#190

[@johnwest7993]

Yours are the best electronics instructional videos on the internet. If I magically had access to these videos back when I was a kid I'd be a retired physicist by now with a 60 year hobby in amateur radio. But better late than never. You're filling in many of the gaps in my Swiss cheese of electronics knowledge. Thank you.

[@TheDradge]

When I left college as an electronic engineer I built thyristors and breakover diodes in a semiconductor plant. Your videos and explanations are reminiscent of the Forrest Mims books back in the day. Excellent and well put together videos!

[@w2aew]

I learned a lot from the Forrest Mims books!

[@TheDradge]

@@w2aew I still have the books! Bought from Radio Shack in the 70s and diligently studied. I remember my interview at the semiconductor plant in 1982. My boss to be gave me a sketch pad and asked me random questions like "draw me a single transistor class A amplifier circuit with example biasing resistor values." That "back of the envelope" method helped to get me the job and I hope your videos help get new kids coming through a grip on electronics. Breaking things down into smaller parts helps to get a handle on the common techniques.

[@TheDradge]

@@w2aew books by R.M. Marston were also essential reading.

[@warrenking1815]

It still nice to see the old analog Simpson meter. Still in use . Good video well explained!

[@1DR31N]

The best explanation about electronics I've seen. Wished I had a teacher like you. I wish all the success you deserve.

[@aatifshah8476]

same

[@maxfarr4142]

I've been a student of EE for 25 years and your style is the most impressive I've seen so far! I love the simply laid out notes and common themes. I'm on the binge and I want a vid specifically detailing the inside of an opamp from you. All the others I watch aren't cutting it!

[@doncoltrane3565]

I recently retired and now have time to learn about electronics. You are a good teacher, and I appreciate your videos. Thanks for all your hard work. 73!

[@francescolucarno6214]

The HP RPN calculator is a really nice touch. I am still using my trusty HP35s.

[@CodeJeffo]

Best practical electronics & education channel out there. Hands down.

[@josevasquez4335]

Professor, so much thankful for your videos. I never never have had the opportunity of learning with such a perfectly explanations. THANKS.

[@W1RMD]

Valued info! Thanks! I love how the majority of your projects involve less than a $5 of a handful of parts.

[@PavelLarsson]

This channel is fantastic! The explanations of important fundamentals are clear and the practical applications are always mentioned. I follow along and experiment with each topic covered. Thank you so much for your effort and I wish you all the best!

[@tonybell1597]

Cheers Alan, simple, straightforward, and very useful! Back to my bench!

[@BersekViking]

Your VOM is in parallel with the current source and this causes a reading error. I guess it is a 20kohm/V meter, so in the 10V range it has an impedance to 200k, that matches the impedance you calculated.

[@w2aew]

Yikes!! You are absolutely right!! Although I grew up using analog meters like the 260, how quickly we can forget about the meter's impedance after getting spoiled by the 10Mohm input impedance of modern DMMs. Ugh, what a bonehead error! I'll have to do a follow up video (with egg on my face). Thank you for keeping me honest!

[@shadowwolf225]

As a subscriber of w2aew's videos I'd like to say kudos for the sharp eye and a professional and polite comment on what was done wrong. You are the antithesis of stereotypical youtube comments. Thank you.

[@shadowwolf225]

***** This is good stuff! You, one of the gods of youtube electronics, made a simple to overlook mistake and from just a comment made a whole video explaining the error and give credit to the one who found it. I'm impressed at the level of sportsmanship/professionalism here. Thanks for this. This type of discourse is REAL education.

[@w2aew]

Agreed!

[@1959Berre]

When real professionals meet they usually behave like gentlemen. But you are right, this kind of chivalry is becoming the exception in YT comments.

[@paultyson5145]

Brilliant. Thanks for explaining these mysterious little circuit blocks!! Now I see how they help increase the gain in audio stuff.

