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2 Objectives Define the basic oscillator circuit • • Identifyelements 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
3 Word Wall Oscillation: an effect that repeatedly and regularly fluctuates about a mean value • • Oscillator: circuit that produces oscillation • Characteristics: frequency, wave-shape, stability amplitude, distortion, Oscillators By S.M.Mehzabeen 3
4 Oscillator In our daily life Pendulum of a clock. 4– 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
5 Overview of the OscillatorOne 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 frequencies high • Most oscillators are amplifiers with feedback Oscillators By S.M.Mehzabeen 5
6 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
7 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
8 Conditions for OscillationFeedback 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
9 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
10 Input Characteristics• Steady source of direct current (DC) • In many applications, the DC source requires a filter Oscillators By S.M.Mehzabeen 10
11 Output Characteristics • Amplitude • Frequency • Waveform type• Stability • On some oscillators, the capability to change frequency Oscillators By S.M.Mehzabeen 11
12 Oscillator Amplitude • Usually determined by the gain available from the amplifier • Supply voltage and circuit losses affect amplitude Oscillators By S.M.Mehzabeen 12
13 • Common feedback circuits used to determine oscillatorOscillator 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
14 Waveform Type • Generally, determined by: – Feedback circuitry– Output filter circuitry – Amplifier gain, or – Changes to input voltage Type • May be sinusoidal (sine wave), square wave, or triangular wave Oscillators By S.M.Mehzabeen 14
15 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
16 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
17 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
18 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
19 Typical Uses of the Oscillator • Digital devices such as computers,calculators, require watches, and iPods all oscillators to generate waveform operation the rectangular required for Oscillators By S.M.Mehzabeen 19
20 Typical Uses of the Oscillator • Variable oscillators, known as signalgenerators, are to generate and waveforms troubleshooting used frequencies needed for and the testing of electronic equipment Oscillators By S.M.Mehzabeen 20
21 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
22 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 signals. periodic Oscillators By S.M.Mehzabeen 22
23 • An oscillator converts DC power from power supply to ACNeed 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 • There are several if an external AC input signal is present.) approaches to design of oscillator circuits. The approach to be discussed is related to the feedback using amplifiers. Oscillators By S.M.Mehzabeen 23
24 Need of an Oscillator • A frequency-selective feedback path around an amplifier is placed to return part of to the amplifier input, which results the output signal in a circuit called an approximately a linear oscillator that produces 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
