Modern Physics 3 - Semiconductors & Communication Systems Class 12| P-N Junction Diode | Truth Table

by: Vedantu

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hey good evening my friends welcome to the last session for business sprint for grade 12 board exams today so I'm sure that you enjoyed all the previous sessions and really excited for this one as well for the newcomers here this is Bob deep thing I am a physics master teacher at Vedanta if you have not seen all these first classes if you would have sent in those or you did not have information about it please do make sure that you see them they are really going to help you today is our last day we are going to do modern physics part 3 which includes semiconductors semiconductors are the reason you and I are conversing right now by the way all these computers electronic devices mobile phones it's impossible without these semiconductors here plus the communication system so that combined here so before we get started make sure that you have subscribed to the channel press the bell icon so that you can get notifications about the upcoming awesome courses that the team is designing right here please also share this information about the free education with everybody these courses are free and will always be free so the strategy today is again as always simplification whatever you thought yay is going to be simplified eeeh because if it isn't simplified enough then you're not studying it the right way a girl here for real though the procedure itself is wrong the teaching procedure is not right so we are going to simplify everything what you thought yet so why board exams today's the last session so I am going to repeat this for the 11th time that 75% marks are needed if you're thinking of IIT is an deities or centrally funded engineering institutes and if you're bored export gives you percentage you have to be top 20 percentile there for medical students you need 50% in physics chemistry and biology combined but more important than that which I highlight every time is clearing the concepts when you're studying for competitive exams shortcuts elimination and all these eliminate this this choice this choice you're not studying everything because you can still get through that and that is not a bad thing but what I'm trying to emphasize is clearing concepts is going to help you and whether or not you take physics in your college as a major but but this is going to help you in life also so clear the concepts how do you clear the concepts derivations long explanations all these are done only in the board exams so that is the reason I'm asking you to emphasize on your board exams for next few weeks here and then you will have enough time for your neat and j-f real exam no don't worry but for now focus and fully can clear all your concepts clearing concepts is going to help you in the competitive exam concept clear Hoagy ah give me a Sakura Sakura then yes our equation easy enough a shortcut learn connector for speed that's all so what is in store today today we are going to talk about the semiconductors transistors diodes everything around us is all semiconductors these light blue LED bills everything is semiconductors bara number chaos Bathory at twelve marks so we were studying optics we studied in ray optics one day wave optics the other day dohno Camille aqua 14 thought that I said is maximum but today's concepts here cookie South number cos semiconductor Saiga and then we are also doing communication systems today that is going to be five months so 7 plus 5 is 12 months is what we are talking about today so we're doing communication systems also now as all face gave you highlighted over a second whether that is that is specific to this chapter so input output characteristics you have to really understand rectification name you have to understand those graphs of input output characteristics either a curlier then you can explain what is transistor how is it used as amplifier house and used as switch you know you can you can understand anything input output characteristic that is if you have to understand one thing and like focus more on this is the input output characteristics graphs ok gennyca rhythm then you're memorizing things OB are their curly ugly or they are they won4t other ninety percent other night but yeah path ahead to stop but i to emphasize on this one thing for this chapter all derivations as always have to become understood not memorized because when you do the derivations using a plain sheet of paper books Dorothy plain sheet of paper take pen and paper try to derive your cell you should be able to derive anything then you are there the next level comes from the back exercises of MCE or TMC ERT book is the king for the board exams so you have to do all the back exercises without referring to any of the solutions solutions are available at Vedanta dot-com please feel free to download them but only after you've already tried those and if you're stuck up refer to them otherwise use them to match your solutions past 5-years question paper at least 5 year question papers the more the merrier but at least 5 year question papers do not refer to those solutions do them yourself then go back and check the solutions or the marking schemes solutions and papers are available at Vedanta dot-com your one-stop solution to any of your preparations for board exams and competitive exams so the right way to understand this as I have already said the the analogies that we do with the mechanical world around us and we have seen that in electrostatics also this analogy is going to help you a lot transistor Hagee a transistor this is the emitter this is the collector and this is the beam in school is there a source of yourself analyzer are not going to teach you today and align yourself emitter that's the source of charges collector if the drain when charges are going and the base is actually the switch over there dad says ok shallow job so try to analyze your transistor with this picture again you can take a screenshot if you want and self explained yes opening up Cooper how you're opening up or oh that's the best