[@norm1124]

Love this video, looking to understand transistors since ~1 year and making slooooowly progress.

[@dwDragon88]

Great video. Earlier today I built most of the circuits you showed, my favorite was the voltage divider biased BJT with two diodes in place of R2 in the divider. This circuit was remarkably stable, even when I changed the supply voltage or dropped in transistors with different betas. One configuration I tested, a 500 uA sink, showed less than 10 uA change with a 100% swing on the supply voltage. Remarkable.

[@w2aew]

Yes, this is exactly what you'd expect - nice job. Note that I made a mistake with my use of the old Simpson 260. It's input impedance was too low to be used to measure the voltage on the current source output. See my next video for details...

[@tiagomello]

Time really flies when watching your videos. Thanks for the great explanation.

[@Tiky.8192]

I really like your back to basics videos, I always learn something new. Very well done too!

[@RinaldiMeteoric]

Gosh I wish you were my electronics teacher on my graduation. Thank you for the excellent videos.

[@CoolDudeClem]

Nicely explained, although I'll have to replay it a few times for my mind to take it all in!

[@das250250]

One of the best channels to refresh and learn electronics ... As always , Excellent work W2

[@zedlepplin9450]

the slope of the IV characteristics is 1/R @ 6 min. So for a ideal constant current source, output impedance is infinite (1/infinite is 0 which means flat line).

[@HenriTroberg]

Another really great back to basics video, thanks! Also liked the HP 15C calculator.

[@dougfarrell5091]

Fantastic presentation Allen. I really enjoy your presentations and get allot of inside from them.

[@BrendaEM]

Thanks for the video. (My Fluke 87vMax multimeter is back for a safety recall, if I ever see my meter again, if they repair it. Check yours?) This setup gave me some issues because I was down to my intrepid Cen-Tech P37772 meter with the patent infringment yellow bumper, and worn selector switch, which also doesn't do microamps. So I needed to scale the current up the setup. I also used two power supplies to keep my bias divider in one place, but that might be cheating. What I found: I was displeased with the linearity (low input-impedence) Why I found it: likely because I was against the compliance voltage for my altered setup. What made recovered the fun: I replaced R2, with a diode, which brought up the bias voltage, and linearized the setup better. Also, placing 1 though 5 LEDs in the current source with very little change in apparent brightness made it fun, once again, all the way past 12 volts. They didn't smoke at 15. What opportunity I found: By watching the LED brightness when changing the voltage, I could see the limits of the voltage compliance. Oddly, the 2n222 handled the current quite well. Apparently, some have a max continous current of 600mA, others 800mA (likely with some kind of clip-on heatsink.) What opportunity I missed: while I practiced calculating the input impedance a few the circuit, I should have for this setup, but didn't. Thanks, again for the videos.

[@min2oly]

This is very helpful for some working I'm doing on pulse magnet motors and what to do with the excess energy output.

[@min2oly]

I just noticed you also provide a pdf of the ckts - SWEET! thanks!

[@jospicant]

It's a wonderfull video. I hope to see another video about current mirrors using Mosfet, wilson,wildar, wildson modificate, active cascode.... thanks for yours videos

[@w2aew]

I'm glad you liked it. I'll keep these topics in mind.

[@ThingsWhichArentWork]

Thanks Alan - a good tutorial on a subject I'm a bit fuzzy on. I'm going to have to watch this again. Cheers!!

[@gregorymccoy6797]

I very much enjoyed this. These have always been a little fuzzy for me but this cleared it up nicely.

[@Linksimpson]

Really clearly explained, again! Thank you, love to see this continue on into a diff amp too.

[@malgailany]

As usual, clear and informative video. Thank you.

[@Taran72]

WOW! Amazing video! thank you very much for the detailed explanation and for showing the actual use of a current generator. A question I've been asking myself for a long time was "what are current sources used for?"

[@anurasenarathna1703]

Very nicely explained. Thank you very much.

