What's most important when comparing JFET's - Page 1
Forum section: electronics / beginners

author: MathWizard
author_rank: Super Contributor
timestamp: July 23, 2020, 02:12:03 am
content: I'm learning some math for JFET's, and also trying them in LTspice. If you have a datasheet for a real JFET, and the big list of JFETs in LTspice, what's most important for choosing 1 like the datasheet ?If I matched typical I-V ratings, then Vpinchoff, the transconductance, and maybe gate leakage and capacitance, Idss..........will that match enough for basic characteristics ? calculations ?Sooner or later I'll have to try making my own part in LTSpice from a datasheet, but not tonight, but then again, how many stats do u need for a DC and low-freq , small signal model ?.

author: TimFox
author_rank: Super Contributor
timestamp: July 23, 2020, 02:55:37 am
content: The most obvious difference between JFETs is Idss: the channel current with zero gate-source bias. It varies greatly from part to part with a given part number. The circuit won't work if you try to bias the FET to a quiescent current greater than Idss, which would require a positive gate bias and the conduction (or destruction) of the gate-source diode.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 23, 2020, 03:26:24 am
content: There's kinda three surviving (silicon) JFET types:1. Switching. These are specified by Rds(on), Vpo and capacitances. Noise, gain, what's that?2. Low noise amplifier. Specifies Idss, gm, Vpo, capacitances, and noise at some usefully low frequency (say 1kHz).3. RF amplifier. Specifies Idss, gm, Vpo, maybe capacitances, maybe noise at relevant frequencies (say 100MHz), and usually y-params plotted versus frequency. (From the plots, you can estimate equivalent capacitances.)Switching and RF types may have excess noise at low frequencies; they aren't tested for it and they aren't binned for it. Amplifiers may not specify Rds(on) or some other DC parameters.Conversely, the switching types have basically the same small signal parameters (gm and such) as the amplifiers, they just weren't measured for it. A ~20mA 30V JFET will be, what, ~50 ohms and 10mS or whatever around there, whether specified for it or not.In any case, they're all slight variations on the same old thing. There's very little variation in gross properties, after scaling for voltage and current. Bandwidth does vary, as some optimization can be done to improve gm and capacitances over the classic types. The wide variation in Idss, Rds(on) and Vpo between parts in a given type, is characteristic, with the tightest being within an octave (about +/-40%) and the loosest spanning about a decade (e.g. 2N3819's 2-20mA Idss spec).Note that LTSpice models may not model the noise correctly, or at all(?).As for SPICE models, I don't recall all the parameters, or what they relate to; look up a guide, or maybe someone will chime in here. The equivalents to the above parameters will be the broad strokes.There are also current and new JFET types, in SiC, GaN and others, for power and RF applications. Kind of specialized, but they perform well in those niches. You'd want to look up models from the manufacturers directly.Tim

author: MathWizard
author_rank: Super Contributor
timestamp: July 23, 2020, 05:29:56 am
content: Ok thanks, I'm getting the hang of it, and I just calculate Idss from the spice model.I heard range of transconductance of these vary a lot more than BJT's Beta of the same batch, and the turn-off voltages too.Well 1 of the next steps will be to actually BUILD a circuit on a breadboard. I've used JFETs in circuits before, but just treated them as a blackbox if they worked, and gave up when they didn't.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 23, 2020, 06:18:23 am
content: Yeah, beta might vary 1:3 or 1:4 in a part, and is often binned too.gm, Vpo and Rds(on) all track; sometimes you'll see a plot of this. Physically, it's because the gate diffusion is very difficult to control. So they don't control it. I mean, they do what they can, but it's say a few seconds here or there, of baking the wafer, to go from one end of the process spread to the other, it's ridiculously sensitive. So the next best they can do is test and bin; hence many parts are in a family of still-pretty-loose ranges. Example: 2N4391 is the beefiest at 50-150mA Idss (max 30 ohms Rds(on); 2N4392 is 25-75mA (<= 60 ohms); 2N4393 is 5-30mA (<= 100 ohms). Crss (at cutoff) and Ciss are identical across the family -- same fab, same area, just a subtle variation in process. (Note the same spread in Vgs(off).)https://www.mouser.com/datasheet/2/68/2n4391-4393-53044.pdfBuilding circuits can be tricky, as it's one thing to deal with the exactly-unknown but reasonably-stable Vbe of a BJT; it's another entirely to deal with the huge spread JFETs offer. Relatively high supply voltages (say >10V) and generous source degeneration resistance are helpful here. Of course, source degeneration means degenerated gain, so if you can do an AC-coupled amplifier that's a huge help. If you have to trim it out to get DC operation, well, you're going to have to do that by hand, with a trimmer (or a calibration or autozero circuit anyway).Tim

author: iteratee
author_rank: Contributor
timestamp: July 23, 2020, 08:58:06 am
content: Thankfully the usual jfet-matching problems only involve finding individual matched pairs as far as I've seen. Requiring a jfet-based current-mirror followed by a matching jfet buffer by to give a reference to yet another matched jfet amplifier might be a problem.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 23, 2020, 02:03:54 pm
content: Yeah, a common way to make a zero-offset buffer is to use one JFET as source follower, and another (matched) JFET wired as a CRD (current-regulating diode -- gate tied to source) to set the source current. When they're matched, both sit at Idss, i.e., Vgs = 0. And they track with temperature. (This can be done for random parts when source resistors are provided, one of them being adjustable over the full range -- the follower's source resistor adds a DC offset allowing +/- offset to be covered, while the CCS's source resistor adjusts its current. Tempco may vary in this case, I'm not sure.)Tim

