* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project

# Download Oscillators

Survey

Document related concepts

Cavity magnetron wikipedia , lookup

Stage monitor system wikipedia , lookup

Alternating current wikipedia , lookup

Mathematics of radio engineering wikipedia , lookup

Pulse-width modulation wikipedia , lookup

Buck converter wikipedia , lookup

Switched-mode power supply wikipedia , lookup

Spark-gap transmitter wikipedia , lookup

Chirp spectrum wikipedia , lookup

Utility frequency wikipedia , lookup

Resistive opto-isolator wikipedia , lookup

Opto-isolator wikipedia , lookup

Rectiverter wikipedia , lookup

Negative feedback wikipedia , lookup

Time-to-digital converter wikipedia , lookup

Crystal oscillator wikipedia , lookup

Phase-locked loop wikipedia , lookup

Transcript

1 Objectives • Define the basic oscillator circuit • Identify elements of feedback in the oscillator • Identify the conditions for oscillation to occur • Identify input and output characteristics • Identify common uses of the oscillator Oscillators By S.M.Mehzabeen 2 Word Wall • Oscillation: an effect that repeatedly and regularly fluctuates about a mean value • Oscillator: circuit that produces oscillation • Characteristics: frequency, amplitude, distortion, wave-shape, stability Oscillators By S.M.Mehzabeen 3 Oscillator In our daily life – Digital watches, Invertors, Radios , T.V, Computers, Fans, Metal Detectors, Electronic Bells and lots more Pendulum of a clock. – If you push on a pendulum to start it swinging, it will oscillate at some frequency - it will swing back and forth a certain number of times per second. – The length of the pendulum controls the frequency. – In pendulum potential energy is converted in kinetic energy Oscillators By S.M.Mehzabeen 4 Overview of the Oscillator • One of the basic building blocks of electronics • Input is a direct current (DC) power source • Output is alternating current (AC) • Can generate sub-audible frequencies or very high frequencies • Most oscillators are amplifiers with feedback Oscillators By S.M.Mehzabeen 5 Amplifiers as Oscillators? • Most amplifiers will oscillate when conditions are correct Example: Too high of a volume on public address system = loud noise and squeals that are the result of acoustic waves traveling from the speakers to the microphone • The result is oscillation Oscillators By S.M.Mehzabeen 6 Electrical Feedback • Practical oscillators use electrical feedback to produce oscillation • Feedback circuits use resistors, capacitors, coils, or transformers to connect a portion of the output signal back to the input of the amplifier Oscillators By S.M.Mehzabeen 7 Feedback and Amplifier Gain Conditions for Oscillation Feedback alone does not promise oscillation There is always some loss in the feedback circuit Amplifier gain must be greater than this loss Feedback must be in-phase Oscillators By S.M.Mehzabeen 8 In-phase Feedback • In-phase feedback is also called regenerative feedback or positive feedback • When the original amplifier input and output signals are not in-phase, the feedback circuit is used to reverse the phase Oscillators By S.M.Mehzabeen 9 Input Characteristics • Steady source of direct current (DC) • In many applications, the DC source requires a filter Oscillators By S.M.Mehzabeen 10 Output Characteristics • Amplitude • Frequency • Waveform type • Stability • On some oscillators, the capability to change frequency Oscillators By S.M.Mehzabeen 11 Oscillator Amplitude • Usually determined by the gain available from the amplifier • Supply voltage and circuit losses affect amplitude Oscillators By S.M.Mehzabeen 12 Oscillator Frequency • Frequency of operation is normally determined by the feedback circuit • Common feedback circuits used to determine oscillator frequency include: – crystals – resistor and capacitor networks (RC) – coil and capacitor networks (LC) {tank circuit} Oscillators By S.M.Mehzabeen 13 Waveform Type • Generally, determined by: – Feedback circuitry – Output filter circuitry – Amplifier gain, or – Changes to input voltage • May be sinusoidal (sine wave), square wave, or triangular wave Oscillators By S.M.Mehzabeen 14 Oscillator Stability • Sometimes referred to as a stable oscillator • Source of a signal with consistent amplitude • Source of a signal with consistent frequency Oscillators By S.M.Mehzabeen 15 Ability to Change Frequency • Oscillators sometimes have the ability to change frequencies • Crystal oscillator frequency is controlled by changing the crystal – Crystals are usually cut from quartz to generate a specified frequency when operating Oscillators By S.M.Mehzabeen 16 Typical Uses of the Oscillator • Radio and television stations require oscillators to develop the basic signal to transmit their information Oscillators By S.M.Mehzabeen 17 Typical Uses of the Oscillator • Cell phones, electronic keyboards, and remote controls use oscillators to produce the required frequencies for operation Oscillators By S.M.Mehzabeen 18 Typical Uses of the Oscillator • Digital devices such as computers, watches, calculators, and iPods all oscillators to require generate the rectangular waveform required for operation Oscillators By S.M.Mehzabeen 19 Typical Uses of the Oscillator • Variable oscillators, known as signal generators, are to generate used frequencies and waveforms needed for troubleshooting and the testing of electronic equipment Oscillators By S.M.Mehzabeen 20 Need of an Oscillator • An oscillator circuit is capable of producing ac voltage of desired frequency and waveshape. • To test performance of electronic circuits, it is called signal generator. • It can produce square, pulse, triangular, or sawtooth waveshape. Oscillators By S.M.Mehzabeen 21 Need of an Oscillator • High frequency oscillator are used in broadcasting. • Microwave oven uses an oscillator. • Used for induction heating and dielectric heating. • Oscillators are circuits that generate periodic signals. Oscillators By S.M.Mehzabeen 22 Need of an Oscillator • An oscillator converts DC power from power supply to AC signals power spontaneously – without the need for an AC input source (Note: Amplifiers convert DC power into AC output power only if an external AC input signal is present.) • There are several approaches to design of oscillator circuits. The approach to be discussed is related to the feedback using amplifiers. Oscillators By S.M.Mehzabeen 23 Need of an Oscillator • A frequency-selective feedback path around an amplifier is placed to return part of the output signal to the amplifier input, which results in a circuit called a linear oscillator that produces an approximately sinusoidal output. • Under proper conditions, the signal returned by the feedback network has exactly the correct amplitude and phase needed to sustain the output signal. Oscillators By S.M.Mehzabeen 24 Types of Oscillators • Sinusoidal or non-sinusoidal. • An oscillator generating square wave or a pulse train is called multivibrator : 1. Bistable multivibrator (Flip-Flop Circuit). 