 Hello and welcome to the session Oscillator, I am Mrs. Vaidya Higulkarni. Learning outcomes are at the end of session students will be able to explain the requirements of oscillations and they will be able to list out the types and applications of oscillator circuit. Contains are like this, before moving towards the definition of oscillator just recall what is mean by oscillations. Yes, it is an effect that repeatedly and regularly fluctuates about its fixed value or mean value. The circuit which generate these oscillations known as oscillator circuit. This is in general definition of oscillator circuit. But now we are developing this circuit electronically. So, it is a circuit that generates a periodic waveform at its output without any input signal or any external signal source. It produces a continuous input signal of some type without the need of an input signal. So, we need to set up some conditions to provide the oscillations to be generated. Here we have criteria to generate the oscillations. Startup mechanism, it consists of electronic amplifier, feedback network which should be obviously a positive feedback network. So, that you are able to provide signal to the oscillator which gives the noise signal generation and due to that it provides the oscillations at the output. So, this feedback is nothing but input to the oscillator circuit and this oscillator circuit will determine the frequency of oscillation. So, at the output you will get the particular waveform or wave shape of the signal and frequency is determined by the oscillator circuit. In some circuit, oscillator circuit and the feedback network are combined together and perform the task of calculation of frequency as well as feedback to the circuit. Now, we will see how this feedback provides the oscillation. Figure 2 shows the block diagram of oscillator circuit where components are arranged so that a standard form of closed loop system is observed. Where A is the gain of amplifier, beta is the feedback factor, VS is the input signal. As you know, if we convert this block diagram into a simple block diagram, it can be simply written as total gain that is g divided by 1 minus g h. This is the standard rule of control system that is this is gain divided by 1 minus g h. This negative sign is there due to the positive feedback applied in the circuit. So, we can say gain with feedback is equal to output voltage by input voltage which can also be represented as A by 1 minus A beta. But if A into beta is equal to 1, then this will become 0 that is 1 minus A beta factor will become 0. At that time, voltage gain with feedback will become infinite. So, we can say it is also known as V0 by gain. So, this will happen when input signal is upset. So, we can control or hear that we get the output without even the application of input signal. So, we can derive the two basic conditions from this criteria and that is known as Barkhausen's criteria. In 1921, the German physician developed these two conditions and once you satisfy these two conditions, sustained oscillations that is undamped oscillations are observed at the output of circuit. So, in that first condition is loop gain must be unity or slightly greater than 1 that we have seen just now. And second thing, nothing but phase shift of the signal or phase shift of the loop must be 0 degree or 360 degree or it can be a multiple of 360 degree. This is known as criteria for phase shift, nothing but condition for positive feedback. This positive feedback gives you error voltage, this error voltage is amplified and then sustained oscillations will get developed. So, this is Barkhausen's criteria for oscillations. To derive this oscillator circuit, you have to select some components for that you just find out or recall the difference between the amplifier circuit and the oscillator circuit. So, what the amplifier circuit is? Amplifier circuit needs the DC supply for biasing and the input signal for the operation and amplifier circuit will give the amplification of signal at the output which say here input must be present for the amplifier, but in the oscillator circuit we know need to give any input signal just with the biasing signal that is DC input signal we are generating the output signal which is nothing but your sinusoidal signal or sweep signal or any kind of oscillations can be generated at the output. So, if we compare these two, we can find out what could be the components of oscillator circuit means amplifier should be there as well as two signals or two different components must be present then and then oscillator circuit can be generated. So, as you know the block diagram for the oscillator circuit, first we need to have the amplifier that can be with the help of transistor circuit or fade or it can be a operational amplifier also, but it must satisfy the condition of back houses nothing but its loop gain must be equal to 1 or slightly greater than 1. So, this is the very first important design component of oscillator circuit. Second component it must have a feedback network which provides the positive feedback means it must give the feedback networks output which is phase shifted by the output signal of amplifier that is why we need to select the components of the feedback network in such a way that there is a phase shift in the signal that is from output of amplifier and output of feedback network signal must be having phase shift. Then third is frequency determining element as I told you sometimes these both will combine together and you must use the components that is passive components which affects the frequency of oscillations nothing but frequency of signal can be determined with these components broadly oscillator circuits are classified into sinusoidal oscillators and non sinusoidal oscillators. When we say these are the sinusoidal oscillators then this can be developed with the help of RC circuits, LC circuits and the crystal oscillator circuit. RC oscillator are also known as audio oscillators because it ranges from 20 hertz to 20 kilo hertz frequency and for that we have the two types as Vainbridge oscillator and phase shift oscillator. So, these two circuit will generate the audio signal at the output. So, when your requirement is greater than kilo hertz signal or in the range of kilo hertz to mega hertz then we should go for the LC oscillator circuit that is inductor and capacitor can be used and that must provide or that will provide you the frequency in the range of mega hertz. So, types are Hartley oscillator, Colpitt's oscillator. Now recall how the gain can be stabilized in an oscillator circuit is it with the low Q circuit that is low Q factor, is it with the positive feedback, is it with the negative feedback or is it with controlling the gain just think for a while and find the correct answer. Yes and the answer is gain stability in oscillator circuit can be definitely achieved by the controlling gain nothing but by controlling the components or selecting the components of an amplifier. It is not with any feedback network or not with the help of any kind of Q factor or low Q circuits. Applications of oscillator will be broadly in the communication circuit for the local oscillator to transform your RF signal into RF signals to generate the RF carrier in a transmitter. Very important application in the digital circuit to generate the clock signal and it is also used in the television circuit and CRO to test your instrument and that gives the sweep circuits or oscillator can be used as a sweep generator over there. So, oscillator is very important equipment in the testing equipment. So, testing can be done with the help of oscillator circuit. References are like this. Thank you.