NPN vs. PNP Transistors as Common-Emitter Switches

NPN and PNP transistors are two complementary types of bipolar junction transistors (BJTs). They both work the same way: when current flows thru the Base-Emmitter junction, the transistor "turns on", and creates a current path through the Collector-Emmitter junction. The amount of current that will flow through the C-E junction (our "Load" current), is determined by the amount of current flowing thru the B-E junction (our "Control" current). (The C-E current is typically larger than the B-E current, and it's equal to the B-E current multiplied by some value, called "hFE" or "common emitter current gain".)
The only significant difference between an NPN and a PNP transistor is that to turn on an NPN transistor, current flows IN to the Base and OUT of the Emitter; we accomplish this by applying a positive voltage to the Base relative to the Emitter. To turn on a PNP transistor, current must flow IN to the Emitter and OUT of the Base (the opposite direction of NPN), and to accomplish this we a apply a negative voltage to the Base RELATIVE TO the Emitter. That is why you see things like "-5V" in a datasheet for PNP. Usually the Emitter is "common" and thus a voltage reference for the Base.
Bipolar transistors are often used as simple switches in a "common emitter" configuration. For NPN transistors, we tie the Emitter to ground and turn the transistor on by applying a voltage to the base that is higher than ground; the Collector provides a "switched ground" output (either "open" or tied to ground).
For PNP transistors, we tie the Emitter to our positive power supply rail, and turn the transistor on by applying a voltage to the base that is lower than the positive power supply rail; the Collector provides a "switched positive" output (either open or tied to the positive power supply rail).

[@HellaFunnyShorts]

One of the best, simplest, and most straight-to-the-point demonstrations I've seen. Obviously excluding the backwards battery

[@kevpatt]

Thanks!

[@marko5472]

This really helped me understand transistors better than any other videos I have seen. When the teacher is good, also difficult things are easier to grasp.

[@wtfucrazy]

Simple, to the point and not losing yourself in details. Good job.

[@SergeyRyabenko]

Thank you Kevin! This is the most concise yet probably the best explanation I've seen about the topic.

[@amirrezafiroozi4080]

Hands on one of the best explanations available on transistors! Keep up the good work!

[@madamerotten]

I don't want to seem negative, but I am positive that your battery symbol is reversed.

[@kevpatt]

+madamerotten yup... a sketch drawn in haste and then turned around... I will apparently never live this one down. :(

[@laggeroomaepiclagger2454]

3 GUARD

[@nick54thegreat]

Doe!

[@stormshockin]

I actually disagree.. since power seems to actually work from neg to positive. And not from pos to neg. I think it is more accurate this way.

[@nick54thegreat]

he has the Positive terminal of the battery connected to the ground symbol, and has labelled it negative, while the negative terminal of the battery is labelled positive.

[@davebalz4039]

Excellent. Clear. Understandable. No frills. Great teacher.

[@chapinoable]

Of all videos I've watched about transistors, your video made understand it better. Thank You

[@prakashq2020]

I have watched so many videos, I name it BY-FAR THE BEST.

[@jimedgar6789]

Years I have tried to grasp this. Wow... thanks.

[@ElTurbandito]

it is so sad I am paying for university but have to go to youtube to get a professor who doesn't just spew formulae and actually explains the concepts. Thanks for the concise explanation.

[@kevpatt]

You're welcome! Hopefully things will become clearer as you get a handle on the basics. Good luck!

[@jimc52]

Ii love your elegant and simple illustrations and explanations. This was very well done.

[@kevpatt]

Thanks!

[@taneliharkonen2463]

This was THE best explanation of the npn/pnp transistors! Subscribed! :D

[@kevpatt]

Thanks!

[@khordad1216]

This is by far the Best, easiest to undrestand and most awesome explanation (atleast for me) that I found In 24 hours of my study on transistors even in 2021. THANK YOU!

[@umbratherios5614]

FINALLY! A video I can understand and actually learn how transistors work! Thanks man :)

[@nikosadie]

best explanation yet. Now I start to understand the basis of transistor use in amplification.

[@nikosadie]

best explanation yet. Now I start to understand the basis of transistor use in amplification.

[@thenet0120002]

Great video on the basics of transistors clearly goes over the current flow and the function of transistors theory.

[@mikecatlapp5775]

That was the best description of NPN vs PNP I have seen

[@kevpatt]

+Mike Catlapp Awesome, glad it could be of help!

[@BillAnt]

Not sure if it was mentioned, that PNP transistors are commonly used in audio power amplifiers as a complementary pair on the output end, working in a push/pull configuration of the signal.