2. Monostable multivibrator. 3. Astable multivibrator (Free-running). Oscillators By S.M.Mehzabeen 25 Types of Oscillators • Depending upon type of feedback, we have 1. Tuned Circuit (LC) oscillators. 2. RC oscillators, and 3. Crystal oscillators. Oscillators By S.M.Mehzabeen 26 Using Positive Feedback The gain with positive feedback is given as • By making 1 – Aβ = 0, or Aβ = 1, we get gain as infinity. • This condition (Aβ = 1) is known as Barkhausen Criterion of oscillations. • It means you get output without any input ! Oscillators By S.M.Mehzabeen 27 How is it Possible ? • Connecting point x to y, feedback voltage drives the amplifier. Oscillators By S.M.Mehzabeen 28 How is it Possible ? • What happens to the output ? • There are three possibilities. Oscillators By S.M.Mehzabeen 29 Basic principles for oscillation • An oscillator is an amplifier with positive feedback. Ve Vs Vf (1) Vf βVo (2) Vo AVe A Vs V AVs βVo f (3) Oscillators By S.M.Mehzabeen 30 Basic principles for oscillation Vo AV e A Vs Vf Vo AVs Aββ o • The closed loop gain is: A Vs βVo 1 Aβ Vo AVs Vo A Af Vs 1 Aβ Oscillators By S.M.Mehzabeen 31 Basic principles for oscillation • In general A and are functions of frequency and thus may be written as; Vo As A f s s Vs 1 As βs • is known as loop gain As βs Oscillators By S.M.Mehzabeen 32 (1) If Aβ < 1, we get decaying of damped oscillations. Oscillators By S.M.Mehzabeen 33 (2) If Aβ > 1, we get growing oscillations. Oscillators By S.M.Mehzabeen 34 (3) If Aβ = 1, we get sustained oscillations. In this case, the circuit supplies its own input signal. Oscillators By S.M.Mehzabeen 35 Wherefrom comes the starting voltage ? ● Each resistor is a noise generator. ● The feedback network is a resonant circuit giving maximum feedback voltage at frequency f0, providing phase shift of 0° only at this frequency. ● The initial loop gain Aβ > 1. ● The oscillations build up only at this frequency. ● After the desired output is reached, Aβ reduces to unity. Oscillators By S.M.Mehzabeen 36 Tank Circuit • LC parallel circuit is called tank circuit. • Once excited, it oscillates at Oscillators By S.M.Mehzabeen 37 Tank Circuit The energy keeps oscillating between electric potential energy and magnetic filed energy. Oscillators By S.M.Mehzabeen 38 “OscillatorsarethecircuitswhichcovertsDC VoltagefrombatterytoACVoltage” – Without excitation input signal A simple example If you charge up the capacitor with a battery and then insert the inductor into the circuit, here's what will happen Oscillators By S.M.Mehzabeen 39 “OscillatorsarethecircuitswhichcovertsDC VoltagefrombatterytoACVoltage” • The capacitor will start to discharge through the inductor. As it does, the inductor will create a magnetic field • Once the capacitor discharges, the inductor will try to keep the current in the circuit moving, so it will charge up the other plate of the capacitor. Oscillators By S.M.Mehzabeen 40 “OscillatorsarethecircuitswhichcovertsDC VoltagefrombatterytoACVoltage” • Once the inductor's field