#190
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- เผยแพร่เมื่อ 16 ม.ค. 2015
- This video is a back-to-basics tutorial on transistor current sources - what they are, some applications for them, and some circuit examples. The basic characteristics of current sources and sinks are presented and demonstrated, including the measurement of the output impedance of these simple current sources. Here is a link to the note pages used in this video:
www.qsl.net/w2aew/youtube/tran...
Viewer +BersekViking pointed out a problem with my test setup. Can you guess what it is? Here is a link to my followup video:
• #191: Beware of test e... - วิทยาศาสตร์และเทคโนโลยี
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.
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!
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!
I learned a lot from the Forrest Mims books!
@@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.
@@w2aew books by R.M. Marston were also essential reading.
It still nice to see the old analog Simpson meter. Still in use . Good video well explained!
The best explanation about electronics I've seen. Wished I had a teacher like you. I wish all the success you deserve.
same
Love this video, looking to understand transistors since ~1 year and making slooooowly progress.
Professor, so much thankful for your videos. I never never have had the opportunity of learning with such a perfectly explanations. THANKS.
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!
Valued info! Thanks! I love how the majority of your projects involve less than a $5 of a handful of parts.
Best practical electronics & education channel out there. Hands down.
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!
The HP RPN calculator is a really nice touch. I am still using my trusty HP35s.
Cheers Alan, simple, straightforward, and very useful! Back to my bench!
I really like your back to basics videos, I always learn something new. Very well done too!
One of the best channels to refresh and learn electronics ... As always , Excellent work W2
Brilliant. Thanks for explaining these mysterious little circuit blocks!! Now I see how they help increase the gain in audio stuff.
Time really flies when watching your videos. Thanks for the great explanation.
Fantastic presentation Allen. I really enjoy your presentations and get allot of inside from them.
As usual, clear and informative video.
Thank you.
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.
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!
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.
***** 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.
Agreed!
When real professionals meet they usually behave like gentlemen. But you are right, this kind of chivalry is becoming the exception in YT comments.
Really clearly explained, again! Thank you, love to see this continue on into a diff amp too.
Gosh I wish you were my electronics teacher on my graduation. Thank you for the excellent videos.
Another really great back to basics video, thanks! Also liked the HP 15C calculator.
Thanks for another great, thorough explanation.
really love these back to basics videos.
Thank you hit the nail on the head I was looking for something on current I like the way you present your vid's.
...and finally I understand current sources. Awesome video. Thank you :)
Thanks Alan - a good tutorial on a subject I'm a bit fuzzy on. I'm going to have to watch this again. Cheers!!
Nicely explained, although I'll have to replay it a few times for my mind to take it all in!
I very much enjoyed this. These have always been a little fuzzy for me but this cleared it up nicely.
Chapeau Maestro! Very clear and well explained...
Another good (and useful) video Alan. Thank you for prparing it.
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.
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...
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?"
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).
another extremely informative video. thank you
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.
Wow!! Great explanation
I'm enjoying your show - especially the b.e.e. stuff.
Thanks Alan! You're the man!
Great video about current soutces.
very clear explanation, thank you
Thanks,very very nice and simple tutorial,
i hope you go forwards and waiting for more basics videos
+Moon Light Shadow Please let me know what topics you'd like to see in future "basics" videos!
calmly taught, thank you.
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.
Thank you very much for this video.
This is really good stuff, thanks!
This is very helpful for some working I'm doing on pulse magnet motors and what to do with the excess energy output.
I just noticed you also provide a pdf of the ckts - SWEET!
thanks!
Well done, excellent video!
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
I'm glad you liked it. I'll keep these topics in mind.
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.
I plan on doing something on this topic soon.
Thanks for the video, it is very helpful.
Current sources are very useful. THANKS
Thanks for the tutorial.
Fantastic video! Thanks
Very cool videos, thanks man.
Great Video, Good Presentation.
Super video. Thank you.
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?
Simply awsome video. Im getting there :-) Thanks for great stuff :-)
great video thank you
Great video. Just correction on current changed is in milliamp, but Allen kept say Microamp.
Very nice video!
awesome, thanks alot for this video
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.
Excellent!
awesome vid ! 🔥
Absolutely awesome!!!
really awsome video.
Thank you for the pdf love your vid's.
You're welcome - I've had a lot of requests over the years, so I try to do it for each video now.
Thank a lot, very interesting.
Great video!, Thanks a lot! I really enjoy your videos.
Can i ask you a question?
In the video at 7:44, what do you mean by "floating power supply"?
A floating or isolated power supply is one that generates the desired voltage between its positive and negative terminals, but neither of these terminals is connected to ground.
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 ,,,
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.