25 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. 3. Monostable multivibrator. Astable multivibrator (Free-running). Oscillators By S.M.Mehzabeen 25
26 Types of Oscillators Depending upon type of feedback, we have • 2.1. Tuned Circuit (LC) oscillators. 2. RC oscillators, and 3. Crystal oscillators. Oscillators By S.M.Mehzabeen 26
27 Using Positive FeedbackThe 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
28 How is it Possible ? • Connecting point x the amplifier.to y, feedback voltage drives Oscillators By S.M.Mehzabeen 28
29 How is it Possible ? • What happens to the output ?• There are three possibilities. Oscillators By S.M.Mehzabeen 29
30 Ve Vs Vf Vf βVo AVs AVs βVo AVe Vf Basic• An oscillator is principles for oscillation an amplifier with positive feedback. Ve Vs Vf (2) (1) Vf Vo βVo AVs AVs βVo AVe Vf (3) Oscillators By S.M.Mehzabeen 30
31 A Vs A Vs βVo Vo Vf 1 1 Aβ Vo AVs Aββ VoBasic principles for oscillation A Vs A Vs βVo AVs Vo AV Vf e 1 Aβ Vo Vo AVs Aββ o • The closed loop gain is: Vo Vs A A f 1 Aβ Oscillators By S.M.Mehzabeen 31
32 s As Vo A s 1 As βs As βs Basic principles for oscillation • In general A and are functions of frequency and thus may be written as; As Vo Vs A s s f 1 As βs • is known as loop gain As βs Oscillators By S.M.Mehzabeen 32
33 (1) If Aβ < 1, we get decaying of damped oscillations. 33 Oscillators By S.M.Mehzabeen 33
34 (2) If Aβ > 1, we get growing oscillations. 34 2014-12-24Oscillators By S.M.Mehzabeen 34
35 (3) If Aβ = 1, we get sustained oscillations. In this case, thecircuit supplies its own input signal. Oscillators By S.M.Mehzabeen 35
36 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
37 • LC parallel circuit is called tank circuit. • Once excited, it oscillates at Oscillators By S.M.Mehzabeen 37
38 The energy keeps oscillating between electric Tank Circuit The energy keeps oscillating between electric potential energy and magnetic filed energy. Oscillators By S.M.Mehzabeen 38
39 “Oscillators are the circuits which coverts DC Voltage from battery to AC Voltage” – Without excitation input signal A simple example If you charge up the capacitor with a battery and then insert the inductor what will into the circuit, here's happen Oscillators By S.M.Mehzabeen 39
40 “Oscillators are the circuits which coverts DC Voltage from battery to AC Voltage” • 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 keep the current in the circuit moving, so it will charge up the other plate of the capacitor. • try to Oscillators By S.M.Mehzabeen 40
41 “Oscillators are the circuits which coverts DC Voltage from battery to AC Voltage” • Once the inductor's field collapses, the capacitor has been recharged (but with the opposite polarity), discharges again through the inductor so it • Frequency will depend upon L and C Oscillators By S.M.Mehzabeen 41
42 L1 L2. Tank Circuit The feedback factor is• When the tank circuit resonates, the circulating current flows through inductance is L1 in series with L2. Hence the equivalent The feedback factor is Oscillators By S.M.Mehzabeen 42
43 Damped oscillations are produced. 43 2014-12-24Oscillators By S.M.Mehzabeen 43
44 • Tuned Collector oscillator.LC • Hartley oscillator. Oscillators • Colpitts oscillator. • Clapp oscillator. • Franklin oscillator • Tuned Collector oscillator. • Armstrong oscillator. Oscillators By S.M.Mehzabeen 44