collapses, the capacitor has been recharged (but with the opposite polarity), so it discharges again through the inductor • Frequency will depend upon L and C Oscillators By S.M.Mehzabeen 41 Tank Circuit • When the tank circuit resonates, the circulating current flows through L1 in series with L2. Hence the equivalent inductance is The feedback factor is Oscillators By S.M.Mehzabeen 42 Damped oscillations are produced. Oscillators By S.M.Mehzabeen 43 LC Oscillators • Hartley oscillator. • Colpitts oscillator. • Clapp oscillator. • Franklin oscillator • Tuned Collector oscillator. • Armstrong oscillator. Oscillators By S.M.Mehzabeen 44 General Form of LC Oscillators Oscillators By S.M.Mehzabeen 45 General Form of LC Oscillators - Description • Amplifier –any active device EX:Vacuum tube,Transistor,FET,Op-amp • Z1,z2,z3 –determine the frequency of oscillation • Z1,z2 serve as ac voltage divider for the output voltage and feedback signal voltage across z1-feedback signal. • 1,3 –input terminals. • 2,3-output terminals. Oscillators By S.M.Mehzabeen 46 General Form of LC Oscillators Z’=z1 II h ie = ZL =z’ +z3 II z2 Z L = II z2 Z L = + II Oscillators By S.M.Mehzabeen 47 General Form of LC Oscillators Vo = - I1(Z1+Z3) = -I1 vf =- I1 Z1 = - I1 vf= β == Av = ZL Oscillators By S.M.Mehzabeen 48 General Form of LC Oscillators Av β =1 =1 General Equation of LC Oscillator: hie (z1+z2+z3) +z1z2 (1+hfe)z1z3 =0 Oscillators By S.M.Mehzabeen 49 Hartley Oscillator • Note that in the collector-tuned circuit, two inductor coils are used. • One end of these coils is grounded. • If we make the tickler coil an integral part of the circuit, we get Hartley Oscillator. Oscillators By S.M.Mehzabeen 50 Hartley Oscillator • LC oscillator • Two inductive reactances • One capacitive reactance in its feedback network. Oscillators By S.M.Mehzabeen 51 Hartley Oscillator Oscillators By S.M.Mehzabeen 52 Circuit Description Amplifierstage: 1.Transistor –Active device in Common emitter Configuration. 2.R1 and R2 biasing resistors. 3.RE-Biasing emitter resistance 4.CE-Emitter bypass capacitors. Oscillators By S.M.Mehzabeen 53 Circuit Description 5.CC1 & CC2 are the coupling capacitors. 6.RFC-Radio frequency Choke(isolation b/w A.C & D.C) Reactance value very high for high frequencies.(open circuit) Reactance value zero for D.C.Conditions. Oscillators By S.M.Mehzabeen 54 Derivation of frequency of oscillation Z1 =jwL1+jwM z2 = jwL2 +jwM z3= General Equation of LC Oscillator: hie (z1+z2+z3) +z1z2 (1+hfe)z1z3 =0 Oscillators By S.M.Mehzabeen 55 Derivation of frequency of oscillation hie ( jwL1+ jwM+ jwL2 + jwM - )+ jwhie (L1 + L2 +2M – ) –(W2 Oscillators By S.M.Mehzabeen 56 Derivation of frequency of oscillation Equating Imaginary part = 0 L1+L2+2M = Oscillators By S.M.Mehzabeen 57 Derivation of frequency of oscillation Equating Real part = 0 Oscillators By S.M.Mehzabeen 58 Derivation of frequency of oscillation hfe L2 + hfe M –L1 –M = 0 Oscillators By S.M.Mehzabeen 59 Colpitts Oscillator • An excellent circuit. • LC Oscillator • Two Capacitive Reactance • One Inductive Reactance In The Feedback Network. • Same As A Hartley Oscillator Except The Tank Circuit. • Widely used in commercial signal generators. Oscillators By S.M.Mehzabeen 60 Colpitts Oscillator Oscillators By S.M.Mehzabeen 61 Its AC Equivalent Oscillators By S.M.Mehzabeen 62 Circuit Description Amplifierstage: 1.Transistor –Active device in Common emitter Configuration. 