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Chapter: Electronic Circuits - Oscillators

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Basic Oscillator Feedback Circuit

1. Without Feedback 2. With Feedback 3. Resonance

Basic Oscillator Feedback Circuit

Where: β is a feedback fraction.

1. Without Feedback


2. With Feedback


Oscillators are circuits that generate a continuous voltage output waveform at a required frequency with the values of the inductors, capacitors or resistors forming a frequency selective LC


resonant tank circuit and feedback network. This feedback network is an attenuation network which has a gain of less than one (β<1) and starts oscillations when A β>1 which returns to unity (A β=1) once


oscillations commence. The LC oscillators frequency is controlled using a tuned or resonant inductive/capacitive (LC) circuit with the resulting output frequency being known as the Oscillation Frequency.


By making the oscillators feedback a reactive network the phase angle of the feedback will vary as a function of frequency and this is called Phase-shift.


There are basically types of Oscillators:\


1.Sinusoidal Oscillators - these are known as Harmonic Oscillators and are generally a :LC Tuned-feedback” or “RC tuned-feedback” type Oscillator that generates a purely sinusoidal waveform which is

of constant amplitude and frequency.


2.Non-Sinusoidal Oscillators – these are known as Relaxation Oscillators and generate complex non-sinusoidal waveforms that changes very quickly from one condition of stability to another such as

“Square-wave”, “Triangular-wave” or “Sawtoothed-wave” type waveforms.


3. Resonance


When a constant voltage but of varying frequency is applied to a circuit consisting of an inductor, capacitor and resistor the reactance of both the Capacitor/Resistor and Inductor/Resistor circuits is to change both the amplitude and the phase of the output signal due to the reactance of the components used.


At high frequencies the reactance of a capacitor is very low acting as a short circuit while the reactance of the inductor is high acting as an open circuit. At low frequencies the reverse is true, the reactance of the capacitor acts as an open circuit and the reactance of the inductor acts as a short circuit.


Between these two extremes the combination of the inductor and capacitor produces a “Tuned” or “Resonant” circuit that has a Resonant Frequency, (fr) in which the capacitive and inductive reactance’s are equal and cancel out each other, leaving only the resistance of the circuit to oppose the flow of current. This means that there is no phase shift as the current is in phase with the voltage. Consider the circuit below.

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