Exploring How JFETs (Junction Field-Effect Transistors) Work! - DC to Daylight
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- เผยแพร่เมื่อ 22 พ.ค. 2024
- In this episode, we're exploring the JFET, or junction field effect transistor, as it applies to the common-source amplifier. We cover graphically approximating the transconductance curve, and how to set the DC bias point. We also show how much this transconductance varies between devices within the same family. We then breadboard our circuit, and verify that measurements approximate our calculations! bit.ly/3QpPO1o
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#0:00 Welcome to DC to Daylight
#0:50 JFETs
#3:35 Graphs
#10:17 Circuit
#12:53 Breadboard
#14:31 Give Your Feedback
#jfet #transistor #transistors - วิทยาศาสตร์และเทคโนโลยี
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I'm happy for this video, it's nice to have a simple explanation of JFETs. I've got a bachelor's and most of a master's in electrical engineering, and have never used JFETs.
The education on them was basically: They're not used that much, don't worry about it for now.
Ha! True they're not used that much in general electronics, but still have niche applications in RF and low noise audio. Thanks for the feedback! -Derek
Thank you. For people who are setting out on their electronics journey, being able to benefit from experience, explanation of practical considerations and clarity of explanation is invaluable ... and much appreciated. Top work.
This channel is really the top notch when it comes to exploration of circuit design and theory. One thing that is somewhat frustrating if new to electronics is understanding how to read datasheets and their graphs. Really well explained and useful. Good job
The internet needed this :) Have been designing a jfet based audio compressor this week, just for shits and giggles, and i really stuggled with tuning in the jfets. This is s very intuitive breakdown. I wish you had uploaded it sooner :,) Thanks!
Good stuff. JFETs are still pretty common in audio effect circuits often as a solid state alternative to vacuum tubes. A lot of op-amps have a FET input as well. In both cases they are very easy to destroy with ESD so it's worthwhile having some protection on the gate in your circuit design.
Hi Justin. Very true I think I have some studio gear with discrete JFET front ends. I used to work in a very dry climate and we used FET based laser diode drivers. We'd blow them left and right due to ESD - even with wrist straps. The cure was antistatic lotion on the wrist prior to putting on the strap. Tip for the day ;) -Derek
This was _very_ informative, but like you say, I can't really remember the last time I had a use for JFET's. Sometime around the late seventies/early eighties.
I _do_ remember building a regenerative receiver using one, which had excellent sensitivity and rejection, but ... that was about it!
Hi Greg. Yeah, they're a little scarce in designs nowadays - I still see them in old ham gear and RF stuff - also in niche audio. -Derek
They are also useful to make oscillators and in general amplify faint sensor data. You can think of them as low noise amplifiers that have little effect on the circuit that is being probed.
Would be interesting to see if it could also be used as a simple oscilloscope probe! Enough circuits get detuned (osciallators) or otherwise disturbed when probing with a normal scope probe.
Gonna try to remember to watch this tomorrow. For the moment I only know that they are good as inputs to amplifiers in some situations, and that they can be set up to be nearly ohmic as voltage controlled resistors
oh, I was just researching this. thanks
Nice explanation!!!!!
awsome tutorials , i always afraid from the JFETs and think it is not that easy, you make it close to be understood for me , Thx and hope you did some videos or series explaining Op-Amps aswell
Awesome!!!
I would love to see a practial comparison of jfet vs MOSFET, and an example of where each is a better choice in circuit then the other
Switching speed. Power handling. Look at some data sheets.
Major use is as really low leakage diodes, using that junction, and shorting drain and source together. Will handle up to 30V for most common Jfet devices, though forward current handling is dismal, but for use as a fast low leakage and low capacitance clamp that is not an issue, as you use them in high impedance circuits.
Interesting. I've read about using them as diodes, but never tried it. Thanks! -Derek
Good video. Would be good to know how to find equivalency, and how to adapt or how to care/expect for one replacement that is not exactly equivalent. The bias likely would have to be recalculated but maybe there's more to care for
thanks
شکریہ
Never really got to grips with JFETs, found this really interesting. Was surprised at the spread of the parameters, wonder if manufactures bought theirs graded or measured them themselves, you could (I imagine with this spread) fit a new part and find it just didn’t ‘appear’ to work.
Nice
Thanks CircuitShepherd! -Derek
2:29 nice flux capacitor 😄
Much appreciated... You can make one too: th-cam.com/video/oMLD13UdEsQ/w-d-xo.html -Derek
nice
Sir what is the voltage between Gate and source shown in your demonstration
Please let me know
So what would be the main differences between a JFET and a depletion mode mosfet? They both seem to operate very similarly.
Hi Dacke. The physical construction is different, as you allude to they are both depletion mode devices, though mosfets can be constructed as depletion or enhanced devices - JFETs are strictly depletion. JFETs are low noise, which is why they still find themselves in op-amps and in things like microphone preamps. JFETs are less susceptible to ESD because they're lacking the delicate oxide insulator found in mosfets. In my experience JFET transconductance tolerance is horrifically all over the map, and mosfet gate parameters are MUCH more tightly controlled - both in physical dimension and electrically (process chemistry and pocket doping). -Derek
Thanks Derek!
6:24 Sauce for the test circuits please? Looks similar to but different from the one at runoffgroove.
