designed to resonate in parallel with a variable capacitor that
can be adjusted between 100pF and 500pF. Determine the upper and
lower frequencies of oscillation and also the Hartley oscillators
bandwidth.
Solution:
 Oscillator Basics Quiz
Oscillators convert dc to __________. ac
In order for an oscillator to work,
the feedback must be __________. positive

The magnitude of the loop gain must be

_________ . unity
Sine wave oscillators have the correct
feedback phase at one __________. frequency
The phase shift of an loop gain at fR
is __________. 0o
 The feedback oscillator is widely used for generation of
__________ signals . Sine Wave

The condition A( jω0 )β ( jω0 ) = 1 is known as

__________ . Barkhausen Criterion

_____, _____ are two tupes of LC feedback filters .

Colpitts , Hartley
The feedback network in Colpitts oscillator is made up of
___ a pair of tapped capacitors (C1 and C2) and an inductor L

The feedback network in Hartley oscillator is made up of

___ tapped inductors (L1 and L2) and a single capacitor C.