[@mouseyou12]

Thank you hit the nail on the head I was looking for something on current I like the way you present your vid's.

[@simonyoungglostog]

Excellent, i can see that I was the last person to comment. I'm understanding more now and noticed the ring of two as an example of a current source. It would be great to see you do a video on that.

[@klave8511]

Great lesson on current sources for tabletop circuits. My area of application is industrial and automotive where temperatures change from -20C to 70C at best. My designs have to cope with -40C to 80C. These basic current controllers won’t be useable without temperature compensation. Just consider the 0.6v Vbe reference voltage these circuits use, over a small 50C temperature range the voltage will change by 2mV per degree, that’s 100mV change for a 600mV reference! Even the self heating can render the current out of range. Throw in todays trend to run everything from a single battery or a USB voltage and you suddenly have a much more interesting design. I consider any circuit design to be incomplete if it doesn’t have some temperature compensation or a good reason not to use it. I’m sure you have a few design options for temperature correction, perhaps an update video one day.

[@TMuel1123]

Thank you for that awesome video. I love these back to basics videos. You have mentioned that the gain of an amplifier could be massively increased with a current source. It would be great if you could do a video about that topic to show how that works.

[@w2aew]

I plan on doing something on this topic soon.

[@monodipsarkar9805]

Wow!! Great explanation

[@kevinnguyen5011]

Great video. Just correction on current changed is in milliamp, but Allen kept say Microamp.

[@AnneThurston-w8w]

Great video about current soutces.

[@wk2w168]

Thanks for another great, thorough explanation.

[@Infinitesap]

really love these back to basics videos.

[@armandine2]

Just been building your second "sink" circuit - 0.6V across a 120 ohm emitter Resistor with a view to create a 5mA constant sink. That particular circuit was in an exam paper , GCE A level - June 1981 AEB Electronic Systems Paper 1. Testing the sink current in an actual circuit I'm getting about 10mA. The paper also asks for graphs of collector current and output voltage when you charge a 1000uF capacitor.

[@w2aew]

Well, something sounds "off" if you're measuring 0.6V cross that emitter resistor, but also seeing 10mA current.

[@armandine2]

@@w2aew I thought it could be my "random" silicon npn transistor - I used a C4881 (TO-220) which measured 171 beta on my Peak tester. It appears to have survived being soldered on to a strip-board. The bias config is 2 diodes in series with a 300-ohm 1-watt resistor - dropping 15V - 1.2V.

[@armandine2]

I will measure again - with a load

[@w2aew]

@@armandine2 Always a good idea to backup your measurements with a second method. For example, if you're measuring 10mA collector current, double check that by measuring the voltage across the emitter resistor, as well as the resistor value - all should lead to the same conclusion - if not, then something is wrong.

[@w2aew]

@@armandine2 Make sure that load doesn't saturate the transistor.

[@TheDansana]

I'm enjoying your show - especially the b.e.e. stuff.

[@MrKaizen75]

Chapeau Maestro! Very clear and well explained...

[@Cesar_III]

So, without the VOM it is actually a higher impedance. Great! It would be nice to see the same video with the VOM in parallel to the voltage source.

[@w2aew]

Yes - in fact I made a followup video that points out this error in measurement technique... kzbin.info/www/bejne/gJOmemeCqNOkntE

[@moonlightshadow5107]

Thanks,very very nice and simple tutorial, i hope you go forwards and waiting for more basics videos

[@w2aew]

+Moon Light Shadow Please let me know what topics you'd like to see in future "basics" videos!

[@fredfabris7187]

You do an incredible job of walking a dumbo like me through this subject. One question........ you said a couple of times that the constant current source can improve the gain of a transistor, the reason isn’t so obvious to me.

[@alanwolke6253]

A current source is a very high impedance - so when it is used as the load in the collector of an amplifier (instead of RC), the gain will be very high (since gain is gm*RC)

[@geneglondo]

...and finally I understand current sources. Awesome video. Thank you :)

[@AJSquirrel53]

So awesome! Thank you!