2.R1 and R2 biasing resistors. 3.RE-Biasing emitter resistance 4.CE-Emitter bypass capacitors. Oscillators By S.M.Mehzabeen 63 Circuit Description 5.CC1 & CC2 are the coupling capacitors. 6.RFC-Radio frequency Choke(isolation b/w A.C & D.C) Reactance value very high for high frequencies.(open circuit) Reactance value zero for D.C.Conditions. Oscillators By S.M.Mehzabeen 64 DERIVATION OF FREQUENCY OF OSCILLATION Z1 = Z2 = Z3 = jwL • General Equation of LC Oscillator: hie (z1+z2+z3 ) +z1z2 (1+hfe)z1z3 =0 Oscillators By S.M.Mehzabeen 65 DERIVATION OF FREQUENCY OF OSCILLATION • General Equation of LC Oscillator: hie (z1+z2+z3 ) +z1z2 (1+hfe)z1z3 =0 hie ( + = 0 j hie ( + -wL ) +( – Oscillators By S.M.Mehzabeen 66 DERIVATION OF FREQUENCY OF OSCILLATION • Equating Imaginary part = 0 + = wL =wL =w2 L w2 = fr = = fr = Oscillators By S.M.Mehzabeen 67 DERIVATION OF FREQUENCY OF OSCILLATION • Equating Real part = 0 = = =c2 L hfe = - 1 = w2c2L 1+hfe = hfe = Oscillators By S.M.Mehzabeen 68 Oscillators By S.M.Mehzabeen 69 Solution : Oscillators By S.M.Mehzabeen 70 Oscillators By S.M.Mehzabeen 71 CLAPP OSCILLATOR • To Achieve Frequency Stability Colpitts Oscillator Circuit Slightly Changed-clapp Osc • Addition Of One More Capacitor C3 Is Introduced In Series With The Inductance. KEY POINT: • C3 Much More Smaller Than C1 And C2. Oscillators By S.M.Mehzabeen 72 CLAPP OSCILLATOR - CIRCUIT DIAGRAM Oscillators By S.M.Mehzabeen 73 Circuit Description Amplifierstage: 1.Transistor –Active device in Common emitter Configuration. 2.R1 and R2 biasing resistors. 3.RE-Biasing emitter resistance 4.CE-Emitter bypass capacitors. Oscillators By S.M.Mehzabeen 74 Circuit Description 5.CC1 & CC2 are the coupling capacitors. 6.RFC-Radio frequency Choke(isolation b/w A.C & D.C) Reactance value very high for high frequencies.(open circuit) Reactance value zero for D.C.Conditions. Oscillators By S.M.Mehzabeen 75 Derivation of frequency of oscillation • Provides improved stability • Permits capacitive tuning of the oscillator if C3 is variable. 1 fr 2 1 Lc3 1 fr 2 L( c1c2c3 ) c1c2 c1c3 c2c3 c1c2 c3 c1 c2 Oscillators By S.M.Mehzabeen 76 ADVANTAGES of Colpitts oscillation • The frequency is stable and accurate. • The good frequency stability. • The stray capacitances have no effect on C3 which decides the frequency. • Keeping C3 variable, frequency can be varied in the desired range. Oscillators By S.M.Mehzabeen 77 Franklin oscillator • Two Transistor Stages With Some Common Terminal(emitter) • Each Stage Provides 180 Phase Shift • Both Stages Provides Amplification As Well As Phase Inversion • The Resonating Circuit Is Isolated From Active Device Path By C1 And C2. • Resonating Circuit Isolated From Input Of First Stage And Output Of Second Stage. Oscillators By S.M.Mehzabeen 78 Franklin oscillator Oscillators By S.M.Mehzabeen 79 SERIES RESONATING CIRCUIT Oscillators By S.M.Mehzabeen 80 PRACTICAL FRANKLIN OSCILLATOR CIRCUIT Oscillators By S.M.Mehzabeen 81 Franklin oscillator - CIRCUIT DESCRIPTION • Parallel Resonating Circuit Formed By L And C. • Rb Coupling Resistance • Rf Feedback Resistance. • Attenuation Caused By These Two Decides Loop Gain • Parallel Resonating Is Most Popular Than Series Resonating Circuit. Oscillators By S.M.Mehzabeen 82 DERIVATION OF