Recommended MOSFETs, JFETs - Page 1
Forum section: electronics / beginners

author: qu1j0t3
author_rank: Regular Contributor
timestamp: December 26, 2018, 10:58:22 pm
content: I am playing around with a voltage follower and current amplifier circuit, and I noticed that I have almost no P-ch FETs.Furthermore, just about all the JFETs available on Digikey are "switching" type, but I am more interested in amplifier type (Mouser seems to have more).Does anyone have recommended P-ch (or N-ch) part numbers of either MOSFET or JFET type for follower/amplifier applications? Low voltage/low current in my case. I'd like to have a few of both N and P in stock for experiments.(This question almost belongs on the "from no parts to stockpile" thread, but posting as a topic for better visibility…)TIA & compliments of the season

author: Psi
author_rank: Super Contributor
timestamp: December 26, 2018, 11:14:49 pm
content: might be some ideas herehttps://www.eevblog.com/forum/chat/jellybean-list/

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 27, 2018, 12:34:42 am
content: JFETs aren't fundamentally different; they are selected based on testing. When you buy a, say, PN4392, you're buying a part that was tested to a switching spec, so it has so-and-so pinchoff voltage, Rds(on) and leakage.You can infer the small-signal parameters from Vpo and Rds(on), if a bit sloppily.A 2N5486 may not even have an Rds(on) rating, though it's pretty obvious what that value is from the curves. It will however carry the assurance of so-and-so transconductance, capacitance, Y parameters, whatever, that you need in the design of an RF amp/osc.And some RF parts may carry a noise rating, but noise depends on frequency too; expect 1/f or popcorn noise at low frequencies (audio, LF). Conversely, low noise (LF) parts may be noisy at HF, though they're sometimes too big (e.g., J105) to use at high frequencies anyway so that's okay.There are very few JFETs that you actually need/want, and this is reflected in the ever-shrinking market. They're great parts when you need them, it's just very rare that you do.Tim

author: oPossum
author_rank: Super Contributor
timestamp: December 27, 2018, 12:44:43 am
content: If this is for audio / low frequency application, then...http://www.linearsystems.com/product.html

author: qu1j0t3
author_rank: Regular Contributor
timestamp: December 27, 2018, 12:50:04 am
content: Quote from: oPossum on December 27, 2018, 12:44:43 amYes, currently, but I'm also considering what to keep in stock for the future.

author: qu1j0t3
author_rank: Regular Contributor
timestamp: December 27, 2018, 01:17:15 am
content: Quote from: T3sl4co1l on December 27, 2018, 12:34:42 amThanks Tim. I'll also accept recommendations for suitable MOSFETs (P/N) for amplifier/follower, to keep in stock.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 27, 2018, 04:37:22 am
content: There was another more recent thread about that too:https://www.eevblog.com/forum/beginners/jellybean-hobbyist-general-purpose-transistors/msg1322846/Tim

author: Zero999
author_rank: Super Contributor
timestamp: December 27, 2018, 09:12:22 am
content: Yes discrete J-FETs are rarely used these days and the same goes for low power MOSFETs.For a voltage follower, just use an op-amp. It can be a J-FET input op-amp, such as the TL072, if you want a high input impedance.MOSFETs are normally used for switching large currents, either on or off, rather than in the linear region.If you want 10 or so N and P channel J-FETs for experiments, just order whatever you can get in a through hole package of, from a widely known distributor: Digikey, Mouser, Farnell etc.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 27, 2018, 09:29:56 am
content: p-Channel JFETs are very rarely used. So here it's about looking what is available, if one really needs one.n-Channel JFETs are still used sometimes, but not very often. THT parts are getting rare, but there are still eve new SMD parts.So if it needs to be THT the choice is usually limited.The variation in the parameters is quite large. So most circuits that use JFETs are not very picky about the exact type. It is mainly for high performance that the type of JFET matters. With JFETs in TO18 some, made for switching have the gate at the case. This can be a bit odd when used in an amplifier.

author: Wolfgang
author_rank: Super Contributor
timestamp: December 29, 2018, 02:22:00 am
content: For grounded gate RF amps, gate at the grounded case is ideal. This reduces unwanted feedback capacitance.