Electronics IIIBy

Anthony Oware-Agyekum

GTUC TAKORADI CAMPUS

Outline1. Introduction to Oscillators

2. Barkhausen Criterion

3. Classification of oscillators

4. R-C Phase shift oscillators

5. Wien Bridge oscillators

6. Hartley oscillators

7. Colpitts oscillators

8. Clap oscillators

Outline

9. Crystal oscillators

10. Amplification stabilization

Introduction An oscillator is a feedback system which uses a positive

feedback and without any external input signal, generates an output waveform at a desired frequency.

In other words, an oscillator is a signal generator which generates an output signal which oscillates with constant amplitude and constant required frequency.

An oscillator does not require an input signal to oscillate.

Introduction Positive feedback

In positive feedback, part of

the output is fed back to the

amplifier as its input which is

in phase with the original input

signal applied to the amplifier.

In the case, the phase of the

feedback signal is the same as

the input applied, and thus its

called a positive feedback.

A

b

input output+

Basic Feedback ConceptThe output signal is:

where A is the amplification factorFeedback signal is

where ß is the feedback transfer functionAt summing node:Closed-loop transfer function or gain is

AXoX

oXbfX

fi XX X

AA

XX

i

o

b

1fA

bbb 11

AAthen fAA

Introduction Barkhausen Criterion In Barkhausen criterion, the feedback network introduce a

180° phase shift and the inverting amplifier also introduces additional phase shift of 180°.

This means that the total phase shift around a loop is 360°. Vf thus drives the circuit, and works as an oscillator without

any external input signal.

Barkhausen criterion Barkhausen criterion states that:

1. The total phase shift around a loop, as the signal proceeds from input through amplifier, feedback network back to input again, completing a loop is precisely 0° or 360°.

2. The magnitude of the product of the open loop gain of the amplifier and the magnitude of the feedback factor β is unity

Satisfying the conditions above, the circuit works as an oscillator producing a sustained oscillations of constant frequency and amplitude.

No input signal is needed to start oscillation. Aβ˃1 is made greater than 1 to start oscillation, in practice and the

circuit adjust itself to Aβ =1 to get sustained oscillation.

BarkhausenThe effect of Aβ on oscillation

Aβ˃1

When the total phase shift around a loop is 0° or 360° and |Aβ| ˃1, then the

output oscillates but the oscillations are of a growing type. Thus the amplitude of

oscillation decreases exponentially and oscillation finally ceases.

Growing oscillation

Barkhausen The effect of Aβ on oscillation

Aβ=1

When the total phase shift around a loop is 0° or 360° and |Aβ| =1, then the

oscillations are with constant frequency and amplitude which is termed as

sustained oscillation

Sustained oscillation

BarkhausenThe effect of Aβ on oscillation

Aβ<1

When the total phase shift around a loop is 0° or 360° and |Aβ| ˂1, then the

output oscillates but the oscillations are of a decaying type. Thus the amplitude of

oscillation increases.

decaying oscillation

BarkhausenThe effect of Aβ on oscillation

To start oscillation, |Aβ| is kept higher than unity and the circuit adjust itself to get Aβ=1 for sustained oscillation.

BarkhausenStarting Voltage

Under the influence of normal room temperature, the free electrons of the resistance move randomly in various directions.

The movement of the free electrons generate a voltage called noise voltage across the resistance which are amplified.

The amplification is made possible when |Aβ| is kept greater than unity at the start.

The amplified voltage appears at the output terminals. The part of this output is enough to drive the input of the amplifier circuit. The circuit adjusts itself to get |Aβ| =1 with a phase shift of 360° for

sustained oscillation.

ClassificationClassification of oscillators

The y are classified based on the nature of the output waveform, the

parameters used, the range of frequency etc. Based on the Output Waveform

Under this condition, the oscillators are classified as sinusoidal and non-sinusoidal.

◦ sinusoidal oscillators generate purely sinusoidal waveforms at the output.

◦ non-sinusoidal oscillators generate an output waveform as triangular, square, sawtooth etc.

Based on the Circuit Components◦ The oscillators using the components resistance and capacitance are called RC

oscillators.

◦ The oscillators using the components inductance and capacitance are called LC oscillators..

In other instances, crystal is used, which are called crystal oscillators

Classification of oscillators Based on the Range of Operating Frequency

• If the oscillators are used to generate the oscillations at audio frequency range which is 20Hz to 200kHz, the oscillators are classified as low frequency or audio frequency.

• If the oscillators are used at the frequency range more than 200kHz to gigahertz and above then they are known as high frequency or radio frequency oscillators. The RC oscillators are used at low frequency range while the LC oscillators are used at high frequency range.

Classification

RC Phase shift Oscillators RC Phase shift Oscillators It consists of an amplifier and a feedback network consisting

of resistors and capacitors arranged in a ladder form.

RC Phase shift Oscillators

RC Phase shift OscillatorsRC feedback network RC network is used in the feedback path. The feedback network has to introduce

180° phase shift and the amplifier also producing a phase shift of 180° to obtain a total phase shift of 360°.

Thus in RC phase shift, one RC circuit produces a phase shift of 60°. This means that to get 180°, three RC circuits have to be connected in cascade form. The feedback bnetwork consists of three RC sections each producing a phase shift of

60° giving a total phase shift at the feedback to be 180°.

RC feedback network

RC Phase shift OscillatorsFrequency of oscillation The frequency of oscillation is given by

• To satisfy Barkhausen criterion, the frequency of oscillation is given by

RC Phase shift OscillatorsAdvantages The circuit is simple to design Can produce output over audio frequency range Produces sinusoidal output waveform. It is a fixed frequency oscillator

Disadvantage

The phase shift oscillator is considered as a fixed frequency oscillator, for all practical purposes

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