[@benthere8051]

Current sources are very useful. THANKS

[@judo-rob5197]

Thanks for the tutorial.

[@wagsman9999]

very clear explanation, thank you

[@DAVET38]

Another good (and useful) video Alan. Thank you for prparing it.

[@robbowman8770]

Nice video - I always forget how useful these are outside of ICs. Have you done a video on the "long tailed pair" before? Would make a good follow up ,,,

[@w2aew]

Yeah, I was thinking of a video on a diff pair, then maybe even using some current source active loads to make a crude discrete op amp.

[@robbowman8770]

***** Sounds great - diff amps are important, and also fun!

[@quantomic1106]

Thanks Alan! You're the man!

[@1inDnile]

calmly taught, thank you.

[@paulp1204]

@ 12:00 mark, I am interested to know why using the current sink with an impedance of 190k is an advantage over simply using a 190k resistor in the amplifier application you mention.

[@w2aew]

Because, in order to use a 190k resistor, the collector current would have to be very low in order to not saturate the transistor. The low collector current will lower the transconductance (gm), and therefore lower the gain...

[@robinharris4706]

Brilliant video with much for me to learn...including from the comments! I'm working through building these circuits to solidify my learning but am not sure how to set up a floating power supply. Would you be able to provide a brief explanation on how you set that up please?

[@444guns]

awesome, thanks alot for this video

[@simonyoungglostog]

Super video. Thank you.

[@cuchulainkailen]

11:33 .... re: Using the current sink as a load for an amplifier. Dumb question: Is this a good strategy for a LNA block that must amplify very low-V (uV) raw sensor output (assuming the opamp ckt is well designed for noise)?

[@hardrocklobsterroll395]

another extremely informative video. thank you

[@RexxSchneider]

It's interesting to see that the output impedance of the 2N2222A of 190K across the range 2V to 10V when sinking around 0.5mA as calculated at 11:34 is almost the same as the output impedance of 192K across the range 5V to 10V, which is given by 5V/(419-396)μA. That's useful because some devices have a "knee" at lower collector voltages where the incremental output impedance drops off. It's also worth noting that the 2N2222A datasheet gives a wide range of output impedances at Ic=1mA and Vce=10V of between 30K and 200K. At Ic=10mA, Vce=10V, that falls to between 5K and 40K, meaning that the 2N2222A is not so useful as a constant current supply at higher currents.

[@w2aew]

Hah! There's actually a mistake in this video when it comes to measuring and calculating the output impedance! It was discovered after the fact, and I posted a followup video. Note that the output impedance is much higher.... kzbin.info/www/bejne/gJOmemeCqNOkntE

[@RexxSchneider]

@@w2aew I noticed the discrepancy between the apparent emitter current of 384μA and the 419μA collector current, but I just surmised that the 1K emitter resistor was high by about 8%. The actual value of that resistor would have been the first thing I'd measure. It's been so long since I used an analogue meter, its loading wouldn't have occurred to me. So, the interesting point now is that the 2N2222A seemed to have an output impedance far in excess of the values in its datasheet, but I then realised that the emitter resistor effectively provides negative feedback in a common base configuration, stabilising the collector current against variations due to changes in Vce. Something to explore.

[@Jose4416]

great video thank you

[@starlight8260]

Well done, excellent video!

[@hammershigh]

Excellent!

[@micpax1]

Fantastic video! Thanks

[@SomeGuyInSandy]

This is really good stuff, thanks!

[@bobdoritique7347]

Thank a lot, very interesting.

[@cohof]

Could you possibly consider a video on different types of oscillator.Specifically why most designs don't oscillate as expected,or work every time in a simulation program but when constructed never do ?

[@robr8554]

Is there a way to temperature compensate a current mirror used as to measure high side current? The current mirror I am using had a pnp and npn matched pair. The issue i see is the offset and slope changes as a function of temperature. Is there a way to compensate it?

