#374 

I wish I could make a video like this. I teach at a 4your US university and I tried to make a video similar to your technique. But I could never do that! The video begins with a crystal clear explanation and very nice voice tone completely balanced, and the it gently proceeds to testing and validations and finally a relaxing moments of summary and conclusion. Sir, I want to tell you that you have a very unique talent that can't be copied or mimicked by others ❤

I've had a few projects where I wanted to use JFETs as current sources, but I found JFETs to be very hard to get hold of these days, so I've been using LM334 instead.

I LOVE this kind of video! We need to come back to basics eventually to bring back what's almost lost for this new generation.

Excellent video, as always. After I saw this, I compared the cascode arrangement to an arrangement with J111, J112, J113 in series with grounded gate and a degenerative resistance. The latter provided a higher impedance source with the same burden voltage in LTSPICE. Of course, as you mentioned, the manufacturers never figured out how to make JFETs with consistent performance, so a lot of testing would be required to see if this really works in practice. This was always a problem when manufacturing low noise transducer preamps. Of course you could pick out ones with similar characteristics, but then you would have to hand solder them to the boards. Then, the medical imaging people came in with big bucks and insisted on low noise JFETs that had consistent characteristics. So now they are available for about $100 apiece for a JFET with 0.8 nV per rt Hz performance that has been binned by the manufacturer prior to inclusion on a reel. I guess this is acceptable on an MRI machine, but not in any of my applications...

Great video. Siliconix used to produce a range of JFET current sources. I don't know if they are still available. They were a JFET with gate to source resistor integrated into the package and just 2 leads brought out. Worked great and helped reduce the component count for designs.

Thanks for another great explanation video. You re-awaken some things I learned years ago. I often get more from your videos than from some of my engineering courses 50 years ago! Thanks

This is very much the sort of tutorial style I appreciate. Discrete component fundamentals get glossed over in many modern day projects that focus on ICs or Controllers. I can pop open my textbooks but this is so much more palatable.

Some years back I made a Triax cable tester which had a 9v battery supply and LED indication. It needed a current limiting circuit so I used the cascode constant current source you have described. It was easy to set up and the current remained constant over the typical range of battery voltage variation.

Alan your videos are the absolute best. The continuing education you provide through these videos is priceless. As I often do follow along at home this was an inexpensive exercise. The ones that require more expensive equipment i.e. mini circuit devices are great too but a bit more expensive. The good thing about those videos is they justify my purchase of some cool test equipment! Keep up the great work i look forward to the next one. 73 de W3AL.

FETs and BJTs where topics in trade school, but i always felt like either the teachers didn't understand them beyond the Wikipedia article. Stuff like this is what you'd want to know. But they would come up with more complex solutions, that, yes would do a more precise job and so on. But this really helps to understand the behavior of parts better. And (and this is where my job comes into play) it helps you understand circuits in devices that you are tasked to repair. The world is full of devices where they pull off that one trick that stops you in your tracks when troubleshooting. Thanks

Alan, these are a tremendous resource and I find myself accessing your videos as "refreshers" the more senility over takes me. Making your notes available is such a bonus as well! I even downloading the notes and print them on three-holed graph paper for that Forrest Mom's effect. Very gratefully, William, k6whp

Yor series is a Great Learning Resource for Everyone! Thanks!

Each time I see a new Back to Basics video from you just makes my day! Each one is really a pleasure to watch and you make this so easy and straightforward to set up on the Bench and get to work ourselves. Thanks so much Alan!!

Another wonderful BtB, thank you. As always, great theory without excessive maths, and practical application.

Great to see B2B videos again. I just love you printing and your ALAN-CAD sketches.

Thanks for this basic video; it´s good to repeat "old" knowledge not to forget it in our digital days!

Great job Alan. I may try this at home. Thankyou.

Thank you! Have followed you for years and learned a lot.

Another video from Alan, well I know what I am doing for the next 15 minutes!!

Nicely explained, thank you! And I'm oddly happy that the quiet rattly scrapey sound that occurs at least once in the background of all your videos is still there after all these years. In a world of change and uncertainty, some things can still be relied upon 😅

Great video again! The link to the Application note needs to be 70596 instead of 70595. 70595 is about Biasing. The information about the cascode could have been a bit more detailed in the relationship between the top and bottom Fet as the top Fet should have equal or higher Idss so the bottom Fet doesn't starve.

Thanks for another very interesting and informative video, Alan!

Another great video! Your back to basics vids are always helpful and clearly explained!

I am at work, watching this in between the calibrations and what I see.. I use the same exact bench multimeter model by Keitley 😃

I love these back to basics videos. Thank you.

Thank you for posting this video on FET's ! I just found a bad one in my IFR -1000S. Gary Grove / 73's

Great presentation from the "Master Jedi". You are the bees knees Sir !

