I have just spent 2 weeks (not full time), figuring this out myself for pnp and npn transistors. Through your inspirational videos, i got inspired to create night lights for my kids with rechargeable batteries and a bare bones atmega328 that will power up them selves during the day and hopefully will run for months/years. As i wanted to create a dark sensor using a solar cell to charge batteries during the day and switch the circuit off while charging and turning it back on when night falls. Awesome stuff Julian, love it !

Always enjoy the clear presentations on your channel.

Ha ha ha - I've been doing electronics work for 40 years also and never knew what the CE voltage drop was before today. I've been mostly in digital logic so I never really needed to know since everything in my world is either slammed to ground up pulled up to 5v and there's never any 'fine tuning' as it were, but it's good info because I do build and/or troubleshoot analog stuff every now and then. Thanks for the video :)

And you are an outstanding teacher, you have that gift !!!

Hi Julian!! I learned it at school, those transistor have about 0.2 Vce sat, depending of the current.

😆, Nice annotation! I don't notice many others do it like you did Dave's video. Well timed indeed sir....

I highly recommend you read the chapter about transistors in The Art of Electronics, the explanation is superb, easy to understand and very in-depth.

It's actually much more simple and intuitive than all of the educational mathematics models. You may note that when the transistor is 'off', it's 'dropping' all of the voltage it sees. A BJT may be viewed as a current-controlled resistor in actuality. The more current flowing from base to emitter (in an NPN), the less 'impedance' it will provide to collector-emitter current. For a circuit of nine volts, all of that voltage must be 'dropped' by some form of impedance, thus performing electrical 'work'. It's analogous to calculating solar cell MPPT. So you may note that when a battery is said to be 'disconnected', it's not actually fully so, it's just experiencing the impedance of the atmosphere. The voltage then across the transistor will be proportional to how much is dropped by other parts of the circuit relative to its own impedance which is set by base current. An 'ideal' transistor would drop zero voltage at currents below its set 'allowance'. A real transistor however usually has a minimum voltage drop, which in this case was around 200mv.

I've mainly used transistors as switches. Never got around to using it as an amplifier. I should probably try it, after all life is short.

Back in the sixties I got a hold of a copy of the "GE Transistor Manual", these days it's available as a download online. There's a great deal of stuff in there that will remove that vagueness, and perhaps enlighten you on some things you weren't aware of. For example back then I didn't think that solid-state components would ever "wear out", but in fact if you're dissipating any significant power then thermal cycling will create an issue that will cause the device to fail over time. Lots of other good stuff in there, too...

Dave and Derek mentioned in the same video. good stuff ;)

Yes, it's incredible how much there is to know about electronics, even what are considered "basic" components. The saturation C-E voltage can be quite low, but it depends on how much current is flowing through. Last week I actually used a couple of 2N3055 power transistors as blocking diodes in one of my solar systems. When the base is connected to the collector they basically behave as diodes. But with a Schottky like (or perhaps even better than) performance in respect to the voltage drop. The only thing to watch out for is that unlike proper diodes they have a very low PIV indeed. I was able to get away with it due to there being a 12 volt (nominal) system voltage, it wouldn't have worked with 24 volts as then reverse breakdown would have occurred.

How cute where did you get those plastic letter "B, E, C" pods? Nice job

I confirm something someone else already wrote, Vcesat is roughly 0.2V.... school days stuff :)

You've put so much current (voltage really) through the base you are working the transistor in saturation mode and your Vce voltage will always be around 200mV for your typical BJT transistor. Fascinating devices. Vcc (battery) - IcRc-Vled = Vce. Now you only needed like 80-100uA on base to light that led fully (Ib*hfe = Ic) in your case 80uA*280 = 22.4mA which is enough to light it brilliantly. Rb = Vcc-Vbe/Ib -> 8.3v/80uA = 103k resistor, anything over that you're pushing more current through it than you should be needed creating a so called early effect. By designing your circuit so that the voltage at the collector is roughly Vcc/2 = 4.5V you get the most from the small signal gain from your transistor. If you ever wanted to measure a really small signal from say a microphone or temperature sensor ( mV ) you pass it through a capacitor into the base and depending on the ratio of your collector resistor and emitter resistor you amplify the signal by 1-200 fold and now you have a V signal from collector to ground instead of the millivolts coming from a microphone. Sorry for the wall of text.

thanks for sharing, learned at again

hi julian , thanks for the video. please tell me , how do you connect the probes to the aligator clips? i have the same transistor tester , do you think it can be wrong when it says that a transistor pr mosfet is ok? it would be very usefull to show us how to test an optocoupler and other devices like this. thank you!