45 General Form of LC Oscillators 45 2014-12-24Oscillators By S.M.Mehzabeen 45
46 • Z1,z2,z3 –determine the frequency of oscillationGeneral 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 • 1,3 –input terminals. • 2,3-output terminals. across z1-feedback signal. Oscillators By S.M.Mehzabeen 46
47 ZL =z’ +z3 z2 General Form of LC Oscillators II Z L = z2 II Z L = + IIZ’=z1 II h e = i ZL =z’ +z3 II z2 Z L = z2 II Z L = + II Oscillators By S.M.Mehzabeen 47
48 General Form of LC Oscillators vf= vf = 1 Z1 = 1 Vo = 1(Z1+Z3) = -I1 -β == Av = ZL Oscillators By S.M.Mehzabeen 48
49 General Form of LC Oscillators Av β =1 hie (z1+z2+z3) +z1z2Equation of LC Oscillator: hie (z1+z2+z3) +z1z2 (1+hfe)z1z3 =0 Oscillators By S.M.Mehzabeen 49
50 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
51 Hartley Oscillator • LC oscillator • Two inductive reactances• One capacitive reactance in its feedback network. Oscillators By S.M.Mehzabeen 51
52 Hartley Oscillator Oscillators By S.M.Mehzabeen 52
53 Circuit Description Amplifier stage: 1.Transistor –ActiveConfiguration. device in Common emitter 2.R1 and R2 biasing resistors. 3.RE-Biasing emitter resistance 4.CE-Emitter bypass capacitors. Oscillators By S.M.Mehzabeen 53
54 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
55 z3= hie (z1+z2+z3) +z1z2 (1+hfe)z1z3 =0 Derivation of frequency ofZ1 =jwL1+jwM z2 = jwL2 +jwM of frequency of oscillation z3= General Equation of LC Oscillator: hie (z1+z2+z3) +z1z2 (1+hfe)z1z3 =0 Oscillators By S.M.Mehzabeen 55
56 Derivation of frequency of oscillation h e ( wL1+ wM+ wL2 + wM )+ j j- i wh e (L1 + L2 +2M – ) –(W2 j i Oscillators By S.M.Mehzabeen 56
57 Derivation of frequency of oscillation Equat ng mag nary part = 0L1+L2+2M = i I i Oscillators By S.M.Mehzabeen 57
58 Derivation of frequency of oscillation Equating Real part = 58 Oscillators By S.M.Mehzabeen 58
59 hfe L2 hfe –L1 Derivation of frequency of oscillation + M –M = 59 Oscillators By S.M.Mehzabeen 59
60 Colpitts Oscillator An excellent circuit. LC OscillatorTwo 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
61 Colpitts Oscillator Oscillators By S.M.Mehzabeen 61
62 Its AC Equivalent Oscillators By S.M.Mehzabeen 62
63 Circuit Description Amplifier stage: 1.Transistor –Active deviceConfiguration. in Common emitter 2.R1 and R2 biasing resistors. 3.RE-Biasing emitter resistance 4.CE-Emitter bypass capacitors. Oscillators By S.M.Mehzabeen 63
64 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
65 Z3 = jwL hie (z1+z2+z3 +z1z2 (1+hfe)z1z3 DERIVATION OF FREQUENCY OFOSCILLATION Z1 = Z2 = Z3 = jwL • General Equation of LC Oscillator: hie (z1+z2+z3 ) +z1z2 (1+hfe)z1z3 =0 Oscillators By S.M.Mehzabeen 65
66 hie (z1+z2+z3 +z1z2 (1+hfe)z1z3 DERIVATION OF FREQUENCY OF OSCILLATION• General Equation FREQUENCY OF OSCILLATION of LC Oscillator: hie (z1+z2+z3 ) +z1z2 (1+hfe)z1z3 =0 h e ( + = 0 i h e ( + wL ) +( – j - i Oscillators By S.M.Mehzabeen 66
67 DERIVATION OF FREQUENCY OF OSCILLATION• Equating Imaginary part = 0 OF OSCILLATION + = wL =wL =w2 L w2 = r = = f r = f Oscillators By S.M.Mehzabeen 67
68 DERIVATION OF FREQUENCY OF OSCILLATION• Equating Real part = 0 OF OSCILLATION = = w2c2L = =c2 L 1+hfe = hfe = 1 hfe = - Oscillators By S.M.Mehzabeen 68