How to determine the maximum operating frequency of a JFET?
In this case we need to be concerned about input frequency response which is 1/2piRC, where R is the source resistance in parallel with the circuit input resistance, and C is the input capacitance Ciss (which is Cgd in parallel with Cgs - found in datasheet) of the JFET.
The output frequency response should also be assessed, same basic formula but where R is drain-resistance || load-resistance || rd, and C is the output capacitance Coss (which is Cds + Cgd). -Derek
If you are lucky, the reference datasheet supplies a frequency bandwidth which is the product of the gain times the frequency that can properly handle the transistor. As an example, if it is 1MHz, then with an amplification of 10, you can go up to 100kHz, but with a gain of 100, you can only go up to 10kHz..
A decent job. Thanks.
Funny how 90$ books cannot beat a 15 minute video which took, say 15 hours from start to finish.
Thanks Rob! -Derek
No local feedback... And I was wondering if 'emitter ummm... source degeneration' lowers the gain too much, how would half or so impact the sonic performance and gain, say like a hundred ohms in series with the 100uF Cbypass? (:Just a thought:)
how about normal jfet and jfet with built-in resistors?
Interesting! I have never seen jfets with built-in resistors. Can you point me to a part number, so I can look further? -Derek
JFET "can" be seen as a variable resistor controlled by voltage at the gate.
When the Drain to Source resistor is minimum when Vg = Vs ( voltage at the gate is "zero" with respect to the source) and the resistance increases as Vgs ( = Vg - Vs) decreases (increases in absolute value). They can thus be used to vary the frequency of an RC sub-circuit, playing the role of R (with fix C) .
While a DMOS can be used for the same purpose, JFET are still more common than DMOS.
could you make a video about negative voltage, please..? like for opamps, jfets, etc they ask for negative voltage, but I've never been able to find good information how to deal with negative voltage on a real circuit, like do you put a 1:1 transformer? have two rails? how do you deal with all that in real life? do i even understand properly? i think that negative voltage is pulling ex. 12 V instead of pushing 12 V of electromotive force, but i don't know how to easily implement it... like just an opamp, what are the various, and best ways to have your negative and positive voltages on the same circuit, and on the same PCB... ?
i would really like to have the final page on negative voltage, it's a big hurdle to me...
thanks if you consider making one 🤫
A voltage is always a difference of electrical tension between TWO points.
What about THE voltage at a (one single) given point?
Well, a little bit like temperature, in Fahrenheit, it can be in the positive while the same temperature, in Celsius, is in the negative.
It is a matter of REFERENCE, of where is the ZERO. Because when you say that the voltage at ONE point is, say, 6 Volt, the SECOND point is implicitly your local GROUND, which is where YOU chose to say that your ZERO is. (Oversimplified definition of what is the ground, I know)
You can use a voltage divider (not very energy efficient, and not as good as it sounds) to establish three "rails", that is, one at +12V, another one at +6 Volt (two equal resistors of low value) and one at 0Volt, OR, as you wish, can SIMPLY say that the ground is at the middle and get +6V, 0V and -6V.
The voltage divider is not always good, in particular if you use resistances, in the useful circuit, which are less than 10 times the resistance value used in the voltage divider.
There are dual rails IC which do a much better job at "creating" negative voltage with respect of the ground define at one pin of the IC, taking a single "positive" voltage and returning and a positive voltage, and a negative voltage, through an internal oscillator (or otherwise) circuit.
@@snnwstt ah, that was quite enlightening, thank you... 😀
If you need more gain, you can always follow the JFET with a common base; together they are cascode connected.
Hi Byron. True, there are several ways the gain can be increased - most devised in the early transistor days. Cascoding from my understanding, is to mitigate miller effect. Could be worth taking a look into it.. - thanks for the input!! - Derek
@@AmRadPodcast Cascode also mitigates the Early effect. The Early effect limits the output impedance of the transistor amplifier's current source.
this is hardcore.
This is the goal. ;) -Derek
Just for dummies like me: why does the P-type BJT arrow point inward, but the P-type JFET arrow points outward?
The arrow in the transistor points from the p type silicon to the n type silicon inside of the transistor. So for a pnp bjt the arrow goes from the emitter (p) to the base (n), and similar for the JFET where it goes from the soure/drain (p) to the gate (n).
@@tjego5806 That sounds to me to mean that the arrow would point in the same direction in both P-Type devices ("... to the base...to the gate..."). But the symbols show reversed arrow directions.
He looks like MCU grandmaster :)
3:26 not good he channel never closes it gets very thin resulting in a high density J of electrons and more so the current does not stop it stalls at that level Idss defined by Vgs=0V and Vpinch as such for another level of Vgs e.g -2V pinch will happen at a different voltage for Vds with the end result another saturation current this could be verified by doing a parameter sweep or simply solving Shockley equation for Vp 8:40 that gm does not vary much at all it`s 2Idss/Vpinch = 3.84 and 3.54 even looking at those lines they are practically paralels which means they have the same slope and the same transconductance which is the derivative of the transfer characterisc
Sir mosfet teach sir
Mosfets are definitely on the to-do list. Thanks! -Derek
Speak american please. Where do i put the electricity
Put 115v between the left and right ear 👂. Phase and neutral can be any one of them. Now switch on and everything will be clear to your brain 🧠.