*****
Sounds great - diff amps are important, and also fun!
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 ?
Thank you very much
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?
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.
Most important would be to match Vbe, and then beta.
Thanks. Makes sense though if the bases are connected together in the current mirror circuit, the Vbe should be close.
Thank you
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?
Would it be possible to demonstrate dependent voltage/current sources? Also, are you planning on doing a BJT differential amplifier with a current source?
I do plan on a diff amp, and show how current sources can be used...
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)?
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.
Yes - in fact I made a followup video that points out this error in measurement technique...
th-cam.com/video/JbCI4Lsnqho/w-d-xo.html
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?
gran video como todos los que usted hace mister gracias.
Thank you.
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?
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 :-)
+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.
Thanks شكرا
Really well done videos! I do have a question....At about 16:06 in the video you say that you take the first current source output and tie it to the PNP current mirror input. This happens to be the emitter on the PNP....my question is : What did you tie the collectors of the PNP mirror to?
Take a careful look at the schematic at 15:47 - the input of the PNP current mirror is the base-collection junction labeled 'IN". The emitters are tied to the positive power supply.
@@w2aew ...my apologies......In prior video you always had a number (ie +10V, etc) labeled and I didn't recognize the 'circle' without the label....my bad....thanks
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.
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)
@ 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.
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...
Hi Pro. At 9:28, is the BJT in saturation or linear region? Why there is a 1.0K resistor at emitter?
Thank you for these knowledgeable videos.
It is in the linear region. The 1k resistor helps to set the current based on the resistor divider at the base and the base-emitter drop. It makes the value of current mostly independent of transistor beta.
When a BJT is in linear region, is Vbe (base-emitter drop) a constant value for different collector current values?
If the emitter resistor is removed, and use a pot to replace 2.2K. Can I tune the pot to get a adjustable current source?
Thank you.
In the linear region, the Vbe is nearly constant, but will vary a little. Your suggestion would make it very difficult to adjust the current source to a particular value - very unstable and will vary with temperature. Much better to leave the degeneration resistor (1k in emitter) in the circuit, and then adjust the base voltage with a pot.
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.
Well, something sounds "off" if you're measuring 0.6V cross that emitter resistor, but also seeing 10mA current.
@@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.
I will measure again - with a load
@@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.
@@armandine2 Make sure that load doesn't saturate the transistor.
hi Alan
great video but i have a question.
i didn't get why you put a 470 ohm resistance in parallel with supply.
will you please shed some light on that
Because the power supply can't "sink" current. The 470 ohm resistor ensures that it is always "sourcing" current.
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.
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.
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.
thank you for the great video
What is the purpise of mirror circuit in power amp circuit?
Can you explain why it was necessary to put a 470 ohm resistor across the outputs of the power supply to keep it sourcing current? Since a bench power supply typically has readouts for volts and current, I suppose one could "adjust" those readings to compensate for the resistor, correct?
In this setup, the power-supply would have to "sink" current (current flowing into the + terminal), and linear power supplies like this can't sink current. So, by placing a small resistor across the supply, the output will always "source" current and force the set voltage across the resistor. There is no adjustment to any readings necessary since we're not measuring the output current of the supply.
@@w2aew Thank you for your quick reply! I have a switching power supply. Can I ask a follow-up question? Do switching power supplies sink current into the + terminal? I am only a hobbyist, so it would take a great deal of research to try to figure this out, and the odds are good I would come up with the wrong answer!
@@claude77573 It likely can not sink current. Also, if you're going to replicate this experiment, you'll have to check the specs on your supply to see if the outputs are truly floating from ground.
Do you have a video you'd recommend for the variable, floating power supply? I'm confused by how its set up.
1) I normally see supplies hooked up with the negative to the ground.
2) It seems to be putting its positive out on the +10V rail. In my mind that would cause a short between the variable supply's voltage (assuming its any value other than 10V) and the 10V rail. But this must not be the case here.
Thanks for any insight
Please see this video, it might help to clear this up for you:
th-cam.com/video/mJL11UF1arQ/w-d-xo.html
Thanks, that helps with the floating aspect.
So the last thing I'm struggling with may just be me not understanding notation. At 8:07 you are talking about the variable supply. When I looked at the diagram I assumed + terminal of the variable is going in opposition to the + rail. Does the symbol not represent the direction of the voltage? (By which I mean the - variable terminal is at 10V and the variable + terminal goes down toward the ammeter)
The variable power supply basically sets the voltage that is generated between the + and - terminals of the supply. Since the + end of the supply is connected to the +10V supply rail, the - end of the supply is XX volts below +10V. For example, if the variable supply is set to 3V, then the - end of the supply will be at 7V in this circuit (10-3).