Oscillator Circuits - Oregon State University

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Oscillator Circuits1

II. Oscillator OperationFor self-sustaining oscillations: the feedback signal must positive the overall gain must be equal to one (unity gain)2

If the feedback signal is not positive or the gain is less than one, then the oscillations willdampen out.If the overall gain is greater than one, then the oscillator will eventually saturate.3

Types of Oscillator CircuitsA.B.C.D.E.Phase-Shift OscillatorWien Bridge OscillatorTuned Oscillator CircuitsCrystal OscillatorsUnijunction Oscillator4

A. Phase-Shift OscillatorFrequency of the oscillator:f0 12πRC 6(the frequency where the phase shift is 180º)Feedback gain β 1/[1 – 5α2 – j (6α – α3) ] where α 1 / (2πfRC)Feedback gain at the frequency of the oscillator β 1 / 29The amplifier must supply enough gain to compensate for losses. The overall gain must beunity. Thus the gain of the amplifier stage must be greater than 1/β, i.e. A 29The RC networks provide the necessary phase shift for a positive feedback. They alsodetermine the frequency of oscillation.5

Example of a Phase-Shift OscillatorFET Phase-Shift Oscillator6

Example 17

BJT Phase-Shift OscillatorR′ R hieh fe 23 29RCR 4RRC8

Phase-shift oscillator using op-amp9

B. Wien Bridge Oscillatorβ V i Vd VbZ2R411 R3VaZ 1 Z 2 R3 R4 Z 1Vo 1 1Z2R4β 0 R3 R1 C 2 R4 R2 C1Z2Z, i.e., 1 should have zero phase at the oscillation frequency When R1 R2 R and C 1 C 2 C thenZ1 Z 2Z2So frequency of oscillation isf0 12π ( R1C1 R2C 2 )f0 R31, and 2R42πRC10

Example 2Calculate the resonant frequency of the Wien bridge oscillator shown abovef0 11 3120.7 Hz2πRC 2 π(51 10 3 )(1 10 9 )11

C. Tuned Oscillator CircuitsTuned Oscillators use a parallel LC resonant circuit (LC tank) to provide the oscillations.There are two common types: Colpitts – The resonant circuit is an inductor and two capacitors. Hartley – The resonant circuit is a tapped inductor or two inductors and onecapacitor.12

Colpitts Tuned Oscillator CircuitFET based Colpitts OscillatorFrequency of oscillations:f0 CC1, where C eq 1 2C1 C 22π LC eq13

Transistor Colpitts oscillator.14

Op-amp Colpitts oscillator.15

Hartley Tuned Oscillator CircuitFET Hartley OscillatorFrequency of oscillations:f0 1, where Leq L1 L2 2 L122π LeqC16

BJT based Hartley Oscillator17

D. Crystal OscillatorsThe crystal appears to the rest of the circuit as a resonant circuit.18

Crystal Resonant FrequenciesThe crystal has two resonant frequencies:Series resonant: RLC determine the resonant frequency. The crystal has a low impedance.Parallel resonant: RL and CM determine the resonant frequency. The crystal has a highimpedance.The series and parallel resonant frequencies are very close, within 1% of each other.19

Series Resonant Crystal OscillatorFET Crystal Oscillator20

Parallel Resonant Crystal OscillatorBJT Crystal Oscillator21

Crystal Oscillator using Op-amp22

E. Unijunction OscillatorFrequency of oscillations:f0 1RT CT ln[1 ( 1 η)]where η 0.4 to 0.6η is a rating of the unijunction transistor.23

Unijunction Oscillator WaveformsVBB VB 2 VB1VP ηVBB VB1 VDThe unijunction oscillator (or relaxation oscillator) produces a sawtooth waveform.24

25

Unijunction transistor (UJT): basic construction.26

UJT – Unijunction TransistorThe UJT is also basically a switching device.Schematic Symbol:27

UJT Basic OperationEven though the UJT is a switching device it works very differently from the SCR varietyof devices.The equivalent circuit indicates that the UJT is like a diode and a resistive voltage dividercircuit. The resistance exhibited by RB1 is variable; it is dependent on the value of currentIE.28

UJT Characteristic CurveA voltage is applied across the UJT (VBB) and to the Emitter input (VE). Once VE reaches apeak value (Vp) the UJT begins to conduct. At the point where VE Vp, the current IE is atminimum. This is the threshold value of VE that puts the UJT into conduction. Onceconducting, IE increases and VE decreases. This phenomenon occurs because the internalresistance labeled RB1 in the equivalent circuit decreases as the UJT conducts more andmore. This is called negative resistance.29

BJT based Hartley Oscillator. 18 D. Crystal Oscillators The crystal appears to the rest of the circuit as a resonant circuit. 19 Crystal Resonant Frequencies The crystal has two resonant frequencies: Series resonant: RLC determine

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