As always, interesting stuff. Thanks Alan📡

Fascinating, never realy used a JFET so I learned something today...cheers !

Simply excellent!!! Thank you.

Wish my uni profs could teach like you. Excellent video! 👏

Always an excellent educator... many thanks.

Love the basics series. Excellent content.

This was a really clear and useful explanation. When I did analog board design, I'd almost always use a bipolar current mirror as the core of my current sources, but I can see how this this might have saved me a few components. Nice.

Nice video as always! I think it would have been nice to use the curve tracer to characterize the current sources, not just the plain JFET. This way, we could have seen a graphical representation of the output impedances of the various topologies that you've covered. :D

I enjoyed your video a lot, watched most of them, learnt a lot, thank you very much, a fan from china.

Wonderful presentation as always. Thank you.

Fantastic content as always ... Thank You for sharing ... Cheers :)

useful trick with the cascode if you have large power supplies otherwise the compliance of the output is a serious limit for small VDD.

Nice and helpful video. Best wishes Sir.

I've used the BF245C for classic 20mA LEDs as current limiter for years...

If you put your audio into a free program like Audacity, you should be able to use the noise removal effect to get rid of things like a small fan constantly going in the background

Good info. Nice presentation. Thanks!

Very nice video. Thanks so much.

Fracking youtube just stop showing videos from certain people, you've made A LOT of videos since I last saw one from you - I knew something was missing in my life :)

You didn't explicitly explain that having a source resistor provides negative feedback; to be precise any tendency to increase Id will result in an increase in Vgs, thus opposing that increase (and obviously the same applies for any tendency to decrease Id). One of the results is to sharpen the knee of the curve relating Id to Vd as well as flattening its slope, which represents the output impedance. The flatter knee allows Vd to be less while remaining a useful constant current source. The cascode circuit has the other disadvantage that the lower JFET only sees the Vgs of the upper transistor as its Vds, and that puts its operating point on the knee, giving a lower output impedance. That's why the cascode doesn't offer as much improvement as you might hope.

Thank you for this

Thanks Alan! So my question is if there is so much variation in components you could theoretically substitute an mpf 102 in place of a j 310 within reason and assuming that you stay within max ratings. Am I correct with this thinking? Take care and I always enjoy your videos. 73's!

Merci for this video, very interesting.

Another great instructional video. I learned how to cascode JFETs from this program. Therefore, it seems that JFET cascode device can easily be used on top of other devices such as BJTs, and MOSFETs without much biasing network. As you know we go to great length to bias the BJT and MOSFET cascode device to make sure the casecode device stays in saturated region; whereas, here you do not need much biasing network, and the cascode device always stays in saturated region naturally. I wonder if you have two matched JFETs, how can you make current mirror out of them? Anyone knows how? Oh by the way I used JFET current limiter for safely discharging high voltage capacitors, instead of resistor, I use LED, when the LED goes off I know the capacitor is discharged to few volts, and another JFET current limiter with resistor in the source, which is connected in parallel with the LED current limiter will complete the discharging the capacitor.

I have used JFETs a couple of times and am aware of the large variability between devices. Why are these so hard to make to narrower tolerances?

I made many CC circuits using your past video on mirroring using NPN (2N2222A or 2N2904) tranys, with each circuit driving a slow flashing colour LED. Maybe I missed it in the video, but why use JFET instead of bipolar, since both are operating in their linear region?

It's channels like this that remind me that the internet maybe wasn't such a bad idea afterall.

Thank you for the upload! Out of curiosity, what application would something like this be used for?

There were current "diodes" available 50 years ago. They were probably made by Motorola. Are there similar devices sold today by some companies?

Thanks! I have been meaning to play with JFET's since I was a kid but because I mostly use junk parts I don't think I've even seen one in real life. I need to buy some. Was that your gas heat turning on? Already that cold?

This is really cool, Alan! I could see this being useful driving LEDs, particularly as the season of holiday lights approaches. Not being an EE, I wonder how much heat this would dissipate? I assume it would be a function of the FET's "on" resistance?

Perfect

Great content!!

Would have been nice to see the circuits back on the curve tracer as 2 pin devices

Thanks Alan. Really helpful content as always. 73, Nick M0NTV

Love the vid.

Super (AP2SM)

When the FETs output voltage is NOT Flat, this is the FETs output impedance? I'm confused when the FETs output voltage is NOT flat what types of problems can it cause with other circuits?

👍👍👍👍

I like your American spelling of "Exercise".

👍

But what is uniquely different about a JFET? I had bought some JFETs from China, they were fraud devices, they were really BTJs. They did not work in my device (a DIY capsule microphone that needed a balanced circuit). The knock-off was un-balanced. I bought a genuine JFET and it worked.

👍❤