harmonic oscillators

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  • 1.Presentation OnHarmonic OscillatorsBy:-Vishal ThakurPage 1

2. What is an oscillator???An oscillator is an electronic circuit thatproduces a repetitive electronic signal, often a sine wave or a squarewave. They are widely used in electronicdevices .Page 2 3. Harmonic Oscillators The harmonic, or linear, oscillators arethose oscillators that producesa sinusoidal output. The basic form of a harmonic oscillator isan electronic amplifier connected in apositive feedback loop with its output fedback into its input through a filter. Page 3 4. Essential Parts of any Oscillator Tank Circuit (R-C , L-C , etc.) Transistor Amplifier (180 phase shift) Feedback Circuit Page 4 5. Barkhausen Stability Criteria The loop gain should be unity. i.e A = 1 Phase shift around loop is zero or anintegral multiple of 2 (PositiveFeedback).Page 5 6. Some Harmonic OscillatorsTuned Collector Oscillator It is called the tuned-collector oscilla-torbecause the tuned circuit (L-C circuit) isconnected to the collector of the transistor. The tuned circuit constitutes of thecapacitor C and inductor L.Page 6 7. When the supply is switched on, a transientcurrent is produced in the tuned L-C circuit. It induces voltage in L1 by mutual inductionwhich causes corresponding vari-ations inbase current. These variations in base current areamplified times and appear in thecollector circuit. A part of this amplified energy is used tomeet the losses that occur in the tank circuitand the rest is radiated out.Page 7 8. Frequency of oscillation , f = 1/2 (L.C) Page 8 9. Colpitts Oscillator In this type of oscillator , the transistoramplifiers emitter is connected to thejunction of capacitors, C1 and C2 which areconnected in series and act as a simplevoltage divider. When the power supply is firstlyapplied, capacitors C1 and C2 charge upand then discharge through the coil L.Page 9 10. The amount of feedback depends on thevalues of C1 and C2 Feedback Coefficient , = C1/C2Page 10 11. Hartleys Oscillator Widely used as local oscillator in radioreceivers. Hartley oscillator circuit is similar toColpitts oscillator circuit, except thatphase-shift network consists of twoinductors L1 and L2 and a capacitor Cinstead of two capacitors and oneinductor. Feedback Fraction , = L1/L2Page 11 12. F = 1 / 2[C (L1 + L2 + 2M)]Page 12 13. Phase Shift Oscillator The phase shift oscillatorproduces positive feedback by using aninverting amplifier and adding another180 of phase shift with the three high-pass filter circuits. The most common way of achieving thiskind of oscillation is using three identicalcascaded resistor-capacitor filters. Page 13 14. Page 14 15. Wein-Bridge Oscillator It is one of the most popular type ofoscillators used in audio frequency ranges. It is a two-stage amplifier with an R-Cbridge circuit. This type of oscillator is simple indesign, compact in size, and remarkablystable in its frequency output. Furthermore, its output is relatively freefrom distortion and its fre-quency can bevaried easily.Page 15 16. The overall gain is high because of use oftwo transistors. When C1 = C2 = Cand R1 = R2 = R Page 16 17. Crystal Oscillator This is a type of oscillator that is controlledby a crystal. The big advantage of a crystal oscillator ishigh frequency stablility. Common crystalsused are Rochelle salts and quartz. The natural frequency of a crystalsvibrations is found to be more constantthan the oscillations in a LC circuit. Natural Ferquency = K/T where K = cut angle constant and T = thickness of crystalPage 17 18. The LC circuit is the electricial equilavent ofthe crystal. It has limited power output. Crystals will overheat or crack when fedwith too much voltage. The current flowingthrough a crystal generally should not bemore than 100mA (.1A) Frequencies :- Series resonant Frequency , fs =1/2 (L.C) Parallel Resonant FrequencyPage 18 19. Quality factor = L/(R C) Crystal Circuit Equivalent Circuit Page 19 20. Page 20