Harmonic Oscillators

Presentation On

Harmonic Oscillators

By:-

Vishal Thakur

What is an oscillator…???

An oscillator is an electronic circuit that produces a repetitive electronic signal,

often a sine wave or a square wave. They are widely used in electronic devices .

Harmonic Oscillators

• The harmonic, or linear, oscillators are those oscillators that produces a sinusoidal output.

• The basic form of a harmonic oscillator is an electronic amplifier connected in a positive feedback loop with its output fed back into its input through a filter.

Essential Parts of any Oscillator

• Tank Circuit (R-C , L-C , etc.)

• Transistor Amplifier (180° phase shift)

• Feedback Circuit

Barkhausen Stability Criteria

• The loop gain should be unity.

i.e Aβ = 1

• Phase shift around loop is zero or an integral multiple of 2π (Positive Feedback).

Tuned Collector Oscillator

• It is called the tuned-collector oscilla tor because the tuned circuit (L-C circuit) is connected to the collector of the transistor.

• The tuned circuit constitutes of the capacitor C and inductor L.

Some Harmonic Oscillators…

• When the supply is switched on, a transient current is produced in the tuned L-C circuit.

• It induces voltage in L1 by mutual induction which causes corresponding vari ations in base current.

• These variations in base current are amplified β times and appear in the collector circuit.

• A part of this amplified energy is used to meet the losses that occur in the tank circuit and the rest is radiated out.

Frequency of oscillation , f = 1/2π √(L.C)

Colpitt’s Oscillator

• In this type of oscillator , the transistor amplifier’s emitter is connected to the junction of capacitors, C1 and C2 which are connected in series and act as a simple voltage divider.

• When the power supply is firstly applied, capacitors C1 and C2 charge up and then discharge through the coil L.

• The amount of feedback depends on the values of C1 and C2

• Feedback Coefficient , β = C1/C2

Hartley’s Oscillator• Widely used as local oscillator in radio

receivers.

• Hartley oscillator circuit is similar to Colpitt’s oscillator circuit, except that phase-shift network consists of two inductors L1 and L2 and a capacitor C instead of two capacitors and one inductor.

• Feedback Fraction , β = L1/L2

F = 1 / 2π√[C (L1 + L2 + 2M)]

Phase Shift Oscillator

• The phase shift oscillator produces positive feedback by using an inverting amplifier and adding another 180° of phase shift with the three high-pass filter circuits.

• The most common way of achieving this kind of oscillation is using three identical cascaded resistor-capacitor filters.

Wein-Bridge Oscillator

• It is one of the most popular type of oscillators used in audio frequency ranges.

• It is a two-stage amplifier with an R-C bridge circuit.

• This type of oscillator is simple in design, compact in size, and remarkably stable in its frequency output.

• Furthermore, its output is relatively free from distortion and its fre quency can be varied easily.

• The overall gain is high because of use of two transistors.

• When C1 = C2 = C

and R1 = R2 = R

Crystal Oscillator• This is a type of oscillator that is controlled

by a crystal. • The big advantage of a crystal oscillator is

high frequency stablility. Common crystals used are Rochelle salts and quartz.

• The natural frequency of a crystal's vibrations is found to be more constant than the oscillations in a LC circuit.

Natural Ferquency = K/T

where K = cut angle constant

and T = thickness of crystal

• The LC circuit is the electricial equilavent of the crystal.

• It has limited power output.• Crystals will overheat or crack when fed

with too much voltage. The current flowing through a crystal generally should not be more than 100mA (.1A)

• Frequencies :-• Series resonant Frequency , fs =1/2π

√(L.C)• Parallel Resonant Frequency

• Crystal Circuit • Equivalent Circuit

• Quality factor = √L/(R √C)

Harmonic Oscillators

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