author: Kleinstein
author_rank: Super Contributor
timestamp: July 23, 2020, 03:41:20 pm
content: The tempco of the JFET current sink does vary with the source resistor used. The usually is a specific resistor value where the TC crosses zero so with a suitable / adjusted resistor at source one can get a low TC. AFAIR this is somewhere around Ugs at about 0.5-1 V above the threshold.Which parameters matter depends on the use: for the switch it is R_on and maybe leakage parameters.For am amplifier it is usually noise (correlates with Gm), capacitance and maybe the 1/f noise.There is quite some spread in Idss / threshold - these are correlated. So there is mainly one parameter that really changes. Usually Gm largely follows one curve as a function of the actual current - just the needed gate voltage changes.

author: MathWizard
author_rank: Super Contributor
timestamp: July 23, 2020, 09:21:21 pm
content: For the math, I'm going through these todayhttps://home.kku.ac.th/rujchai/analog/FETBiasing.pdfhttps://leachlegacy.ece.gatech.edu/ece3050/notes/mosfet/fetdiffamp.pdfI'm trying to model a dual JFET diff pair, that is the input to an integrator. I guess soon I'll know why they chose to do it this way for a large signal input. I guess to make it into a small signal, but then aren't diff.pairs really bad at large signal inputs ? I guess I'll find out.

author: David Hess
author_rank: Super Contributor
timestamp: July 23, 2020, 11:13:33 pm
content: Quote from: T3sl4co1l on July 23, 2020, 03:26:24 am4. And the 2N4117/2N4117A series.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 24, 2020, 12:18:32 am
content: Heh, kinda. JFETs usually have pretty low leakage, but the '4117 is the go-to picoamp (or less) part. Unsurprisingly it's a very tiny junction, which means little gm, Idss, and high Rds(on) (a few kohms isn't it?).Tim

author: David Hess
author_rank: Super Contributor
timestamp: July 24, 2020, 04:00:57 am
content: You identified the other three categories of JFETs that I would have selected, excluding power JFETs which have not been produced for a while, but low leakage is definitely the fourth.There used to be a pretty similar selection of 3 and 4 lead small signal MOSFETs but they were never as popular as JFETs. Linear Systems and one or two other specialty companies still make them but availability is questionable.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 24, 2020, 06:02:37 am
content: Dual JFETs (especially the monolithic form) are becoming rare, as there is not that much need for these anymore. They were popular before good quality OP with Fet inputs were available. Today modern JFET input OPs are really good and can replace discrete differential pairs in quite a lot of applications.Quote from: MathWizard on July 23, 2020, 09:21:21 pmOne can use differential pairs also for large signal swings (e.g. non inverting audio amplifiers). Especially with JFETs it can there help to use some bootstrapping, so that the drain source voltage stays low and constant. This is one of the few cases were discrete JFET pairs still make sense - though one can use bootstrapping with an OP too.For modeling a pair one can use 2 equal ones with an added source for the offset due to different threshold.

author: schmitt trigger
author_rank: Super Contributor
timestamp: July 24, 2020, 03:38:24 pm
content: Tim: a related question to your three categories outlined in your post #3.What would you select if you want to use the JFET as a constant current?I know, I know. There are already CC "diodes" in the market which are optimized for this application. But I wonder what parameters are important here.Obviously IDss and Vpo.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 24, 2020, 04:18:06 pm
content: Do they still make CC diodes..?What's the CCS for? If it's for low noise AF application, you'd want that kind of part, certainly. Anything else I suppose it really doesn't matter.Tim

author: schmitt trigger
author_rank: Super Contributor
timestamp: July 24, 2020, 04:53:41 pm
content: Mouser still lists them as available.https://www.mouser.com/Semiconductors/Discrete-Semiconductors/Diodes-Rectifiers/Current-Regulator-Diodes/_/N-ax1mlMy question was purely academic curiosity, understand the parameters which would be important in this particular application. I know output impedance is very important in CC devices, but how would that relate to a parameter which you had previously mentioned?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 24, 2020, 04:59:02 pm
content: Output conductance doesn't relate to the other properties as far as I know. It'd be, what, channel length modulation, same as MOSFETs? (SPICE LAMBDA parameter)Source degeneration resistance does help improve this, in the same way that it improves BJT Early effect (which is a little embarrassing for non-degenerated current mirrors, but FETs are generally much better behaved than BJTs in this parameter equivalent).Tim

author: Kleinstein
author_rank: Super Contributor
timestamp: July 24, 2020, 05:52:15 pm
content: The temperature coefficient as a simple CC diode with no source resistor depends on the threshold. For a low TC the threshold should be relatively low (e.g. 1 V range). The other parameter to chose is the current, that can scatter quite a bit. One may be lucky with a few amplifier types like SK209 that are available with relatively tight groups (color groups).