Starter Guide to BJT Transistors (ElectroBOOM101 - 011)

hi today we will learn about transistors yes i know transistors are like michael jackson of electrical components and everyone knows about them although really it's the capacitors who are the king of pop i know you might be scared of transistors complexity but you know what they really are they're just a bunch of variable resistors a bunch of electrical potentiometers well if the potentiometer was on mushrooms but really they're just resistors you can adjust by providing them with a suitable input mind you their resistance is not linear you can't connect them backwards like regular resistors there are different types and sponsors sponsors oh yeah this video is sponsored by brilliant learn complex math science and computing easily using brilliance interactive courses from my link below more at the end okay how do i make it simple there are many different types of transistors main ones being bjt and mosfet in this video i only talk about bjt god how do i make it simple okay hear me out bjt stands for blow be a bipolar junction transit store it's just a name for a type of transit store that makes sense to me as much as it does to you don't worry about it it is made of silicon semiconductors same as a regular silicon diode if you have watched my diode video you know how diodes work we have a positive or p and a negative or n semiconductor fused together and that's a diode now imagine we also fuse an n to the p end of the diode or a p to the n end of the diode and that's it we have a bjt if a pn junction was a diode then these are like two diodes in series and act as such not quite yes this behaves similar to two back to back diodes but this is not equal to this also it looks symmetrical on paper but for example this is an actual cross section of a bjt transit store forget what you saw it doesn't matter don't look at it look into my eyes don't panic that cross section cannot hurt you we just want to know how the transistor behaves so we can use it let's focus on npn so it is kind of like two diodes but although that's true the transistor is designed such that in proper operation only one of those diodes is in use and so this is its symbol the terminals for a bjt are called base the terminal connected to the middle semiconductor emitter and collector the reason behind the naming convention is nobody cares despite the fact that this is an npn arrangement flipping the collector and emitter doesn't work well i've seen it work but pretty badly and not to the component specifications so no flipping you saw the cross section and it is not symmetrical don't look okay so here's how it works if you place a positive voltage across the collector emitter terminals then if you pass some little current through the base emitter diode by placing say 0.6 volts across it magically it is like opening a valve between ce and a much larger current flows from c to e the more current into the base the opener the collector emitter valve gets and so more collector current that's why i said bjt is like an adjustable resistor end of story no there's more let's give it a try i have a generic 2n3904 transistor i place a one kilo ohm resistor series with the base and adjust the input voltage the voltage value across the resistor is equal to the base current in milliamps i place a 1 ohm resistor from collector to a 5 volt supply the voltage value across it is equal to the collector current the base current is around half a milliamp and collector current is 66 milliamps change the base current to around one milliamp and the collector current is around 87 milliamps so like i said we typically have a large collector to base current ratio which is a factor we called beta or as more commonly used in data sheets hfe for some reason just remember the names and of course the emitter current is the sum of base and collector currents but with the base current being much smaller than the collector current we often assume emitter and collector currents are equal but this beta thing is only almost constant if the transistor is on and is in active region what is active region you ask well the behavior of the transistor changes based on the collector emitter voltage let's measure same setup as before but i'm providing a fixed base current and instead of a fixed collector supply voltage i have an alternating voltage between 0 to whatever i measure the collector emitter voltage and collector current here you go x-axis is vce and y-axis is ic you see the collector current rises with collector emitter voltage kind of like a resistor in this area up to around 0.2 volts after which the collector current is almost constant for a large range of vce and then changing the base current you can adjust the collector current so this is what we have for the current versus voltage a graph changing based on the base current the region after this dotted line where the collector current is almost constant is called the active region and the region where the collector current starts dropping because vce becomes too small is called saturation region the regions are named as such after nobody cares i don't remember why and this is not a history lesson we just want to use it in active region the collector current changes so little based on the ce voltage that for simplicity mostly we assume that the collector current is just constant in electrical engineering you'll notice that to keep sane we simplify by ignoring small little things and designed to get close to what we need which is mostly good enough and if it is not we use adjustable components to fine tune to almost exactly what we need and this quarter area of operation is what the bjt transistor is designed for if the vce is