way to learn now the question of the one here say is what type of charges are left near or on either side of the junction diode what does the junction diode it's a PN Junction map briefly cover Karuma very very briefly because of the lack of time I'm going to cover very briefly how this all happens so PN Junction are not in detail pn-junction you take one side you inject impurities to make this p-type p-type is what holls holls holls holls and I and then n-type is what electrons electrons electrons electrons up chemistry map or other diffusion what is diffusion when there is a lot of lot of atoms here or molecules here they're going to diffuse into no density area that's exactly what happens some books a positive charge is negative charges attract next that's not what is happening what is happening is diffusion the majority oh sorry diffuse don't get yeah without diffusion get diffuse will get o Kyogre you happy easy idiom electron Tokui ha ya thinkin on negative charge gonna get positive onions to get more electrons diffuse already are negative or oh yeah you're positive or rubia Jeff Yeager ah ha ha - what is going to happen now positive negative there is an electric field that is getting build up here processive to negative but electric field think about negative charges they get force in the backward direction so more electrons trying to come here they're going to phase resistance by this electric field but electron Sedna I can still do that there are some electrons they can still do that but when they do that the electric field gets stronger Bucky or obvious fear we couch or dodging it keep building up until that activity reduces to very minority level so the diffusion current has reduced not zero it has the reduce the current is still there because electrons going this way there is a diffusion current going this way try to understand this chapter like this ticket or yoga atmosphere the VA room temperature that is going to break one of the so electron hole pair if created let's say here because of thermal agitation there is electron hole pair created here now electron will go in this direction because it's in an electric field electron goes this way hole go this way but that means there's a current flowing this way no electrons are going that way this is the drift current my friend that was the diff current is another current which is called the drift current I'm just covering this in two minutes so then you this is just a pointer how you have to understand this chapter so now what is happening there is a diffusion current because these are steady diffusing but then because of thermal agitation once in a while electron hole pair is created there is a drift current in equilibrium this is equal to this and that is why there is no charge flowing net charge mean current is still the net current of zero a witness of cubic ayah it not obviously but if you can analyze this way all the upcoming questions are going to be very very easy for you so let's look at this what type of charges are left near or on either track yes yay positive ions will be a negative ions okay that's all they're not free charges for each or just a but those three charges take no opening here because they're gonna those go frame Kastigar now answer me a clue use the word diffusion and it'll be all this is happening because of the diffusion current so you're supposed to use those words diffusion current in this so let's go to the next question here define AC signal amplification factor so briefly cover car again very very brief introduction this is not a full coal I'm just trying to briefly introduce you there is a transistor here we're talking about transistor here right so let's take a transistor a now this is the emitter this is the base and this is the collector now let's make a quick circuit here with common emitter common emitter secure Haga some of the electrons let's say this is NPN type electrons go in 100 electrons go in there let's say 100 electrons go in there what would happen is couple of them a cadeau a happier jacket no electrons come out here baki Kassar a good virtual earring in 98 come here now electrons are charges so the rate of flow of charge is the current what we are talking about is there is some emitter current has been split into base current and the collector current law of conservation of charge is what we are talking about so what we can guarantee that in emitter current is always going to be equal to collector cover the current plus the base current this is guaranteed here and a law of conservation of charge of who okay I have a 198 - now when you try to analyze all these amplification factors and all you need to very basic ratios here noted on you should have already studied this but let's see that what is alpha alpha is about how much is the collector current for a unit of emitter current okay and when you are talking of dynamics and I know it will be change in collector current versus the change in emitter current another ratio that is very useful as beta that is how much is the collector current for the base current and usually we're talking of changes there because you are talking about amplification when I change beta or sorry very change the base current how much does the collector current change and you can clearly see 49 times so beta here is 49 just an example and this is exactly what they're talking about beta so you're supposed to respond this in your own words obviously so I'll just move out here so that you can see that so this is just the samples always talk about Delta's you're not actually interested in the actual number there you're more interested in the differences the fluctuations how much does that current change when I'm changing the base current how much does the collector current change so you're always interested in the changes you're not interested in the actual numbers in there so the next question or the void would present an odd gate of not gate K out there not get care through the other falls or your Falls the other true oh yeah hello that's a not gate not negate current as approach true input input 0 if you give true it gives you false if you