FREQUENCY OF OSCILLATION 1 YL Rs jLs Yc jc Rs jLs YT 2 j C 2 2 Rs Ls YT YL YC Rs jLs YT 2 j C 2 2 Rs Ls Rs jLs YT 2 2 jC 2 2 2 2 Rs Ls Rs Ls = 1 j C Rs jLs Oscillators By S.M.Mehzabeen 83 DERIVATION OF FREQUENCY OF OSCILLATION • Equate Imaginary Part to zero: Ls C 0 2 2 2 Rs Ls Ls 2 2 Ls Rs 2 c Rs2 1 2 LsC Ls Ls Rs 2 2 Ls 2 c 1 Rs 2 2 LsC Ls 2 2 1 1 Rs ( f 2) LsC Ls 2 Oscillators By S.M.Mehzabeen 84 DERIVATION OF FREQUENCY OF OSCILLATION f 1 2 LsC Rs 2 2 Ls 1 • The impedance at resonance condition is determined from the real part. 1 zr Yr Rs 2 2 Ls 2 2 Ls2 1 zr Rs Yr Rs Rs Oscillators By S.M.Mehzabeen 85 DERIVATION OF FREQUENCY OF OSCILLATION • Substituting value 2 Ls zr CRs Rs 2 Ls 1 zr Rs ( 2 ) Rs LsC Ls Ls zr Rs Rs CRs Ls zr CRs 2 Oscillators By S.M.Mehzabeen 86 FREQUENCY RANGE • 100MHZ TO 3 GHZ Oscillators By S.M.Mehzabeen 87 APPLICATION • RF Oscillator • Precision Frequency Meter-used For The Measurement Of Frequency Oscillators By S.M.Mehzabeen 88 ARMSTRONG OSCILLATOR Oscillators By S.M.Mehzabeen 89 TICKLER OSCILLATOR • Small Secondary Winding Is Called Tickler Coil. • Hence Another Name Tickler Oscillator. Oscillators By S.M.Mehzabeen 90 CIRCUIT DESCRIPTION • LC Oscillator. • Employs Transformer Action. • Primary Acts As A Inductor(L) • Voltage Across The Secondary Used As A Feedback. • Biasing Point Selected By R1,R2,RE. Oscillators By S.M.Mehzabeen 91 CIRCUIT DESCRIPTION • C1 And C2 Coupling Capacitors. • C3 Emitter Bypass Capacitor. • Feedback Signal Applied To The Base.(Q) • Transformer 180 Phase Shift .Transistor Q 180 Phaseshift. • Overall Phase Shift 360 Satisfies Barkhausen Criterion. Oscillators By S.M.Mehzabeen 92 FREQUENCY OF OSCILLATION • The Sustained Oscillation Depends On Inductance(l) And Capacitance(C) Oscillators By S.M.Mehzabeen 93 DRAWBACKS • Rarely Used. • Compared To Other LC Oscillator. • Transformer Used • Circuit Costlier And Bulkier. Oscillators By S.M.Mehzabeen 94 TUNED COLLECTOR OSCILLATOR Oscillators By S.M.Mehzabeen 95 WHY? • The Tuned Circuit Is Placed In The Collector Of The Transistor. • Hence Called Tuned Collector Oscillator. Oscillators By S.M.Mehzabeen 96 CIRCUIT DESCRIPTION • LC Oscillator. • Employs Transformer Action. • Primary Acts As A Inductor(L) • Voltage Across The Secondary Used As A Feedback. • Biasing Point Selected By R1,R2,RE. Oscillators By S.M.Mehzabeen 97 CIRCUIT DESCRIPTION • Collector Drives The LC Resonating Circuit. • Feedback Signal Induced From The Primary Applied To The Base.(Q) • Transformer 180 Phase Shift .Transistor Q 180 Phaseshift. • Overall Phase Shift 360 Satisfies Barkhausen Criterion. Oscillators By S.M.Mehzabeen 98 FREQUENCY OF OSCILLATION • The Sustained Oscillation Depends On Inductance(l) And Capacitance(C) Oscillators By S.M.Mehzabeen 99 DRAWBACKS • Rarely Used. • Compared To Other LC Oscillator. • Transformer Used • Circuit Costlier And Bulkier. Oscillators By S.M.Mehzabeen 10 0 Oscillators By S.M.Mehzabeen 10 1 RC Oscillators • Three types : 1. RC Phase shift Oscillator. 2. Wein Bridge Oscillator. 