 Hello and welcome to this session in which we are going to learn about oscillators. My name is Sachin Gengze, I am Professor and Head, Department of Electronics Engineering at Valtran Institute of Technology, Sulaipur. The learning outcome of this session, after completing this session, you will be able to explain working of a popular oscillator named Wainbridge Oscillator. And then the student or you will be able to design a simple Wainbridge Oscillator given the frequency of waveform to be generated, you will be able to design this particular simple oscillator using op-amp. As we know the function of the oscillator is to generate alternating current or voltage waveform. Oscillator is a circuit that generate repetitive waveform of fixed predefined amplitude and fixed predefined frequency without any external input. But when I say without any external input, of course I need to apply a DC power supply for this oscillator circuit. Oscillators they can be used in many of the electronic devices, wherever we require a clock and we know that for almost all digital circuits we require the clock and then these clocks can be generated by the oscillator. Then in computers and peripherals also the oscillators can be used. Common examples of signal generated by oscillator include signal broadcast by the radio and television transmitter, we know for any kind of modulation we require a carrier waveform and then these carrier waveforms are generated using oscillators. Then the clock signals that regulate computers and clock and the sound produced by the electronic beepers and the sound produced in video games they are also generated using the oscillators. As far as the types of the oscillators are concerned, the oscillators can be classified either based on the types of the component used or the frequency of oscillation or the type of the waveform generated. We know that there are three types of the oscillator based upon the components used. The oscillators can be designed using R and C that is register and capacitor or LC or the crystal. The most simple and the most popular oscillators which are used for low frequencies they are designed using RC oscillator. So in an RC oscillator circuit, the filter network is a network which consists of registers and capacitor. RC oscillators are mostly used to generate lower frequencies in the range of audio that is from say that is up to say 20 kilohertz and there are two popular type of the RC oscillator circuit. One is a phase shift oscillator and the other is called as the vain bridge oscillator. In today's session, we are going to discuss about a simple vain bridge oscillator which can be designed using operational amplifier. So a vain bridge what is a vain bridge oscillator? A vain bridge oscillator is an oscillator that generates a sinusoidal waveform. It can generate a large range of frequencies. So the we can design the vain bridge oscillator to generate any frequency in the audio range. Interestingly, the oscillator is based on the bridge circuit which was developed by Max Wain in 1891. So it is a vain bridge, that vain bridge circuit is very old it is it was designed in 1891. Some interesting history of the vain bridge oscillator. The modern circuit is derived from derived by Hewlett in 1939 when he was doing his Stanford University master's degree thesis. Now this person William Hewlett he figured out how to make the oscillator with a stable output amplitude and the low distortion. Interestingly after master's the Hewlett started a company with a David Packard and then we know that this is a very famous company very popular company and it's called as the Hewlett Packard and the first product now although now many products are available from HP or Hewlett Packard. The first product which was launched by the Hewlett Packard after when they started these two young people they started the company. The first product was called as HP 200A and it was a precision vain bridge oscillator. So this is again some historical picture you can see that this is the front end of that HP 200A which was started or which was launched by Hewlett Packard as a first product and this is how inside that Hewlett Packard vain bridge oscillator was looking at. So what is a vain bridge oscillator coming back to the vain bridge oscillator what is a vain bridge oscillator because of the simplicity and stability it is one of the most commonly used audio frequency oscillator. Then vain bridge oscillator consists of a vain bridge circuit connected between the amplifier input and output terminal and this vain bridge circuit consists of series RC network in one arm and parallel RC network in another arm and because of this vain bridge circuit the total phase shift around the circuit is of 0 degree and we know that when this total phase shift is of 0 degree or 360 degree the oscillations occur. When the oscillations occur the oscillation occur when the bridge is balanced and we know that the bridge is balanced at the resonance. So this is what is the circuit as we can see here I am using op-amp as a non-inverting amplifier because I am just applying a feedback to the inverting amplifier then there is a non-inverting amplifier and then there is an additional feedback there is an additional feedback one consists of a series RC network and then there is a parallel RC network also. So the total phase shift around over here is of 360 degree and because of that at the resonance the sustained oscillation start. Now as far as the frequency of oscillation is concerned which occurs at the resonance which is given by formula F0 is equal to 1 upon 2 pi RC if I substitute the value of pi in this case then it becomes 0.159 divided by RC. Then the gain required for sustained oscillation AF is equal to 3 as we know this is a non-inverting amplifier the gain is 1 plus RF by R1 and hence I can say that 1 plus RF by R1 must be equal to 3 so RF is equal to 2 R1. So let us have a look at a simple oscillator design. So design a wind bridge oscillator for 965 hertz. So I think that now you it is just you have to just there are two formulae and then you have to just substitute into that and I think that now you have completed that design. Let us have a look at the answer of this question. So let us design a wind bridge oscillator of 965 let us design a wind bridge oscillator for 965 hertz. We know that F0 which is the frequency of oscillation is given by 0.159 divided by RC. Now we already know F0 is equal to 965 so what I can do is I can then I can have this R is equal to 0.159 divided by CF0. Now this is again a typical electronic design problem in which there are two unknowns namely we already know the value of F0 is 965 but we have we don't know the value of R and C. So this is a typical electronic design problem in which we have more variables than the number of equation. Now we solve this kind of problem by assuming the value of one of the component and we usually assume the value of capacitor because we have a limited the capacitors available. So let us the value of capacitor be equal to 0.05 microfarad. So the value of capacitor is 0.05 microfarad. Now if I substitute the value of 0.05 microfarad into this R into this equation so I get R is equal to 0.159 divided by C into F0 so I substitute the value of all these and then I get the value of R is equal to 3.3 kilo ohm. Now so that is what I have got the value of R then I have to find out the value of R1 in order to avoid the loading effect I assume that R1 is equal to 12k but we have already seen that RF should be equal to twice R1 and then RF I get is equal to 24k. So the 24k if not available we can assume a potentiometer of 50k right. So I think that that is a pretty simple design in which I can find out given a frequency I can find out the values of the different component like R and C. So what are the advantages of wind bridge oscillator. Now we can see that the wind bridge oscillator require very few components like it requires an op-amp and few resistors and few capacitors so the design of the wind bridge oscillator is very simple and then further by using the decayed resistance boxes the frequency range can be selected. So by putting instead of putting one R if you put if you put decayed resistance boxes over there then I can select one of the R from that and the frequency can be selected. Then over the range by using this decayed resistance boxes we can select the range of the frequency and then out of this range of the frequency one of the particular frequency can be selected by selecting the variable capacitor right. So by varying the resistance register through decayed decayed box and then putting the variable capacitor we can generate any frequency of on the lower side right. What are the disadvantages of the wind bridge oscillator the maximum frequency output is limited because of the amplitude and phase shift characteristics of the amplifier. As we know every op-amp has a limitation of the slew rate and then it rather that slew rate limits the highest frequency waveform that can be handled by that particular op-amp. So same is true for the wind bridge oscillator of course but if you want to generate higher frequencies and if that slew rate is not supported by the op-amp we can always select op-amp with a higher slew rate. The design is bulky because circuit require more number of electronic component. So that is what is the disadvantage of the wind bridge oscillator. So with that we come to the end of this session in which we discussed about the design of the wind bridge oscillator. The reference for this session is op-amp and linear integrated circuit by Ramakan Kaikwa. And with that I end this session thank you very much for attending this session. Thank you.