give false it gives true now we are supposed to you'd use NAND gate what is NAND gate and gate loves the nor gate who's who use curtain nor gate when an and and you should be comfortable with these actually because you might as some of the students have might have taken computer signs also so you might also know that but let's make an and gate and try to understand how this works as an odd kid so let's make inputs here and let's say there is one input not so there is one input a here now let's try to make that truth table or try to understand us many a could true car the other if this is true this is true true and not true true true and true is going to give me true but that is going to negate that it's going to make it false so what happened just now I gave true here ultimately I got falls there and let's try with false here if I give it falls here falls and falls becomes false but Oska ulta which is true so I gave false here it becomes true there the game a run nor gate key that I come Kara this is the NAND gate but this is behaving as an or gate not so let's let's move on to the next question or before that let's see how you can yeah that's it a bar here not get burned Gemara okay which mode of propagation is used by shortwaves broadcast services shortwave radio right radios ERP how do we do that basically you take electromagnetic radiations beam Gardea oh but ionosphere hair okay cross need your mother they don't have so much of energy that they can go through that was they reflect oj yeah that's all they do so what is happening is you have your beaming the radiations let's think about radiations going up there's ionosphere there it's going to bring it back to the earth and this is based on the reflection there and this is exactly what these waves are using so this is how you can present it and this is this is how the FM radio and all these am radios they they use these tubby would say t make our theory auspice concur then you're not using satellites for them you're using you're making use of the rift election through the ionosphere party number Quebec communication it's very easy a student wants to use a to PN Junction diodes what is the PN Junction PN Junction can be used as a wall wall chaotic one-way flow cycle the tube may have every the air goes in air cannot come out that is evolved hemara harden a Volvo that blood can go one way it cannot come back so that's a blood wall your water walls water can flow one way cannot come back that's a water wall either electricity keep authorial electrons can go one way cannot come back that is PN Junction used as a wall now so that that's basically what you are talking about not alternating current alternating current inertia inertia inertia inertia you put a wall job inertia of the Jonathan either Jonathan eggie Johnny Vega that's what you're going to use this for alternating current fluctuates in direction it tries to go this way your wall it says please go in and then when it tries to go back it says no no you cannot do that and that's exactly what you're using there so very briefly member lasondra though and then i'll show you how you can respond in the exam also so to PN junctions use carne so let's do step by step even though the question is not asking for half wave rectifier let me explain you how we have rectifier also so there is a transformer here transformer as we have already seen it will only work with alternating current because this is mutual induction if the flux here is changing flux there will be generated we have already covered that so this is alternating current a Mary electronic devices will not work on alternating current there is a big problem now two mega Karuma half wave rectifier on this is not the question so what I can do is I can put a valve here this is a world right so there is there is something here some device that needs current one way the mana is coconut curl they are very easy now see if this is zero volts and this is 12 volts let's say current goes in and a negative cycle may give minus 10 minus 4 jojoba current tries to go this way it doesn't allow that it says when this is positive cycle the current goes in negative cycle current does not so this was your input let's see that this was your input and your output will be all the positive cycles it says okay please yes sure negative cycles will cut cur they go this is half wave rectifier oh it's good next level Billie Kjar this who will make it a full wave rectifier how very very easy same same procedure here I'm going to use two PN junctions this time or for a parabola Otto now so that I can use more spiel there of two diodes we are going to use two diodes one tayo desert Livadia which be memorize nigger nazma which name MRI scan and then this is my device let's say this is my device yes simplified it there cannot be any simpler circuit than this so let's talk about this this is zero words if this is zero words let's say this was positive 12 volts and this was positive six words hello of the co Kia Ora 12 is more than six if 12 is more than six the current starts it tries to go this way and the current can go this way the current is flowing this way and zero or six my compare zero six is more than zero so two current tries to go this way through this diode but no it cannot do that because it's trying to go in the reverse direction so this is off it is not helping no only this is participating and making sure that the current goes this way of negative cycle majority okay over here negative cardia is Scobie negative cardia and now look at it negative 12 negative six negative 6 is more than negative 12 the current tries to go this way it cannot so this one is off now in the negative cycle assume a echoing don't know my zero or -6 my comebacker or zero is more not so the current tries to go this way again the current is going this way now in the positive cycle this is on this is off this is making sure the current goes through this in the negative cycle here off okay hey Anagha up current is going in the same direction thank you to this and this is exactly what is a full wave rectifier the game a recycle T of cycle Yahweh Yahweh you do