3. Twin T Oscillator. Oscillators By S.M.Mehzabeen 10 2 TRANSISTORIZED RC FEEDBACK NETWORK Oscillators By S.M.Mehzabeen 10 3 RC FEEDBACK NETWORK Oscillators By S.M.Mehzabeen 10 4 Derivation for RC phase shift oscillator. Let c1 =c2=c3=c R1=R2=R R3+(Rth II Rin’)=R Oscillators By S.M.Mehzabeen 10 5 Derivation for RC phase shift oscillator. Oscillators By S.M.Mehzabeen 10 6 Derivation for RC phase shift oscillator. Oscillators By S.M.Mehzabeen 10 7 Derivation for RC phase shift oscillator. Oscillators By S.M.Mehzabeen 10 8 Derivation for RC phase shift oscillator. Oscillators By S.M.Mehzabeen 10 9 Derivation for RC phase shift oscillator. Oscillators By S.M.Mehzabeen 11 0 Advantages and Disadvantages of RC oscillator Advantages Disadvantages • Doesn ’ t require any • Fixed frequency oscillator. bulky and expensive inductors. • Only one frequency fulfill barkhausen criterion . • Pure sine wave output is possible. Oscillators By S.M.Mehzabeen 11 1 Oscillators By S.M.Mehzabeen 11 2 Solution : Oscillators By S.M.Mehzabeen 11 3 WIEN BRIDGE OSCILLATOR Oscillators By S.M.Mehzabeen 11 4 WIEN BRIDGE OSCILLATOR Z S R ZC R 1 1 sRC sC sC 1 1 1 1 sC Z P R ZC R ZC R 1 R 1 sCR Oscillators By S.M.Mehzabeen 11 5 WIEN BRIDGE OSCILLATOR Input Loading 1 1 Z1 Z P Z S ZP Z S 1 sC R1 sCR 1 sCR R 1 sCR sCR (1 sCR)2 1 Output Loading Z2 ZS R ZC 1 sRC sC Oscillators By S.M.Mehzabeen 11 6 Amplifier Gain Ar V 0 V 0 Vi IS Vi I S To get V0 , we use Vi Vi V V I 1 R1 V0 R1 R 2 Vi R1 Since 1 I 0, I1 I 2 Vo R1 R 2 and Vo R1 so R1 R 2 R2 R1 Vi Z 1 and IS V 0 Vi R Z 1 1 2 Vi I S R1 R 1 sCR where Z 1 sCR (1 sCR ) 2 Ar so R R 1 sCR Ar 1 2 R1 sCR (1 sCR ) 2 Oscillators By S.M.Mehzabeen 11 7 Feedback factor f Xf Xo If Vo 1 ZS sC 1 sRC Oscillators By S.M.Mehzabeen 11 8 Oscillation Condition Phase of f A r equal to 180 o . It already Then need R is since f Ar 0. sCR f A r 1 2 1 R1 sCR (1 sCR ) 2 only Rewriting f Ar 1 R2 sCR R1 sCR (1 sCR ) 2 sCR R 1 2 R 1 2 sCR s 2 C 2 R 2 sCR 1 R sCR R 1 2 1 2 2 2 2 R1 1 3 sCR s C R R1 R 1 1 2 1 R 1 3 j CR CR Then imaginary term 0 at 1 o RC Then, we can get the 1 3 1 sCR sCR oscillatio n frequency f Ar 1 by selecting the resistors R1 and R2 appropriat ely using R 1 1 2 1 R1 3 or R2 2 R1 Oscillators By S.M.Mehzabeen 11 9 Loop gain sC Ar 1 sCR f Ar sC R2 R1 sCR 1 R1 sCR (1 sCR) 2 1 sCR R sCR 1 2 R1 sCR (1 sCR) 2 Gain with feedback is Arf Ar 1 f Ar Oscillators By S.M.Mehzabeen 12 0 Advantages and Disadvantages of wien bridge oscillator Advantages Disadvantages • It uses both positive and • More negative feedback, it provides better stability and high over all gain. • Frequency can be easily components used costlier. • It cant generate very high frequency. adjusted by varying R or C Oscillators By S.M.Mehzabeen 12 1 TWIN T OSCILLATOR Oscillators By S.M.Mehzabeen 12 2 Crystal Oscillator • The Piezoelectric Effect Quartz exhibits piezoelectric effect. – When a changing mechanical stress is applied across the crystal to cause it to vibrate, a voltage develops at the frequency of mechanical vibration. – Conversely, whenanacvoltageisappliedacrossthecrystal,it vibratesatthefrequencyoftheappliedvoltage.Thegreatest vibrationoccursatthecrystal'snaturalresonantfrequency. – Which is determined by the physical dimensions and by the way the crystal is cut. Oscillators By S.M.Mehzabeen 12 3 Crystal Oscillator Oscillators By S.M.Mehzabeen 12 4 Crystal Oscillator • From equivalent circuit it is clear that it consists of series as well as parallel resonant circuit • At series resonance inductive reactance is equal to capacitive reactance Cs • At parallel resonance inductive reactance is equal to capacitive reactance Cm • So,crystal can be used in hartley or colpitts