When a transistor gets enough base current, about .7-.8V base-emitter, it enters saturation, where the collector-base voltage actually becomes negative, that is, the collector voltage with respect to your emitter ground falls below the voltage of the base with respect to ground. Saturation is the state used for switching devices on and off, and for digital logic. Before saturation, with slightly lower base-emitter voltage it operates in what is known as the linear region, where the hFE is a gain in collector current that is reasonably linear with base current. The linear region is used for analog amplification. Below the .65 V or so base-emitter voltage the transistor is in cut-off, that is virtually no collector current flows except a very small leakage. To study and become acquainted with these regions connect a pot to the base to supply and vary the base current and watch the various voltages.

Thumbs up!

+Julian Ilett Thanks for another good video. Just what I needed tonight, as I had my 2N2222 transistor connected wrong. You mentioned that you would link two videos, but the links did not come up on my screen. Could you perhaps link them in the video description? Cheerio!

The condition for saturation (I believe) is that the Vce voltage is less than the Vbe voltage. The effect is that the base is starting to get into forward saturation, this sort of limits how much base current you can have. I have some high voltage low current transistors with not all that much gain, and to get the many amps of current to flow through the load, I have to put nearly 5V directly across the base-emitter junction. That means the Vce voltage bottoms out around 4.3V or so. At this point if I try and further increase the Vbe, the Vce will increase in step with Vbe.

We tend to take these things for granted when MOSFETs are a lot easier to work with.

Lol you did it again! That was funny...

Nice vidéo, very clear and didactic, can you the reference of the transistor tester you are using. many thanks

Julian,Very good video. Thank you. Where did you get the transistor tester?

good video

I have been going crazy trying to grasp other peoples explanations just how a transistor works and am confused by the many variations they use to figure the proper biasing. There has to be a much simpler way and maybe one day I will find a way that I understand and works for me. So far , I have found one article on the web however, there are typo's that need to be corrected. I guess the only way to understand them is to build a circuit like Julian has and experiment.

whenever the transistor goes to saturation (beyond the maximum beta) Vce drops to the minimum value that can be taken from the datasheet, generally Vce(sat) ~= 0.2v

Maybe this U2 emitter resistor is there to prevent U3 emitter to base voltage exceed the -6V absolute maximum that is in the datasheet.

The standard silicon PN junction forward voltage is around 650mA at room temperature, and changes with precise doping but also changes a lot with temperature. In fact, you can make a crude temperature sensor using one. Your initial tester Vbe of 714mV was reasonable, the second result of 780mV suggests the junction temperature has risen during the test, and your measured 800mV in circuit reflects the heating due to the collector current driving the LED. This is why in circuits where matching Vbe is important, like current mirror circuits, it's vitally important to ensure good thermal coupling between the two transistors, which should also be selected from the same manufacturing batch to ensure similar doping. The configuration with a resistor in the emitter but not the base turns the transistor into a switchable current sink, the collector current being approximately equal (given large hFE) to emitter voltage divided by emitter resistance. The IC switches that current on and off.

You left us hanging on the strange circuit with the 220K emitter resistor. Did you figure it out?

Couldn't see the links to the other videos?

I think you are looking for Vce_sat :) but I really should watch the video through first lol

You should retry the experiment with a larger sample size of transistors, to get a more accurate average of the C-E Vf. Good video though.

I tried using a transistor in a pocket loudspeaker, but it combusted when i turned it on after briefly working. I decided to do some research into how to use the devices. I should probably not connect 12 volts to it with no resistor.

You should of added the LED voltage drop for the proper voltage from Collector to Emitter.

Just read the datasheet to get the operating parameters.

And the next video should be "Playing with MOSFETs" =)

how much base resistor value should be if i am using using bc547 at 5v 75ma?

Great video. You failed putting those links though, at least as far as I can tell.

Hi Julian, watching your vids often. You might find W2AEW's site helpful - he has a video on Transistor Bias Circuits and more. I'm still trying to figure out transistors myself. I understand the basics but there is still a lot to know.

I have the same multimeter, love it. But blew both fuses and are more expensive than the multimeter... sad...

you should check out according to Pete from spark fun he does a good one on transistors

I never understood why we use an NPN transistor with the emitter on the negative rail. This means we are pushing 1 mA through the base and loose it. I would always think it is better to put the NPN on the positive rail. This way you can send the 1 mA current from base to emitter as well through the load (the LED). am I right about this? That would be like DUH, why has this been done wrong for 50 years?

I worked as an Electronic Technician for the US Government for 28 years and in all that time, I never had the need to understand how transistors work much less any other discrete component. I learned how the circuits were supposed to function and if they did not work properly, I knew what circuit was the problem. After a while, you just learn common sense trouble shooting.

I watched this video and repeated the experiment. There is hardly any supply voltage to the LED and it barely lights up. It lights up far better just by touching the base with my finger, not sure why. Any idea if I can make it so I can get genuine amplification. It is pretty lame as it is. I checked the voltage on the receiving antenna and it barely registers over .5 volts as far as I am concerned. kzfaq.info/get/bejne/rZdorZNmnqref4k.html

0.7V is the standard drop for diodes, etc..

Vce sat is a spec usually found in datasheets.

This video fully saturated me!