Introduction to Oscillators
0 likes767 views
The document is a comprehensive overview of oscillators, including their definitions, principles, classifications, and applications. It differentiates between oscillators and amplifiers, describes various types of oscillators such as Hartley and Colpitt's, and explains key concepts like Barkhausen's criterion for sustained oscillations. Additionally, it covers the basics of oscillation, damped versus undamped oscillations, and the importance of feedback circuits in oscillator design.
Introduction to Oscillators
- 1. Physics B.Sc. III Sem V (2020-21) 1 Dr. Vaishali V. Deshmukh Assistant professor Department of physics Shri Shivaji Science College, Amravati (MS)
- 3. Outline What is an Oscillator? Comparison between an Amplifier and an oscillator Applications of Oscillator. 3 Principle of Oscllator Circuits Oscllations Basics Damped and undamped oscillations Frequency of oscillations Essentials of transistor Oscillators Barkhausens criteria for oscillations Classification of Oscillators
- 4. What is an Oscillator? 1. It is a circuit which converts dc energy into ac energy at a very high frequency. 2. It is an electronic source of alternating current or voltage having sine, square or sawtooth or pulse shapes. 3. It is a circuit which generates an ac output signal without requiring any externally-applied input signal 4. It is an unstable amplifier. 4 An oscillator is a circuit which produces a continuous, repeated, alternating waveform without any input. Oscillators basically convert unidirectional current flow from a DC source into an alternating waveform which is of the desired frequency, as decided by its circuit components.
- 5. Applications of Oscillators Oscillators are important in many different types of electronic equipment. For example, a quartz-watch uses a quartz oscillator to keep track of what time it is. An AM radio transmitter uses an oscillator to create the carrier wave for the station, and an AM radio receiver uses a special form of oscillator called a resonator to tune in a station. There are oscillators in computers, metal-detectors and even stun-guns. Computers, clocks, watches, radios, and metal detectors are among the many devices that use oscillators. ◎ 5
- 6. Amplifier is an electronic circuit which gives output as amplified form of input. The amplifier does not generate any periodic signal. Amplifier uses negative feedback. Amplifier operates as a multiplier. The Amplifier provides amplified signal. Amplifiers do nothing till input signal is fed to the input. Comparison between an Amplifier and an oscillator Oscillator is an electronic circuit which gives output without application of input.. The oscillator is generating of the periodic electronic signal. Oscillator uses positive feedback. Oscillator operates as a source Oscillator is gives oscillatory signal. Oscillators produce signals from the moment of powered. 6
- 7. Oscllations Basics 7 One of the most commonly used oscillators is the 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 is the main thing that controls the frequency. For something to oscillate, energy needs to move back and forth between two forms. For example, in a pendulum, energy moves between potential energy and kinetic energy. When the pendulum is at one end of its travel, its energy is all potential energy and it is ready to fall. When the pendulum is in the middle of its cycle, all of its potential energy turns into kinetic energy and the pendulum is moving as fast as it can. As the pendulum moves toward the other end of its swing, all the kinetic energy turns back into potential energy. This movement of energy between the two forms is what causes the oscillation.
- 8. Damped and Undamped Oscillations 8 Oscillation whose amplitude reduce with time are called damped oscillation. This happen because of the friction. In oscillation if its amplitude doesn't change with time then they are called Undamped oscillation.
- 9. Principle of Oscllator Circuits 9 The oscillatory circuit, also called the L-C circuit or tank circuit, consists of an inductive coil of inductance L connected in parallel with a capacitor of capacitance C. The values of L and C determines the frequency of oscillations produced by the circuit. The most important point is that both the capacitor and inductor are capable of storing energy—the capacitor stores energy in its dielectric field whenever a pd exists across its plates while the inductor stores energy in its magnetic field whenever current flows through it. 1 2 3 4
- 10. Frequency of Oscillations ◎ output of the sine function is a smooth curve alternating between –1 and 1. This type of a behavior is known as oscillation, a periodic movement between two points. ◎ Periodic motion is a repetitious oscillation. ◎ The time for one oscillation is the period T. ◎ The number of oscillations per unit time is the frequency f. ◎ These quantities are related by f=1/T ◎ Oscillators are electronic circuits that generate a continuous periodic waveform at a precise frequency ◎ Actual frequency of the oscillations is the resonant freq of the tank ckt. 10
- 11. 11 Where: ƒr is in Hertz, L is in Henries and C is in Farads. Then the frequency at which this will happen is given as: This equation shows that if either L or C are decreased, the frequency increases.
