Date post: | 08-May-2015 |
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Presentation On
Harmonic Oscillators
By:-
Vishal Thakur
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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 .
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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.
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Essential Parts of any Oscillator
• Tank Circuit (R-C , L-C , etc.)
• Transistor Amplifier (180° phase shift)
• Feedback Circuit
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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).
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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…
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• 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.
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Frequency of oscillation , f = 1/2π √(L.C)
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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.
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• The amount of feedback depends on the values of C1 and C2
• Feedback Coefficient , β = C1/C2
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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
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F = 1 / 2π√[C (L1 + L2 + 2M)]
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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.
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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.
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• The overall gain is high because of use of two transistors.
• When C1 = C2 = C
and R1 = R2 = R
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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
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• 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
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• Crystal Circuit • Equivalent Circuit
• Quality factor = √L/(R √C)
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