[@MrMac5150]

Nobody but you, made it easy to understand, I could never understand, the shunt effect. good video, I hope you have one on Mosfets. Thanks.

[@kevpatt]

MrMac5150 Thanks! Yes, I will be posting more videos on mosfets and other components.

[@MrMac5150]

Kevin Patterson You have got my attention on your channel, it is excellent.

[@kevpatt]

Glad to have you. Thanks!

[@nkacarova4000]

You are the best man! So simple you give us light with how it works! So many videos here in youtube for this topic, but this is the best one!

[@MarkWarbington]

Watched ten transistor videos before this one. Yours was the best. Excellent job!

[@kevpatt]

+Mark Warbington Thanks!

[@radostar7139]

Agree

[@wakefieldyorkshire]

Yup I second that.

[@rayhantec4833]

I agree too

[@electronicstuff6355]

Me fourth

[@PhilJonesIII]

Five vids in and you are the one that nailed it. One guy even showed how to wire an NPN when he was in fact using a PNP......confusing as hell. I was following along on my breadboard. Nice video, clear explanation. Keep it up.

[@kevpatt]

Thanks! Glad it was helpful!

[@markybogbrushify]

agreed! video eeds more votes!!

[@DupczacyBawol]

Theres 80% of idiots who pretend to know and understand and 15% of those who intentionally upload the Internet with false information and knowledge.

[@JamesKelly89]

One way of memorizing which symbol is NPN is that the arrow is; Not Pointing iN

[@markschuurman4000]

And my way to memorize: nPn needs Positive on its base, pNp needs Negative. And the arrow points the direction of current. What's so hard on that?

[@kennmossman8701]

The 'P' is in the middle....and rhymes with Pee

[@DaisyLech]

Thank you! This is by far the best explanation.

[@kevpatt]

You're welcome!

[@johnr625]

Very well explained, one of the best on KZbin. Thank you

[@blockeduser5769]

Great Explanation NPN feeds a ( - ) to the load PNP feeds a ( + ) to the load.

[@PreludeSon]

Very good explanation, easy to understand and the drawing helps a lot.(battery symbol is flipped)

[@kevpatt]

+InfinityOrchid Thanks! Yeah I drew the sketch and turned it around, forgot about the symbol. Quite embarrassing. :(

[@utkf16]

Great explaination very clear and pitched just right for someone, like me, fairly new to electronics

[@StephhTheAwesome]

Let me send you $0.10 for a decent pencil....

[@raghavvsrinivasan9645]

Seriously, I have seen like about ten videos and none of them had such a simple and easy circuit. Great video, great explanation. Eager to see your next videos. Do something with LDRs. I also subscribed ;)

[@kevpatt]

Thanks!. Do you mean LDOs (Low Dropout [Voltage] Regulators)?

[@goozebump]

thank you you explained this very easily than some others

[@CesarGarcia-ft7wu]

very good explanation...long time ago he...but still helping...thank you Kevin.

[@abdulrahmanmohamed8298]

Nice explaination bro.. .😀 finally understanding how transistor works thnx

[@kevpatt]

Glad it was helpful!

[@kavindugilshan]

this video is very suitable for beginners !! very clean explanation thank you very much :)

[@habibthebikershop4059]

Thank you for the lesson 😄👍🏿

[@TheBaconWizard]

Great tutorial, nice and clear. Thank you.

[@mrakjunior]

Very helpfull thank you very much. Other videos on the internet are still complicated and miss this point of view.

[@mikemixon9937]

Explained very well. Thank you.

[@erichfeit7779]

Yes, I agree . Your battery symbols are reversed. But the rest of your teaching is excellent. 75 year old electrical teacher here.

[@khaben6986]

a very useful video I've watched about transistors thank you for that ! ^^ ...but I've a question about the active mode I don't really get the idea of biasing in forward and reverse ! when we forward bias the emitter-base junction and we reverse bias the collecter-base junction then I see that the base has two applied potentials on it !!? in case of npn transistors, we've the one of the input stage which is the positive terminal of Vee, and in the output stage the negative terminal of Vcc ! what does this mean ? can anyone explain this for me and thank you in advance ^^

[@kevpatt]

It can be confusing to think about two different potentials (voltages) present at the base at the same time. However, you should rather think of _currents_ only. If there is forward _current_ through the Base-Emitter junction, a proportional forward _current_ will be allowed to flow through the Collector-Emitter junction.

[@sandeephooda9253]

I haven't see any simple explanation then this one. Thanks Kevin. I was wondering if you user battery positive and negative representation correctly. Generally I have seen positive terminal line length greater than length of line representing negative terminal. Any how great explanation.