- 13. Essentials of Transistor Oscillators 13 Fig. below shows the block diagram of an oscillator. Its essential components are: Tank Circuit: It consists of inductance coil(L) connected in parallel with capacitor(C ). The frequency of oscillations in the circuit depends upon the values of inductance of the coil and capacitance of the capacitor. Transistor Amplifier: The transistor amplifier receives d.c. power from the battery and changes it into a.c. power for supplying to the tank circuit. The oscillations occurring in the tank circuit are applied to the input the transistor amplifer. The output of the transistor can be supplied to the tank circuit to meet the losses. Feedback circuit: The feedback circuit supplies a part of collector energy to the tank circuit in correct phase to aid the oscillations. ie. provides positive feedback.
- 14. Barkhausens criteria for oscillations ◎ Conditions which are required to be satisfied to operate the circuit as an oscillator are called as “Barkhausen criterion” for sustained oscillations. ◎ The Barkhausen criteria should be satisfied by an amplifier with positive feedback to ensure the sustained oscillations. ◎ For an oscillation circuit, there is no input signal “Vs”, hence the feedback signal Vf itself should be sufficient to maintain the oscillations. ◎ The Barkhausen criterion states that: ◎ The loop gain is equal to unity in absolute magnitude, that is, | β A | = 1 ◎ The phase shift around the loop is zero or an integer multiple of 2π: ∠ β A = 2 π n, n ∈ 0, 1, 2,…. ◎ The product β A is called as the “loop gain”. 14
- 15. Barkhausens criteria for oscillations 15 VIN =input voltage Vo =output voltage A= forward path gain β = small fraction of O/p signal is fed back to I/p A small change In DC power supply or noise component in oscillator circuit can start oscillation and to maintain oscillation in circuit must satisfy Barkhausen’s criterion. From the diagram we can conclude that, feedback voltage (Vf)=βAVIN The loop gain is equal to unity in absolute magnitude, that is, | βA|=1 | βA| > 1: In this condition, feedback is greater than the input voltage Thus addition of input wave and feedback wave will result in larger amplitude wave and as oscillation goes on the amplitude will increase and this can be harmful for device. | βA| < 1: In this condition, feedback is less than the input voltage Thus addition of input wave and feedback wave will result in smaller amplitude wave and as oscillation goes on the amplitude will gradually decrease and oscillations will die out. | βA|=1, In this condition, feedback equal to the input voltage Thus addition of input wave and feedback wave will result wave having amplitude of input and as oscillation goes on the amplitude will remain constant and hence a sustained oscillation is achieved.
- 16. Classification of Oscillators Oscillators can be classified in a variety of different ways. Some of the more common classes are: ◎ Operating frequency band (Audio, Radio). ◎ Output waveform (Sine wave, Square wave, Triangle wave, Sawtooth wave). ◎ Components used to set the frequency (RC, LC, crystal). ◎ Configuration of those components (Phase Shift, Wein Bridge, Hartley, Colpitts). ◎ Purpose of the oscillator (Local oscillator, Beat Frequency oscillator, system clock, signal generator, function generator). ◎ Available tuning range (fixed, adjustable, wide range). ◎ Technology used (Analog, Digital, CMOS). 16
- 17. On the basis of type of component used for the feedback 17 LC Oscillators 1. Hartley 2. Colpitt’s RC Oscillators 1. Phase Shift 2. Wein Bridge
- 18. Essentials of a feedback LC Oscillator 18 A resonator which consists of an LC ckt. It is also known as frequency-determining network (FDN) or tank ckt. An amplifier whose function is to amplify the oscillations produced by the resonator. A positive feedback network (PFN) whose function is to transfer part of the output energy to the resonant LC ckt in proper phase. The amount of energy fed back is sufficient to meet I2R losses in the LC ckt. The essential condition for maintaining oscillations and for finding the value of frequency is βA= 1+j0 or βA< ɸ = 1<0 It mean that The feedback factor or loop gain | βA|=1. The net phase shift around the loop is 0⁰ ( or an integer multiple of 360⁰). In other words feedback should be positive.