know buddy-buddy conquer Nepal a positive cycle it's near secure negative the isness are here to basically they are taking turn having Mary Mary up Terry why I know so this is a full wave rectified the only problem you can see is the potential difference is only six but then you can make its 24 enter that's fine give the truth table for a NAND gate and a walker live in NAND gate chaotic so X or Y no input data oh and give Galarraga and gate to Calgary girls go and correct Hannah 0 0 will be 0 0 1 will be 0 1 0 will be 0 1 1 will be what AB NAND gate NAND gates Coulter cartega NAND gate 1 1 1 you don't cut so let's go to the solution for this how you can present it is the 3 mark question and I think this is what I wanted to show you so body body had a body body they'll take 2 so let's see I know out here so that you can see in the full solution here and again this is a sample solution you make your own solutions there you write it in your own words do not memorize anything physics my memorization will not help you understanding will help you so this is what pro today but it's always a good idea to make the circuit diagram for that also symbol for that so let's go to the next question here it talks about give three reasons why modulation of a message signal is necessary for long-distance transmission modulation care here you have some message you want to transmit parallel to a chapter teat separate transducer carrots it's converting all that into electrical signals now I'm not going to many details but there are three reasons that are asking number one reason I just show you very briefly so that you can never forget it and why so we it's much beyond the curriculum here so the length of the antenna has to be roughly one-fourth of the wavelength for effective transmission that's the reason number one you should get so your length of antenna I know transmitting antenna whose key length should be one-fourth of the wavelength of wave length ki baate am wavelength wavelength is wavelength is velocity divided by frequency velocity kidney these are electromagnetic radiations three times 10 to the power 8 divided by divided by kitna humans can understand 20 Hertz to 20,000 Hertz maximum Lilia just a wavelength or comment 20,000 chars Eros okay cha 0 CH or 0 cut against a Tokyo area a doe teen char / doe ha yeah curaga but there are 15 kilometres is what you're talking about 15 kilometres is the wavelength if you did not modulate the signal the highest frequency would have this wave like this cup 1/4 karo 3.75 kilometres lumba antenna chi is it really practical no that is what what you do is instead of taking this frequency you modulate the frequency Giada Haj appear if frequency is more wave length is less wave length is less short antenna will work that's one reason for that second reason first reason okay our team pooching second reason first we attack a for effective transmission the wavelength curve 1/4 is the internal and secondly when you transmit that the power is proportional to 1 by square of the wave length again I'm telling you these details you do not need at least you have to be able to show that the power of transmission is proportional to 1 by the square of the wave length if that is the case if I am reducing the wave length there I am actually increasing the power of transmission there and that means your signal is going to be able to be transmitted long distances that is the reason number 2 the reason number 3 is very very interesting let's say this is the room and I'm speaking and you're standing over there you can hear me right because I'm speaking with some frequencies of the range of what humans do speak now bring in two people here both of them are speaking Toma Hadoop and a sound mix so J now think about there are hundred people standing in this room and everybody is speaking and you're standing on the far end of the room can you really understand I'm saying no yogi you are receiving or your ears are receiving all these sound waves from here here here how will your ears know get Mary Valley sound waves Omega sub K frequency the same way the Harvey Lee even understand that and that is the exactly reason number three when you tune in your ad let's say you have a radio here now Big FM Red FM chili FM will g FM whatever reference there are they're all transmitting let's say you're not modulating you're only transmitting the frequency is that humans speak let's say everybody is transmitting the same frequencies your radio what will it do to sorry because we match okay Jackie and that's why what you do is you give one frequency as the courier wave you're going to modulate that signal into a carrier wave big FM quake frequency the vo snow tomorrow signal Dahlia red FM caduceus Medallia of not frequency who carry 92.5 FM by megahertz pair what Jovi team curry whose frequency cup Laker will be a better so let's see how can present all this in your example practical antenna length number one right practical antenna and three point seven five kilometres number 30 antenna on our way okay the never now that's the thing how many of those and that's not really practical anyways effective power radiated proportional to 1 by lambda square inversely proportional to the square of that so if you're reducing lambda you're actually increasing the power transmitted the number three Germany will have multiple frequencies okay now this is one of the examples of modulation this is amplitude modulation this is the carrier wave this is the frequency ninety two point five megahertz volley this is the carrier wave this is going to carry that signal this is the message wave what you're singing speaking whatever this is the message that has to be sent so this super imposes on this and it m it modifies the amplitude you can see that this is amplitude model modulation so the envelope of this this is the message or your receiver cry that is going to be able to filter all that so that technology we are not going into details but this is the concept of amplitude modulation and similarly you should be able to identity you should be able to make the graphs of frequency modulation and phase modulation