pushed too high the collector current starts to go up combined with the high voltage overheats and kills the transistor if vce goes negative well do you remember when i said bjt was like two back to back diodes well a negative bce results in the collector-based diode to turn on pulling large currents from base like this which is useless and bad and we avoid bad so far this was all about the npn bjt but how about pnp well pnp is just a complement of npn with the diodes reversed and behaves exactly the same except voltages and currents are in reverse here's its symbol if there is around a negative 0.6 volts of base emitter voltage pulling current out of the base we would have a collector current flowing say beta times larger than the base current and the collector current to bce graph looks like this voltage and current are just in reverse and that's it that's all you need to know for now this much information is good enough for most your applications and above that will get complicated in a hurry we will get there if you prove yourselves worthy for now though we should know enough to throw some good circuits together let's do an amplifier well i haven't told you enough to design a proper amplifier but this doesn't stop us from my favorite process trial and error here's the circuit it's not a proper amplifier but a simplified class a amplifier good for a one-off example we already know it is an amplifier because it amplifies the base current to beta times at collector but we want to amplify voltage the input voltage goes to base and output comes out of collector with some gain we don't know enough yet to calculate gain doesn't matter for now first thing we do is we bias the circuit which is setting up dc operating voltages and currents around the circuits and the ac signals will ride over those dc values this circuit is a common emitter meaning that the transistor emitter is connected to a dc fixed voltage which here is ground we choose what are the best bias values for our operation starting from collector voltage the best voltage for an amplifier is right in the middle of the supply rails that way we get the maximum ac voltage swing without distorting the signal see the maximum a transistor can do is either being fully on or off which means the output voltage here can go close to zero or supply voltage so if the collector voltage is close to one of the supply rails the output voltage clips at a lower amplitude if our supply is 5 volts then collector voltage is 2 and a half volts and for the collector current let's pick 10 milliamps so with two and a half volt on collector resistor this means the resistor would be 250 ohms but closest i have is a standard 270 ohms which means collector current is around 9 milliamps for the same two and a half volts good so now we can calculate the base current looking at the data sheet yeah hfe not only changes significantly by the collector current even at a fixed ic there can still be a huge variation that's why we should design a circuit that doesn't rely so heavily on the inaccurate transistor parameters but anyway let's assume hfe is around 200 so ib should be around 45 microamps and so the base resistor would be around 97 kilo ohms i have 100 kilo ohm though and my resistors can be 5 off let's just put the circuit together okay looking at the collector voltage it's two volts or three volts across the resistor so this means my collector current is closer to 11 milliamps so for two and a half volt i should change the resistor to 227 ohms well i have a 220 ohms with 220 ohm resistor i have closer to two and a half volts so now i feed the signal into the base issue is the dc component of a signal generator can load the base voltage and throw the bias off so i add a 10 microfarad capacitor between them to isolate the dc voltages while the ac will get through one thing though how big can the input signal be base emitter is a diode and its voltage versus current is like this if the signal is too large the base current is not linear and the output signal is distorted for minimal distortion we should keep our signal small so the base current remains relatively linear here i have a plus minus 5 millivolt input to the circuit and if i probe the output that is on a two volt scale you see i have a sine wave inverted from the input riding on a two and a half volt bios voltage now if i get rid of the dc component change the scale you see for the 5 millivolt peak input i'm getting around 300 millivolt peak output so i have a gain of around 60. i could connect a microphone to it and amplify my voice well i'm using a subwoofer speaker as my microphone umbrella now only looking at the output voltage if i increase the input you see we start seeing the distortion i was talking about more and more until the signal hits the rails and clips see i'm happy we got this far in electro boom 101 i feel like i can throw circuits together with more confidence knowing that if you follow the series you can figure out what i'm doing and of course don't forget about my sponsor brilliant i always recommend it to everyone because i see how interesting and interactive it is and how it keeps people engaged in learning anything from logic and computing to science and math 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Channel: ElectroBOOM

Views: 619,397

Keywords: educational, electrical, ElectroBOOM, science, electronics, engineering, entertainment, equipment, measurement, experiment, mehdi, mehdi sadaghdar, arc, mishap, physics, Sadaghdar, test, tools, circuit, funny, learn, shock, spark, discharge, transistors, BJT, NPN, PNP, Bipolar Junction Transistor, Didoe, silicon, semiconductor

Id: 2uowMENwiHQ

Length: 13min 57sec (837 seconds)

Published: Thu Sep 22 2022