Oscillator and its type(Wein bridge and Crystal Oscillator).pptx

An oscillator is a circuit that converts direct current from a DC source into a continuous alternating waveform, typically without any external input.

1. Oscillators   Neeta Ukirade( Gatkal ) Pratibha College of Commerce and Computer Studies

2. CONTENTS Introduction to Oscillator 01 Principle of Operation of an Oscillator 02 Types of Oscillator 03 04 Barkhausen Criteria

3. Introduction to Oscillator An oscillator is basically a signal generator that produces a sinusoidal or non-sinusoidal signal of some particular frequency . Oscillator is a circuit which utilizes positive feedback amplifier to generate sinusoidal waveforms of fixed amplitude and frequency . It is the major source of power in electrical and electronic instruments. The amplifier provided with the positive feedback can generate the sinusoidal signal even in the absence of any input.

4. Block Diagram of Oscillator Feedback in electronic circuits ” The process of injecting some portion of output signal of a circuit back as input to the same circuit is known as feedback. “ The positive feedback here implies the addition of some part of the signal from the output with the input signal voltage . This is allowed to pass through the amplifier circuit . The amplifier does nothing other than adding the signal coming from the feedback path and the input signal . Thus, continuous oscillations are generated, and at a time a stage is reached when without any input signal the oscillator circuit generates waveforms.

5. The principle of the oscillator is that when the feedback factor or the loop gain is on then the overall gain of the oscillator circuit will be infinite . Principle of Operation of an Oscillator V i  is the input applied at the terminal of the amplifier having gain A . β is feedback fraction of feedback network The output of the amplifier is V o  and that of the feedback network is V f . Initially, Vi is applied at the terminal of the amplifier with gain A. So, at the output of the amplifier we get, T he signal achieved at the output of the feedback amplifier is given as ,

6. If the amplifier and feedback circuit introduces 0° phase shift. Then both feedback signal , as well as the input signal, will be in phase with each other . Then, the signal at the output of the amplifier will be given as , So, we can write, the closed loop gain of the oscillator with feedback,

7. If loop gain is more than one i.e.,  Aβ > 1 . Then it causes the output to get built up. Thus, each time on passing the loop, increase in the amplitude of the oscillations is noticed. Now, if the loop gain is equal to one i.e.,  Aβ = 1 . Then it causes V f   to be equal to V i . Thus at the output, the signal will be a continuous sinusoidal waveform. In this way, the input is itself provided by the circuit and hence a sinusoidal output is achieved. If the open loop gain is less than one i.e.,  Aβ < 1 . Then after some period of time, the output will die out. This is so because, here AβV i  serves as input to the amplifier, so this will be less than V i  and Aβ will be less than unity.

8. What is Barkhausen Criteria? Barkhausen criteria state the two conditions to achieve sustained oscillations. These are given below : The open loop gain which we have recently discussed must be slightly more than or equal to 1. This means Aβ ≥ 1. The overall phase shift of the circuit must be 0. So, that the input and output signal will be in phase with each other . These two conditions will provide sustained oscillations at the output of the amplifier. This is termed as  Barkhausen Criteria .

9. Types of Oscillator The oscillator is mainly classified on the basis of the signal generated at its output : Sinusoidal or Harmonic Oscillator : Here the achieved signal at the output of oscillator shows continuous sinusoidal variation as a function of time. Non-sinusoidal or Relaxation Oscillator : In this case, the achieved signal at the output of oscillator shows a quick rise and fall at different voltage levels. Thereby generating waveforms such as a square wave, sawtooth wave etc.

10. Wien Bridge Oscillator A Wien-Bridge Oscillator is a type of phase-shift oscillator which is based upon a Wien-Bridge network comprising of four arms connected in a bridge fashion . Here two arms are purely resistive while the other two arms are a combination of resistors and capacitors . In particular, one arm has resistor and capacitor connected in series (R 1 and C 1 ) while the other has them in parallel (R 2 and C 2 ). This indicates that these two arms of the network behave identical to that of high pass filter or low pass filter, mimicking the behavior of the circuit

11. Wien Bridge Oscillator using Opamp Where: ƒr  is the Resonant Frequency in Hertz R  is the Resistance in Ohms C  is the Capacitance in Farads

12. Wien Bridge Oscillator Advantages The circuit provides good frequency stability. It provides constant output. The operation of circuit is quite easy. The overall gain is high because of two transistors. The frequency of oscillations can be changed easily. Disadvantages The circuit cannot generate very high frequencies. Two transistors and number of components are required for the circuit construction.

13. Crystal oscillators Crystal oscillators operate on the principle of the inverse piezoelectric effect. When an alternating voltage is applied to the crystal, it vibrates at its natural frequency. These vibrations are then converted into oscillations . These oscillators are usually made of Quartz crystal. In crystal oscillators, the crystal is suitably cut and mounted between two metallic plates as shown by Figure 1a whose electrical equivalent is shown by Figure 1b. In reality, the crystal behaves like a series RLC circuit , formed by the components. A low-valued resistor R S A large-valued inductor L S A small-valued capacitor C S which will be in parallel with the capacitance of its electrodes C p .

14. Due to the presence of C p , the crystal will resonate at two different frequencies viz ., Series Resonant Frequency , fs which occurs when the series capacitance Cs resonates with the series inductance LS. At this stage, the crystal impedance will be the least and hence the amount of feedback will be the largest. Mathematical expression for the same is given as Parallel Resonant frequency , fp which is exhibited when the reactance of the LSCS leg equals the reactance of the parallel capacitor Cp i.e. LS and CS resonate with Cp. At this instant, the crystal impedance will be the highest and thus the feedback will be the least. Mathematically it can be given as

15. Crystal oscillators can be designed by connecting the crystal into the circuit such that it offers low impedance when operated in series-resonant mode (Figure 2a) and high impedance when operated in anti-resonant or parallel resonant mode ( Figure 2b).

16. Advantages They have a high order of frequency stability. The quality factor (Q) of the crystal is very high. Disadvantages They are fragile and can be used in low power circuits. The frequency of oscillations cannot be changed appreciably.

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