69 Oscillators By S.M.Mehzabeen 69
70 Solution : Oscillators By S.M.Mehzabeen 70
71 Oscillators By S.M.Mehzabeen 71
72 • To Achieve Frequency Stability Colpitts Oscillator Circuit 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
73 CLAPP OSCILLATOR - CIRCUIT DIAGRAM 73 2014-12-24Oscillators By S.M.Mehzabeen 73
74 Circuit Description Amplifier stage: 1.Transistor –Active deviceConfiguration. in Common emitter 2.R1 and R2 biasing resistors. 3.RE-Biasing emitter resistance 4.CE-Emitter bypass capacitors. Oscillators By S.M.Mehzabeen 74
75 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
76 Derivation of frequency of oscillation• Provides improved stability • Permits capacitive tuning of the oscillator if C3 is variable. 1 1 1 fr fr c1c2 c3 L( c1c2c ) 2 Lc3 2 c1 c2 c1c2 c1c3 c2c3 Oscillators By S.M.Mehzabeen 76
77 • The stray capacitances have no effect on C3 whichADVANTAGES 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
78 • The Resonating Circuit Is Isolated From Active Device Path ByFranklin 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
79 Franklin oscillator Oscillators By S.M.Mehzabeen 79
80 SERIES RESONATING CIRCUIT Oscillators By S.M.Mehzabeen 80
81 PRACTICAL FRANKLIN OSCILLATOR CIRCUIT 81 2014-12-24Oscillators By S.M.Mehzabeen 81
82 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
83 FREQUENCY OF OSCILLATIONDERIVATION 1 OF FREQUENCY OF OSCILLATION Yc jc YL Rs jLs 1 Rs jLs YT YL YC = jC YT jC Rs2 2 Ls2 Rs jLs Rs jLs Rs jLs jC YT jC YT Rs2 2 Ls2 Rs2 2 Ls2 Rs2 2 Ls2 Oscillators By S.M.Mehzabeen 83
84 FREQUENCY OF OSCILLATIONDERIVATION OF • Equate Imaginary Ls FREQUENCY OF OSCILLATION Part to zero: Ls Rs2 2 Ls2 C 0 Rs2 2 Ls2 c 1 Rs 2 Ls c 2 Ls2 2 Rs2 LsC Ls 2 1 Rs2 2 1 1 Rs ) f ( LsC Ls2 2 LsC Ls 2 Oscillators By S.M.Mehzabeen 84
85 FREQUENCY OF OSCILLATIONDERIVATION 1 OF FREQUENCY OF OSCILLATION Rs 2 f 1 2 LsC Ls 2 • The impedance at resonance condition is determined from the real part. 1 1 Rs Ls 2 2 2 Ls 2 2 zr zr Rs Yr Yr Rs Rs Oscillators By S.M.Mehzabeen 85
86 DERIVATION OF FREQUENCY OF OSCILLATION• Substituting value Ls 2 2 Ls 1 Rs2 Ls2 zr zr Rs ( ) CRs Rs LsC Ls Ls zr Rs Rs zr CRs CRs Oscillators By S.M.Mehzabeen 86
87 • 100MHZ TO 3 GHZ FREQUENCY RANGE 87 2014-12-24Oscillators By S.M.Mehzabeen 87
88 APPLICATION • RF Oscillator• Precision Frequency Meter-used For The Measurement Of Frequency Oscillators By S.M.Mehzabeen 88
89 ARMSTRONG OSCILLATOR Oscillators By S.M.Mehzabeen 89
90 • Small Secondary Winding Is Called Tickler Coil.TICKLER OSCILLATOR • Small Secondary Winding Is Called Tickler Coil. • Hence Another Name Tickler Oscillator. Oscillators By S.M.Mehzabeen 90
91 • Employs Transformer Action.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
92 • C1 And C2 Coupling Capacitors. • C3 Emitter Bypass Capacitor. 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
93 FREQUENCY OF OSCILLATION• The Sustained Oscillation Depends On Inductance(l) And Capacitance(C) Oscillators By S.M.Mehzabeen 93
94 • Compared To Other LC Oscillator.DRAWBACKS • Rarely Used. • Compared To Other LC Oscillator. • Transformer Used • Circuit Costlier And Bulkier. Oscillators By S.M.Mehzabeen 94
95 TUNED COLLECTOR OSCILLATOR Oscillators By S.M.Mehzabeen 95