[@markiesheli]

Very Nice tut! Thanks!

[@kevpatt]

+markiesheli You're welcome!

[@mpl868]

Why don't you come to my university as my lecturer? Nice presentation for npn and pnp transistor!! I could save a few weeks if I find this video early!

[@dantea6]

best explanation, pretty cool

[@navaneethakrishnannarayana265]

Very nice and clear explanation sir.. Thanks alot. 🌹

[@luvradios]

As an Electronics technician for the last 25 years, my job primarily has been to troubleshoot why something isn’t working, having a basic understanding of electronics principles and experience with tons of failure scenarios has served me well. Scrolling through the videos on here today I found yours to be the only one that brilliantly and simply explained transistors, I loved the imaginary jumper explanation of how the transistor works as a switch. When troubleshooting any electronic device the first rule is simplicity! Example - if your receivers FM band isn’t working doubtful you’ll find the problem in the amp section. Well done sir!

[@IGBeTix-Electronique]

Good explanation but the "generator" symbol seems to be inverted.

[@kevpatt]

+Yannick Sirjean (autonomys) Yes, it was a stupid mistake when I was hastily sketching out the schematic.

[@mihaineken]

Very well explained ! Thank you !

[@kevpatt]

mihaineken You're welcome!

[@TBL_stevennelson]

I tested what you said and it works

[@jimnulty5077]

Good explanation, good video

[@kevpatt]

Thanks!

[@heathmeckley7585]

Very well explained!!!

[@mr.technician2638]

This is the video I wanted to see on youtube, very clear and simple....for me that seeking of understanding working principles of electronics especially with this transistor, for begginer like me....thank you sir to share with us your understanding of this...would you do also on capacitors? Ac motors or dc or on other semiconductor?

[@kurtlittlebridge]

I watched this many time before I finally got it and even though I'm still not sure. However, I feel I have enough of a grasp to implement it in my circuits. Thank you!

[@kevpatt]

+PS Awesome, keep at it. I started learning electronics when I was just a kid, and it's amazing when I look back and realize how little I actually understood and all the misconceptions I had. However, perseverance and study will get you there. Sometimes experimentation is the fastest way to learn. :)

[@naderhumood1199]

Great job Sir Thank you so much. ...

[@xpress021hundred]

Thank you, good sir, transistor basics are a lot clearer to me now! Keep up the good work! I would absolutely love to hear more of your own spin on electronic tutorials in the future! For instance; radio frequency/communication systems explained easy.

[@kevpatt]

+qdnguyenbp I will certainly be making more videos as I have the time. Thanks for the encouragement!

[@maxwarfield6699]

Finally! NPN transistors and PNP explained, in a COMPREHENSIBLE manner, for complete NOOBS like me. Been looking all over the web and all I came across was a bunch of FAST talking know-it-alls preaching to the already converted. Kudos and THANK YOU good sir, for this great video. Cheers!

[@kevpatt]

You're welcome! Glad it was helpful.

[@nickbungus]

Thank you, thank you. Finally, an explanation on how to use transistors and not how they work internally.

[@kevpatt]

+Nick James Thanks, and you're welcome!

[@shadowxoxx]

Keep up the great work buddy.

[@kevpatt]

Thanks!

[@johnishikawa2200]

Good description - but isn't that battery symbol turned around in your schematics? I thought that the long line, or dash, was for the positive terminal, the short one representing the negative terminal. Anyway, the ground connection in a circuit employing npn transistors - say in a transistor radio - would be the negative terminal. Hence there would be one location in the circuit where all connections to ground are soldered, including the negative terminal of say, a nine volt Duracell battery. And for such an appliance that employs pnp devices, simply reverse the polarities. I guess that's right.

[@tigerseye73]

To clarify, I meant to say DC CURRENT flows from negative to positive. Sorry about that.

[@kevpatt]

Yes, electron current flow does do that. However, schematics are drawn according to "conventional" current flow, which is the opposite. It is just a convention.

[@21thTek]

battery symbols on you schematic are inverted , the short dash is the negative pole ( minus ) .

[@Swan584]

Great tutorial

[@michituurbentaler1378]

Great! exactly that explanation i was looking for! Thank you.

[@kevpatt]

+michi tuurbentaler You're welcome!

[@waelalabrouni2669]

nice lecture , good job

[@kevpatt]

+wael alabrouni Thanks!

[@ianos123]

THANK YOU SO MUCH!!!!

[@kevpatt]

You're welcome!

[@tonyrebeiro]

Except for the battery symbol being flipped, the explanation is Very clear and easily understood.