- 19. The Hartley Oscillator 19 The Hartley Oscillator design uses two inductive coils in series with a parallel capacitor to form its resonance tank circuit producing sinusoidal oscillations
- 20. Advantages Disadvantages and Applications of Hartley oscillator The advantages of Hartley oscillator are ◎ The need for components is very less even after including the tapped coil or fixed inductors. ◎ Frequency of oscillation can be varied by varying the inductance or by using a variable capacitor ◎ A single coil of bare wire can be used instead of using two separate inductive coils L1 and L2. ◎ The circuit is very simple and it’s not complex. ◎ Sinusoidal oscillations with constant amplitude can be generated in the Hartley oscillator. The disadvantages of Hartley oscillator are ◎ Sometimes distorted sinusoidal signals will get generated due to the presence of harmonics. This is one of the major disadvantages of the Hartley oscillator. ◎ The Hartley oscillator can’t be used as a low-frequency oscillator because the size of the inductor and the value of the inductor is large. The applications of Hartley oscillator is discussed below ◎ The Hartley oscillator is used as a local oscillator in radio receivers. Due to the reason for a wide range of frequencies, it is a popular oscillator. ◎ This oscillator is suitable for oscillations in Radio Frequency (RF) range up to 30MHz. ◎ This oscillator used for producing sine wave with the desired frequency 20
- 22. Drawbacks of LC circuits ◎ We have observed that the LC combination in oscillators provide 180o phase shift and transistor in CE configuration provide 180° phase shift to make a total of 360o phase shift so that it would make a zero difference in phase. Though they have few applications, the LC circuits have few drawbacks such as, ◎ Frequency instability ◎ Waveform is poor ◎ Cannot be used for low frequencies ◎ Inductors are bulky and expensive 22
- 23. Phase shift oscillator 23 RC phase-shift oscillators use resistor-capacitor (RC) network to provide the phase-shift required by the feedback signal. They have excellent frequency stability and can yield a pure sine wave for a wide range of loads
- 24. 24 Ideally a simple RC network is expected to have an output which leads the input by 90o. However, in reality, the phase-difference will be less than this as the capacitor used in the circuit cannot be ideal. Mathematically the phase angle of the RC network is expressed as Where, XC = 1/(2πfC) is the reactance of the capacitor C and R is the resistor. In oscillators, these kind of RC phase-shift networks, each offering a definite phase-shift can be cascaded so as to satisfy the phase-shift condition led by the Barkhausen Criterion. One such example is the case in which RC phase-shift oscillator is formed by cascading three RC phase-shift networks, each offering a phase-shift of 60o, as shown by Figure 2. Here the collector resistor RC limits the collector current of the transistor, resistors R1 and R (nearest to the transistor) form the voltage divider network while the emitter resistor RE improves the stability. Next, the capacitors CE and Co are the emitter by-pass capacitor and the output DC decoupling capacitor, respectively. Further, the circuit also shows three RC networks employed in the feedback path. This arrangement causes the output waveform to shift by 180o during its course of travel from output terminal to the base of the transistor. Next, this signal will be shifted again by 180o by the transistor in the circuit due to the fact that the phase-difference between the input and the output will be 180o in the case of common emitter configuration. This makes the net phase-difference to be 360o, satisfying the phase-difference condition.
- 25. Advantages Disadvantages of RC phase shift oscillator ◎ Advantages ◎ It does not require transformers or inductors. ◎ It can be used to produce very low frequencies. ◎ The circuit provides good frequency stability. ◎ Disadvantages ◎ Starting the oscillations is difficult as the feedback is small. ◎ The output produced is small. 25
- 27. Advantages Disadvantages and Applications ◎ Advantages ◎ Distortion testing of power amplifier. ◎ It supplies the signals for testing filters. ◎ Excitation for AC Bridge. ◎ To fabricate pure tune. ◎ Long distance can be spanned by the resting beams. ◎ Disadvantages ◎ The Wheatstone bridge is not used for the high resistance. ◎ The circuit needs the large no. of other components. ◎ The limited output frequency is obtained because the amplitude and the phase shift characters of the amplifier. ◎ Applications of Wien Bridge Oscillators ◎ It is used to measure the audio frequency. ◎ Wien bridge oscillator designs the long range of frequencies ◎ It produces sine wave. 27
- 28. Link for Class Test :2 28 https://forms.gle/LguEZhdMgkK2ZCPQ8
- 29. Reference Books 1. Principle of Electronics by V.K.Mehata 2. Basic Electronics by B.L.Theraja 29
- 30. “ “If four things are followed - having a great aim, acquiring knowledge, hard work, and perseverance - then anything can be achieved.” 30
- 31. 31 Thank You! Any questions? You can find me at vaishalideshmukh27@gmail.com