96 • The Tuned Circuit Is Placed In The Collector Of The Transistor. WHY? • The Tuned Circuit Is Placed In The Collector Of The Transistor. • Hence Called Tuned Collector Oscillator. Oscillators By S.M.Mehzabeen 96
97 • Employs Transformer Action. • Primary Acts As A Inductor(L)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
98 • Collector Drives The LC Resonating Circuit.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
99 FREQUENCY OF OSCILLATION• The Sustained Oscillation Depends On Inductance(l) And Capacitance(C) Oscillators By S.M.Mehzabeen 99
100 • Compared To Other LC Oscillator.DRAWBACKS • Rarely Used. • Compared To Other LC Oscillator. • Transformer Used • Circuit Costlier And Bulkier. 10 Oscillators By S.M.Mehzabeen
101 10 1 Oscillators By S.M.Mehzabeen
102 RC Oscillators • Three types : 1. RC Phase shift Oscillator. 2.Wein Bridge Oscillator. 3. Twin T Oscillator. 10 2 Oscillators By S.M.Mehzabeen
103 TRANSISTORIZED RC FEEDBACK NETWORK 10 2014-12-24 3Oscillators By S.M.Mehzabeen
104 RC FEEDBACK NETWORK 10 4 Oscillators By S.M.Mehzabeen
105 Let c1 =c2=c3=c R1=R2=R R3+(Rth II Rin’)=R Derivation for RC phaseshift oscillator. Let c1 =c2=c3=c R1=R2=R R3+(Rth II Rin’)=R 10 5 Oscillators By S.M.Mehzabeen
106 Derivation for RC phase shift oscillator. 10 2014-12-24 6Oscillators By S.M.Mehzabeen
107 Derivation for RC phase shift oscillator. 10 2014-12-24 7Oscillators By S.M.Mehzabeen
108 Derivation for RC phase shift oscillator. 10 2014-12-24 8Oscillators By S.M.Mehzabeen
109 Derivation for RC phase shift oscillator. 10 2014-12-24 9Oscillators By S.M.Mehzabeen
110 Derivation for RC phase shift oscillator. 11 2014-12-24 Oscillators By S.M.Mehzabeen
111 Advantages Disadvantages • Doesn ’ t require any bulky inductors.Advantages and Disadvantages of RC oscillator Advantages Disadvantages • Doesn ’ t require any • Fixed frequency oscillator. bulky inductors. and expensive • Only one frequency fulfill barkhausen criterion . • Pure sine wave output is possible. 11 1 Oscillators By S.M.Mehzabeen
112 11 2 Oscillators By S.M.Mehzabeen
113 Solution : 11 3 Oscillators By S.M.Mehzabeen
114 WIEN BRIDGE OSCILLATOR 11 4 Oscillators By S.M.Mehzabeen
115 WIEN BRIDGE OSCILLATOR 11 R 1 1 sRC Z P R ZC sC Z R Z S C sC 1 sC 1 1 1 1 Z P R ZC sC R ZC R R 1 sCR 11 5 Oscillators By S.M.Mehzabeen
116 Loading Output Input Loading WIEN BRIDGE OSCILLATOR 111 Z1 Z P Z S Z 1 sRC P S Z Z 2 Z S R Z C sC 1 1 sCR sC R1 sCR R sCR sCR (1 sCR)2 11 6 Oscillators By S.M.Mehzabeen
117 Amplifier Gain 11 V0 V0 Vi I S Vi I S V0 Vo R1 R2 VoTo get , we use I I and R1 R2 so Vo Vi V V I1 R1 R1 R1 R2 V R1 R 2 R 2 1 Vi R1 R1 Vi Since I 0, Z and I S V0 Vi R 2 A Z 1 r 1 Vi I S R1 R 1 sCR where Z 1 so sCR (1 sCR ) 2 A 1 R 2 R 1 sCR r R sCR (1 sCR ) 2 1 11 7 Oscillators By S.M.Mehzabeen
118 Feedback factor 11 f X f I f X o Vo Z S sC 2014-12-24 8 1 1 sRC 11 8 Oscillators By S.M.Mehzabeen
119 Oscillation Condition 11 2014-12-24 9 f Ar R sCR (1 sCR ) 2It already is since A 0. Condition Phase of A equal to 180 o . f r f r R sCR Then need only A 1 2 R sCR (1 sCR ) 2 1 f r 1 Rewriting R sCR A 1 R sCR (1 sCR ) 2 2 f r 1 R 2 sCR 1 sCR 1 2 sCR s 2 C 2 R 2 R 1 R sCR R 1 2 1 2 R 1 3 sCR s C R R1 1 3 sCR sCR R 1 2 R 3 j CR 1 1 the CR Then imaginary 1 term 0 at oscillatio n frequency o RC Then, we can appropriat ely get using f Ar 1 by selecting the resistors R1 and R 2 R 1 R 1 1 or 2 2 R1 3 R1 11 9 Oscillators By S.M.Mehzabeen