oscillator in place of the tank circuit Oscillators By S.M.Mehzabeen 12 5 Crystal Oscillator Oscillators By S.M.Mehzabeen 12 6 Crystal Oscillator • Used when accuracy and stability of fo is utmost important. • Where do you need such high stability of frequency of oscillations ? • Instead of an inductor, it uses a crystal of quartz, tourmaline, or Rochelle salt. Oscillators By S.M.Mehzabeen 12 7 Crystal Oscillator • Piezoelectric effect. • The crystal is suitably cut and then mounted between two metallic plates. • The fundamental frequency is given as Oscillators By S.M.Mehzabeen 12 8 Crystal Oscillator Cm (mounting capacitance) = 3.5 pF; Cs = 0.0235 pF; L = 137 H; R = 15 kΩ Oscillators By S.M.Mehzabeen 12 9 Crystal Oscillator • Crystals have incredibly high Q. • For the given values, Q = 5500. • Q as high as 100 000 can be possible. • An LC circuit has Q not greater than 100. • The extremely high value of Q makes fo highly stable. Oscillators By S.M.Mehzabeen 13 0 Crystal Oscillator Oscillators By S.M.Mehzabeen 13 1 Series and Parallel Resonance • First, resonance occurs at fs for the series combination of L and Cs. • Above fs the series branch LCsR has inductive reactance. • It then resonates at fp , with Cm. • For this parallel resonance, equivalent series capacitance is Cp. Oscillators By S.M.Mehzabeen 13 2 Crystal Oscillator Oscillators By S.M.Mehzabeen 13 3 Crystal Oscillator • Normally, Cs is much smaller than Cm. • Therefore, Cp is slightly less than Cs. • Hence, the frequency fp is slightly greater than fs. • The crystal is inductive only between the frequencies fs and fp. • The frequency of oscillation must lie between these frequencies. • Hence the stability. Oscillators By S.M.Mehzabeen 13 4 The fo is between 411 kHz and 412 kHz. Oscillators By S.M.Mehzabeen 13 5 Pierce Oscillator • If we assume that the current through C1,2 is larger than the collector current (high Q), then we see that the same current flows through both capacitors. The voltage at the input and output is therefore vo I1 1 jC1 vi I1 1 jC2 vo C n 1 C2 vi Oscillators By S.M.Mehzabeen 13 6 FACTORS AFFECTING THE FREQUENCY STABILITY • Change in temperature affects –component values of the tank circuit. • Change in temperature affects active device parametres inturn affect the frequency. • Variation in the power supply the another factor affecting the frequency. • Changes in the load connected affect the effective resistance of the tank circuit. Oscillators By S.M.Mehzabeen 13 7 MODIFICATIONS FOR FREQUENCY STABILITY • Enclosing the circuit in a constant temperature chamber. • Maintaining constant voltage by using the zener diodes. • Load effect is reduced by coupling the oscillator to the load loosely • Circuit having high input impedance and low output impedance. Oscillators By S.M.Mehzabeen 13 8 Variation of frequency with temperature.FOR FREQUENCY STABILITY • It is denoted by ‘S’. Oscillators By S.M.Mehzabeen 13 9 Review • Need of an Oscillator. • Types of Oscillators. • Using Positive Feedback. • Barkhausen Criterion of Oscillations. • Starting Voltage . • Tank Circuit. • Tuned Collector Oscillator. Oscillators By S.M.Mehzabeen 14 0 Review • Hartley Oscillator. • Colpitts Oscillator. • RC Phase Shift Oscillator. • Wien Bridge Oscillator. • Crystal Oscillator. • Series and Parallel Resonance Oscillators By S.M.Mehzabeen 14 1