[@kevpatt]

Thanks!

[@BobWa43]

Excellent video, thank you.

[@cuttyflam6136]

best explanation I've seen anywhere, thanks so much

[@kevpatt]

+Cutty Flam You're welcome!

[@thomasthesailorchubby5973]

That's the best explanation I've seen yet.. It seems to me all BJT transistors are Normally open. I need to figure out a normally closed circuit using transistor. I can't find it anywhere. Does anybody have a link to a simple NC circuit ?? "simple" is key here :) I'm a beginner.

[@kevpatt]

+ThomastheSailor Chubby I'll see what I can do... What you're looking for is a "logic inverter". To do "normally closed" with regular transistors will require a small amount of power even during the "normally closed" mode, unlike a mechanical switch. The reason is that pretty much all transistor designs, when used as switches, are "normally open" when unpowered, and it will take a small amount of power to keep it "closed". This power could be negligible with perhaps a MOSFET, or it could be "stolen" from the load being switched. Some interesting designs are certainly possible. I have another video showing NPN and PNP transistors in a circuit simulator, and I believe it shows a logic inverter along the way.

[@AClarke2007]

This video demonstrates the Transistor used as a Positive Feedback device. So, if you keep it positively biased the circuit will always be on until you provide overwhelming negative bias (Negative Feedback). This can be done by adding an extra Resistor at the Base which pulls the voltage level in the opposite direction when a mechanical switch is closed (Voltage divider).

[@andyshyshy]

Best explanation I've seen, thanks so much. And what's the drawback if the LED put in between NPN BJT's collection terminal & GND (low-side) instead of high side?

[@keetor13]

great video. thanks

[@kevpatt]

+Zachary Keeton Awesome! Glad you found it useful.

[@chrisjescobar]

Thanks very well explined

[@844pm2]

Thanks Kevin, I've see through many videos about the transistor, their explanation are complicated but you're making it easier for people to understand

[@kevpatt]

+Forever Joy Thanks!

[@michael636336]

If he can not draw the battery correctly He can be trusted to tell us anything that is true.

[@moegasim]

The clearest explanation found...thanks for sharing

[@MrRanmaedu]

Good video my good sir. The inverted symbol of your power source is in fact inverted but you are aware of it. Thank you for your video

[@kevpatt]

+MrRanmaedu Thanks for the kind remarks.

[@abhimanyu7274]

This is the first time I understood. Thanx

[@kevpatt]

You're welcome!

[@tonyvtech25]

Awesome video super understandable LOL. Finally i understand how transistors work and the difference between PNP and NPN. Thank you very much kevin.

[@kevpatt]

You're welcome!

[@dilanlakshitha2737]

Super explaination

[@adhd_alchemy]

You're a life saver! I am new to electronics. So, considering the lack of a resistor to protect the base in the PNP configuration ...does that make is less reliable/vulnerable to premature failure?

[@arkufahl]

I often hear people say current on the base turns on the transistor, but its voltage that does that... yes/no? Unless you get the voltage minimum, no amount of current will get you through. I'm a beginner, so maybe I'm wrong, but a lot of people seem to want to use voltage and current interchangeably, and that confuses me (personally) when I'm trying to learn something new.

[@paulb8264]

Even with the battery symbol slip (didn't notice till I read some comments) the walk through of the concept was extremely useful and well explained. Thank you very much for this video.

[@RageSondrayy]

your pencil is giving me anxiety

[@seacaptain72]

You keep saying it's current that turns on the transistor, but I've never heard that. We talk about a fully turned on transistor as one with a general 0.7 V requirement. Is this a standard that's used outside of school, to speak of a turn on current instead of a turn on voltage?

[@kevpatt]

It is normal to speak of Bipolar transistors (BJTs) in terms of current. The Base-Emitter junction has a fixed voltage drop, but very low resistance on top of that. So if you put 0.7V into the base, it will be enough to turn the transistor on. If you put 0.8V to the base, how much current will flow? 12V? As long as the voltage is higher than the B-E drop voltage, current will flow, theoretically without limit. That is why we must use a resistor to limit the current. The B-E junction, like all diodes, can be thought of as a "dam" (i.e. water dam on a river). Voltage can be thought of as the height of the water. Current is water flowing. Voltage is the height of the dam. If the water level is not high enough to go over the dam, no water will flow. As water is higher than the dam, the amount of water flowing over is not theoretically limited (only by the width of the dam, theoretically infinitely wide). So, we use a resistor to limit the current to the desired value. Any supply voltage that is higher than the B-E drop voltage can be used.