120 Loop gain 12 sC Ar 1 sCR sC R2 R1 sCR 1 f r sC R2 R1 sCR 1 1 sCR sCR (1 sCR)2 1 R 1 R2 sCR R sCR (1 sCR)2 1 Gain with feedback Ar is A rf 1 f Ar 12 Oscillators By S.M.Mehzabeen
121 Advantages Disadvantages • It uses both positive and • More costlier.Advantages and Disadvantages of wien bridge oscillator Advantages Disadvantages • It uses both positive and • More costlier. components used negative provides and high feedback, it better stability over all gain. • It cant generate very high frequency. • Frequency can be easily adjusted by varying R or C 12 1 Oscillators By S.M.Mehzabeen
122 TWIN T OSCILLATOR 12 2 Oscillators By S.M.Mehzabeen
123 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, when an ac voltage is applied across the crystal, it vibrates at the frequency of the applied voltage. The greatest vibration occurs at the crystal's natural resonant frequency. – Which is determined by the physical dimensions and by the way the crystal is cut. 12 3 Oscillators By S.M.Mehzabeen
124 Crystal Oscillator 12 4 Oscillators By S.M.Mehzabeen
125 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 12 5 Oscillators By S.M.Mehzabeen
126 Crystal Oscillator 12 6 Oscillators By S.M.Mehzabeen
127 • Used when accuracy and stability of fo is utmostCrystal 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. 12 7 Oscillators By S.M.Mehzabeen
128 Crystal Oscillator • Piezoelectric effect.• The crystal is suitably cut and then mounted between two • The metallic plates. fundamental frequency is given as 12 8 Oscillators By S.M.Mehzabeen
129 Crystal Oscillator Cm (mounting capacitance) =Cs = pF; L = 137 H; R 3.5 pF; = 15 kΩ 12 9 Oscillators By S.M.Mehzabeen
130 Crystal Oscillator • Crystals have incredibly high Q.• For the given values, Q = 5500. • Q as high as can be possible. • An LC circuit has Q not greater than 100. • The extremely high value of Q makes fo highly stable. 13 Oscillators By S.M.Mehzabeen
131 Crystal Oscillator 13 1 Oscillators By S.M.Mehzabeen
132 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. 13 2 Oscillators By S.M.Mehzabeen
133 Crystal Oscillator 13 3 Oscillators By S.M.Mehzabeen
134 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. 13 4 Oscillators By S.M.Mehzabeen
135 The fo is between 411 kHz and 412 kHz.13 5 Oscillators By S.M.Mehzabeen
136 • If we assume that the current through C1,2 is larger thanPierce 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 1 vo I1 jC1 1 vo C1 C2 n vi vi I1 jC2 13 6 Oscillators By S.M.Mehzabeen
137 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. 13 7 Oscillators By S.M.Mehzabeen
138 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. 13 8 Oscillators By S.M.Mehzabeen
139 Variation of frequency with temperature.FOR FREQUENCY STABILITY• It is denoted by ‘S’. STABILITY 13 9 Oscillators By S.M.Mehzabeen
140 Review • Need of an Oscillator. • Types of Oscillators.• Using Positive Feedback. • Barkhausen Criterion of Oscillations. • Starting Voltage . • Tank Circuit. • Tuned Collector Oscillator. 14 Oscillators By S.M.Mehzabeen
141 Review • Hartley Oscillator. • Colpitts Oscillator.• RC Phase Shift Oscillator. • Wien Bridge Oscillator. • Crystal Oscillator. • Series and Parallel Resonance 14 1 Oscillators By S.M.Mehzabeen