 Good morning and welcome to a lecture on Oscillators Part 1, hello I am Sachin Gengze, professor and head department of electronics engineering at Walchin Institute of Technology, Sholapur. The learning outcomes of this session includes after completing this session student will be able to explain working of phase shift oscillator which is a type of an RC oscillator and then the student will able to design a simple phase shift oscillator. What is an oscillator? The function of the oscillator is to generate alternating current or alternating voltage waveform. One can define oscillator as a circuit that generates repetitive waveforms which are of fixed amplitude and fixed frequency without any external input. So the oscillator circuit keep on generating a repetitive waveform, the frequency and amplitude of this waveform is already predecided. Now one can ask a question how a circuit can generate the output without any external input but definitely there is an input to the oscillator circuit and that input is in term of the DC power supply connected. As we already know the oscillators find applications in many electronic devices ranging from simple clock generator wherever we require to generate a clock in digital circuit we require oscillator to complex digital instrument like computers and the peripheral. Common examples of the signal generated by oscillator include the signal broadcast by the radio and television transmitter. We know in case of modulation we require a carrier signal and that carrier signal is generated by the oscillators. Clock signals and regular computers and clock clocks they are also generated by the oscillator and for producing sounds by electronic beepers and video games oscillators are also being used. What are the types of the oscillator? The oscillators can be classified based upon the type of components used in which we can find the oscillators can be designed using R and C or L and C that is inductor and capacitor or even a crystal can be used as the frequency deciding component in an oscillator. Depending upon the type of the frequency generated by the oscillator the oscillator can either be of low frequency or of audio frequency or of the radio frequency radio frequency oscillators they produce high frequency waveform ranging from 100 kilohertz up to certain gigahertz and then oscillators are not always generating only sinusoidal waveform but they can also generate they are waveform generators which can generate even square wave or there can be oscillator which are generating triangular wave or a sawtooth wave basically the oscillator they generate a repetitive waveforms of fixed amplitude and fixed frequency so it can be of any type it may be sinusoidal square triangular or a sawtooth. When we talk about the most popular type of the oscillators which are used for the audio frequencies those are the RC oscillator consisting of registers and capacitor. RC oscillator are mostly used to generate low frequencies for example in the range of audio. Common type of there are two common type of the RC oscillator the first one is called as the phase shift oscillator and the second one is called as the wind bridge oscillator. Now let us move ahead and understand one popular type of the RC oscillator called as the phase shift oscillator. So what is a phase shift oscillator? Phase shift oscillator is an electronic oscillator circuit that produces a sine wave or sinusoidal waveform output. How it can be designed? It consists of an inverting amplifier which is either can be designed using transistor or using op amp and then output of this amplifier is fed back to its input through a network which offers certain phase shift which is already we know how much phase shift is and then this phase shift network can be designed using resistor and capacitors. So basically phase shift oscillator consists of two parts one is an inverting if we talk about the phase shift oscillator using op amp then we can say that the phase shift oscillator consists of two part one op amp as an inverting amplifier and the output of the op amp is fed back. Now this feedback network which is there it's also adds some phase shift and then the total phase shift around the loop is of the 360 degree. So please look at the circuit diagram this circuit is for the phase shift oscillator which can be designed using operational amplifier. If you look at this there are two parts this is operational amplifier as an inverting amplifier because you can see that the non-inverting input is connected to the ground the feedback is there at the inverting terminal and then the another you can see that the output is again fed back through three stages of RC so RNC and then the total phase shift around the loop the total phase shift around the loop is of the 360 degree 180 degree of the phase shift is because of the inverting amplifier and remaining 180 degree of the phase shift is because of the 3 RC ladder network. So op amp is used as an inverting configuration provide the phase shift of 180 degree and the remaining 180 degree phase shift is produced by the 3 RC network hence the total phase shift around the loop is of the 360 degree. At some specific frequency when phase shift of the cascaded network is exactly 180 degree see the phase shift of inverting amplifier is fixed of 180 degree but by proper selecting values of RNC at a certain frequency the RC network offers a phase shift of 180 degree at this particular frequency if the gain of the amplifier is sufficiently large then the circuits start oscillating and then the frequency of oscillation of this particular circuit or this particular oscillator is given by F0 which is nothing but the frequency of oscillation equal to 1 upon 2 pi root 6 into RC. Now if I substitute the value of pi and then calculate for all these value I can get that F0 is equal to 0.065 divided by RC where please remember RNC are the values of the register and capacitor. The gain required for at this particular frequency at which the oscillation start is equal to 29 and as we know this is an inverting amplifier the gain of this is given by minus Rf by R1 but that minus we are already considered as a 180 degree phase shift so the gain of this amplifier must be equal to 29 so when one can say that Rf by R1 must be equal to 29 so as to start the rather to has the stable waveform at the output of the oscillator. Now with this I give you a question a very simple design design a phase shift oscillator for 200 hertz so we know we already know the circuit we know the various equations and then using this equation the oscillator can be designed. Let us go ahead and find out how this oscillator can be designed so coming again the same circuit is repeated over here we know the frequency of oscillation is F0 is equal to 0.065 divided by RC now I already know what I want to do I want to design a oscillator for frequency of 200 hertz so I can say that R is equal to point then I can rearrange the this equation R is equal to 0.065 divided by C into F0 now in this equation I know the value of F0 equal to 200 but there are two unknown one is called as R other one is called as the C this kind of problem is very typical in electronic design problems where you have more than one you have only one equation and maybe two unknown or three unknown variables to be found now this can be solved by assuming either the value of R or assuming value of C usually we assume the value of C because we have very limited choices available as far the C is concerned so let us let us assume the C available on our shelf is of 0.1 micro farad so let us assume C is equal to 0.1 micro farad now if I substitute into this the now into this equation R equal to 0.065 the value of C and F0 I can see that R is of 3.25 k now 3.25 k R is not usually available so what I can use is I can use register of 3.3 k now to prevent the loading of the amplifier because of the RC network it is required that this R1 this R1 should be equal to R it should be greater than R and we usually assume this R1 is 10 times of the R so we have already found the value of R 3.3 k so R will be R1 will be equal to 33 k lastly we know that for the sustained oscillation the gain of the circuit must be equal to 29 inverting amplifier so the gain is Rf by R1 so Rf is equal to 29 into R1 we have already found the value of R1 as a 33 k so Rf must be equal to 957 k so what we can do is we can use 1 mega potentiometer over here lastly if we are using 741 it is limited by the limited by the low sleeve rate so if we want to design a oscillator which of the higher frequency why must use the op amp which is having a higher sleeve rate like lm318 or lm351 so with that we come to the end of this session in which we have discussed a design of the phase shift oscillator the reference for this session is op amp and linear integrated circuit by Ramakan Gaikwad thank you very much for attending the lecture and see you for the next lecture thank you