[@blacklightning7227]

awesome vid ! 🔥

[@yaghiyahbrenner8902]

Great Video, Good Presentation.

[@fouadnano]

Thanks شكرا

[@pertileluca]

Very nice video!

[@billwilliams6338]

W2aew, Current source circuits were mostly used for headphone amplifiers cause there was no standard output impedance for headphones? A Constant current source amplifier circuit "can drive any load" impedance that is why they used them for headphones?

[@billigerfusel]

Very cool videos, thanks man.

[@alexeykokh8240]

Thanks for the video, it is very helpful.

[@Infinitesap]

really awsome video.

[@pirateman1966]

I came across your video while searching for a solution to my problem. I built a circuit exactly like what you have on top right hand side of frame, you're pointing at it at 4:53 I've selected 5V as supply voltage. The resistor is 50 Ohms. I plan on charging a 20Ah NiMH cell connected to the output (via a schottky diode to prevent back flow when the source is off). It provides about 310mA, but quickly runs up and gets into a thermal run away. The current increases until the output transistor is destroyed. I've tried with 2x 2N2222, then went beefy, and used 2x TIP42C NPN transistors. Same effect. Both transistors are matched. Tried a resistor as load instead of battery. Same result. Then I installed a heat sink on the output transistor, and it works fine now! Why is this happening on only 300mA draw? How can I stabilize it? Would a MOSFET with its negative temp coefficient stop the thermal runaway? Do they even make current mirrors using MOSFETs?

[@johnnytacokleinschmidt515]

11:45 I'm trying to process this concept. Learning and noticing this current source and current mirror in modern hi-fi solid state amplifiers. I keep rewatching and I still get stuck at why the gain would be different if the load was simply resistive. I'm very slow and just trying to move up comprehension. Thanks

[@w2aew]

In the example cited, if you used a 190k ohm resistor, the current would have be very low to avoid saturating the transistor, which would then result in a lower gm factor - so the overall gain would be lower (gm*Rc). The current source load gives you very high impedance (Rc) at higher currents which gives you higher gm, so the gain can be very high. In fact, in most cases, the gain when using an active load like a current source is usually so high that they're only used with additional negative feedback to set the overall circuit gain. The benefit of having a very high gain inside of a feedback amplifier is much lower distortion.

[@magdalenoarellano1841]

gran video como todos los que usted hace mister gracias.

[@Infinitesap]

Simply awsome video. Im getting there :-) Thanks for great stuff :-)

[@aminemo93]

Absolutely awesome!!!

[@janedoe6182]

Precesion current source based on simple zener? You can make much better using combination of the Widlar mirror and peaking current source. 2 BJT + 3 resistors, no zener, great stabililty, etc.

[@AB-yu2tj]

Thank you

[@waynegram8907]

When looking at a schematic of a passive network or active network, how can you tell if the passive network or active circuit is a current source or voltage source? Can passive LCR or LR or LC networks be a current source or voltage source how can you tell?

[@gloubiboulgazeblob]

Another test : 1 current source "feeding" 1 current mirror with +9V supply. - last example of current source (purists prefer to say it is a current sink, not a current source) : 2 NPN (2N2222A) and 2 * 1k resistors. ---> measured current is ~0.7 mA (collector of second transistor), quite unstable actually (breadboard quality ? I have no clue)... Current is the same if I power that current source with +12V : ~0.7 mA (as expected...) then connecting collector of second resistor directly to collector of the first transistor of the first current mirror example (2 * NPN, 2N2222A, no resistors) Then, connecting 3 LEDs at the collector of the second transistor of the current mirror : they indeed all have the same brightness (same goes whatever the color or size of the LEDs) but the current at the LEDs is about 10 times bigger than the current at the current source.... But I think I made a mistake : if I use a current source with NPNs transistors (sink model), should I use a current mirror that uses PNP transistors (instead of NPNs like I did in my test) ?