[@industrialautomation3470]

If you would like to understand, how it's actually work in field of industrial automation, where you have PLC input card, where you wire sensors, read on. In NPN sensor drawing (0:26), top part is actually PLC input (could be relay or something else). There is permanent 24VDC on PLC input and wire going from PLC input to sensor is going to the collector of transistor which is switching negative (N pn - switching negative). If you take DC voltage multi-meter and connect black probe to 0VDC and red probe to PLC input and there is 24VDC, that's mean you have to use NPN sensor. On other side, if you want to test the sensor, connect red probe to 24VDC and black probe to sensor output, if multi-meter display 24V, then it's NPN sensor. Sensor have to be in ON state of course. IN PNP sensor drawing (4:59), bottom part is actually PLC input (could be relay or something else). There is permanent 0VDC on PLC input and wire going from PLC input to sensor is going to the collector of transistor which is switching positive (P np - switching positive). If you take DC voltage multi-meter and connect red probe to 24VDC and black probe to PLC input and there is 24VDC, that's mean you have to use PNP sensor. On other side, if you want to test the sensor, connect black probe to 0VDC and red probe to sensor output, if multi-meter display 24V, then it's PNP sensor. Sensor have to be in ON state of course.

[@tinkmarshino]

Well Kevin.. at 66 years dumber I get it! I am trying to tech myself a new trick... so much to learn so little time to learn it... thanks Kev.,...

[@kevpatt]

You're welcome! Glad it was useful!

[@LPFan4]

My question is, what does it mean that bjt's work on current rather than voltage? Aren't they concurrent? Can you push current with 0v?

[@kevpatt]

You are correct, it takes voltage to get current moving. However, BJTs are _current_ amplifiers. So if the BJT has a HFE of 100, and you put 0.001 A in at the base, it will allow 0.1 A to flow through the collector. In this way they can be used as amplifiers with predictable results. In contrast, a MOSFET's gate terminal is controlled by the voltage present, and no current flows when it is on (the gate is like a "charged" capacitor, with "pressure" on it). So it is very useful to work with BJTs in terms of Base current, and MOSFETs in terms of Gate voltage.

[@ryanmurphy3011]

In the U.K. especially in industry, PNP based sensors are a lot more common. That’s because you’re switching the positive terminal of the load. It makes much more logical sense to use these transistors for references as it’s more practical. As most people will treat an output from a sensor as a positive terminal and look for a negative reference.

[@jasonmurawski5877]

Thank you for this video, i was having a very hard time understanding the difference between the two with what i read on google, watched this video and instantly understood it.

[@SpecNazMako]

I am beginner.. why u have resistor at the “npn” circuit before led and “pnp” crcit after led? Does it matter?

[@kevpatt]

Position of the resistor before or after LED does not matter.

[@BobSmith-bv9ep]

Finally. Not even nine minutes long and I can actually get it . Why can't they all be this simple? I was seriously becoming convinced that an easy explanation was not out there. Subscribed.

[@kevpatt]

Thanks! Glad it was helpful!

[@Michealfarmer]

Thanks for the great video. Finally learnt something useful about PNP transistors! Cheers!

[@kevpatt]

You're welcome!

[@GamersMedia2723]

thanks kevin petterson this is the best tutorial i have seen yet

[@kevpatt]

Aashu Malik You're welcome!

[@antoniorewa9491]

То чувство, когда чувак на английском объяснил про транзисторы понятнее, чем все, кого смотрел на русском языке...

[@musiconworldoff1996]

In your video I found what I want but you made a mistake that there are terminals of the battery are not right and the symbol of transistor is also wrong you shorted collector base and emitter of the transistor....but other all the things are excellent and your presentation skill is so good

[@kevpatt]

Thanks! Yes, I made a mistake when I first drew it, the battery symbol is inverted. :( See the first comment thread...

[@Plan-C]

Perfect. Cheers.

[@unosec]

good video, everything is well explained except your battery symbol is opposited.

[@kevpatt]

Thanks! Yes, I made a mistake when I first drew it, the battery symbol is inverted. :( See the first comment thread...

[@ibrahimalqayyas8360]

Thank you my friend for this goooooooood video, just 2 comments: 1- The battery symbol is reversed. 2- you forgot the second battery which is in the main circuit that will make the LED to glow. ThaaaaaaaaaaaaaanQ

[@venitusaber2131]

He didn't forget a second battery. In this configuration, the smaller circuit that drives the transistor uses the same battery that powers the LED.

[@ibrahimalqayyas8360]

@@venitusaber2131 Ah yeah got it brother thanq, I have forgotten considering the resistors :))

[@ishowhalal]

You're amazing sir