also the next question is a show that the voltage gain this is the last question today five marks and this is one of the questions that that many students think ok our night nazar a key derivation s away so let's see how we can simplify this this is one of the very simple ways all you have to do is Kirchhoff rules yet so let me show you how to do that because this is given in two pages or one and a half pages and then C at a book and many students find it difficult but here they go yeah I can either oh yeah all you're doing is Kirchhoff rules here I'll show you how you can do that - yeah I'll get common emitters coming here - banana on a chair now you're going to do what I tell you what we are doing to do this is the last question today this is my input circuit this is my output circuit what is the amplification there is a signal here fluctuating signal here that has to be amplified here so just understand what is input signal input signal is between these two ana output signal is between these two basically output signal is same as the potential difference between C and E 2 para input key both curl in the kirchoff's rule junction rule of effect junction a loop rule agape VBA start kiamana so let's do that VBA start cable athos edit mode will automate the VB semester movie be likely unaware and a potential ad be of inertia oh it's it's my my choice do i do bit can't a clockwise or clockwise I'm looping anti-clockwise Mary Marcia you can loop clockwise clockwise to be easy how much of gear time going against the current so I'm going to gain potential how much gain ibrb now because this is the last question here I am going to take at least two or three minutes here and we are going to end the session so we come here of ether say that where we are going from positive terminal to the negative terminal the negative VBB of negative vvv away or you have a portion of ve / Agha is equal to ve like the amana loop rule does not mean you to complete the loop you can stop Midway also take a just write the final potential final potential as ve now think about this is my input key our input fluctuation either say the aquatic side the input fluctuation happening along VBB this is the input fluctuation so let's talk about VB be separated or my strategy for that okar given vb b : lisanna banana to its core the virus re-arranged cargill except MV b be here and above ee there I got the VB minus V vbe banana so that becomes equal to vbe plus ib RB input key last statement at those statements here a vb v coke occurring at fluctuating v vb by putting all that signal that that's fluctuating so that means that the change in the input voltage so that you can find delta here if you find delta v vb that is the fluctuation in input voltage Delta VI is equal to VB Iike fluctuation your vbe to constant and a transistor man potential difference between b and e is a fixed value so that becomes zero here plus the change in this Delta in this RB is not changing so that becomes our B kinds of Delta IB yeah Mary equation one one of the manner input Kelly similarly nice simple my input a second equation B longer for output let's start with VC VC the start here will touch up as it and de clockwise the VC C start Curto VCP gamma Oskie bug against the currents of IC or Siena plus IC or C again from positive terminal of the battery to the negative so again I'm losing potential how much potential VCC losing korone minus VCC ha a be happy Agha that is equal to ve ve now remember what is output output we are taking here C or Iike to eco miserly over the VC minus ve by jaga VCE that is output a fill fluctuation giveth current of fluctuation in VCE is actually what the output fluctuation is output signal so fluctuation in output is equal to fluctuation in VC C that's a constant value there minus RC Delta IC e homeless Kadhim now we are talking about amplification here amplification is how much is output fluctuating for the input fluctuation so the input fluctuation if not her output fluctuation Muji is not yet that is what amplification is about how much is the output voltage fluctuation compared to input so what we are looking for is Delta V over delta VI in dono could divide Gardo what you're going to get is the amplification factor Delta V by Delta VI is coming out to be minus negative sign look at that negative sign Delta IC by Delta IB with a beta Jota or RC minus of RC / RB times of beta negatively both Korean on the probe curve they are negative now what is negative mean negative actually means your hop is a positive cycle that is going to be negative cycle you flip o J given it it doesn't really matter these are sound waves sound waves it knee high frequency it doesn't really matter all you're getting is free so this is how you can prove that this is negative egg dough teacher purchase order I know that's it and now how you present is is it's up to you but like it obviously this is just lines of roof there so let's go back and see how we can do that in exam so here is the circuit diagram back for you and this is a sample way you can prove that you don't follow this word toward you just understand the concept and you present it in your own words your language your grammar so again this was my last session for the Sprint here and I hope you enjoyed this please spread out the word tell your friends and relatives these lectures will still be available for free on YouTube channel so if you missed any of the any of the sessions here you can always go back and you can always go back and revise you can tell your friends classmates they can always come back make sure you subscribe yogyakarta me or a picture we back here we have more courses coming up and they will be free they will be free they're awesome they're more they're better maybe in the others so make sure you press the bell I can also share the information with your friends and and make sure you install the the the new app that was launched recently because you can download these slides as a PDF there so good luck and I believe in you you're going to rock the exam so I'll see you soon bye bye for now


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