[@mouseyou12]

Thank you for the pdf love your vid's.

[@w2aew]

You're welcome - I've had a lot of requests over the years, so I try to do it for each video now.

[@miketoreno4969]

Thank you.

[@2326TOM]

Thank you very much

[@bwack]

I love these fundamental electronics videos. Can I ask you how to match transistors used in a current mirror? Is it ok to compare the measured Hfe? Or are there other parameters needed.. (like thermal coefficients..) ? Thanks in advance.

[@w2aew]

Most important would be to match Vbe, and then beta.

[@bwack]

Thanks. Makes sense though if the bases are connected together in the current mirror circuit, the Vbe should be close.

[@stefano.a]

Good video, however, in the measure experiment, I don’t understand why you haven’t connected directly a voltage source between collector and ground to fix the voltage of current source.

[@waynegram8907]

When engineers say an Op amp is a LOWER Characteristic op amp, what do they mean by "lower characteristic? In OP AMP schematics they often use transistor or FET current sources and current mirrors and differential , Common mode rejection. If you look at common Op Amp schematics for common part number some use differential inputs using transistors while others use FETs inputs plus it looks like they are configured in a current source, current mirror, differential configuration for inputs#1 and inputs#2. If the circuit designed is for an op amp LF348 and LF347 which is JFET inputs, you can't use a replacement like an LM741, TLO84, LM324 because those op amps aren't JFET inputs which I'm not sure why it wont work in circuits as an cross reference part. It would be nice if you made a video lesson about common OP amp part number schematics to go over the FET/transistor configurations inside the op amps of the current sources, current mirrors, etc

[@Skokiii]

Thanks for the video, helped a lot! I just came across another current source configuration called the "wilson current mirror". Could you do video about this one and explain why you would pick a wilson current mirror over the basic two transistor current mirror configuration :-)

[@w2aew]

+Rolfrolfsen Wilson current mirror provides more accurate current mirroring (less error between input and output current) and a higher output impedance (more ideal current source, less load dependent) than a simple two-transistor mirror using degenerative feedback. Might make for an interesting video in the future.

[@duffyb11]

Would it be possible to demonstrate dependent voltage/current sources? Also, are you planning on doing a BJT differential amplifier with a current source?

[@w2aew]

I do plan on a diff amp, and show how current sources can be used...

[@ivayloiliev6641]

How are current sources used to increase the gain of an amplifier when used instead of a resistive load? I didn't get that part...

[@w2aew]

Quick example: for a common emitter amp without emitter degeneration, the small signal gain is given by gm*RL. Since gm is a function of collector current, the maximum gain is limited to typically well under 100x. Since a current source can be used as an active load (instead of a passive load like a fixed resistor RL), the output impedance of the current source becomes the load (RL_equivalent). Thus, for the same collector current (same gm), the gm*RL_equivalent product can be very large, leading to very high gain. This is how op amps achieve extremely high open loop gain.

[@ivayloiliev6641]

***** Got it. As always thanks for a great video!

[@hichamtassi1753]

Hello, nice video ! how to do to have a constant 1amp 14V power supply to charge a battery. WIth classical PSUs, when the battery reaches 13.5v, it takes ages to go to 14v because it draws very few current. Pushing 1Amp all the way will make it charge better and faster. Thanks.

[@w2aew]

Different battery chemistries have different charge circuit requirements. Lead acid batteries need a constant voltage float charger, while NiCad generally use a constant current charge. Lithium and LiFePO have other requirements. Using the proper charger circuit for your particular battery chemistry will give you the best results, and safest results.

[@rapsod1911]

I am curious about something you told about slew rate. Value of current inside op amp current mirrors limit slew rate.

[@jospicant]

slew rate is function of Capacity and Current if and opamp use a bigger current of polarization (this current normaly is obtained of a current mirror) you will improve the slew rate.