Why the D S of jfet is shorted ? - Page 1
Forum section: electronics / projects

author: namster
author_rank: Regular Contributor
timestamp: October 14, 2019, 07:16:06 pm
content: Hi , i just wanna know why for biasing a protection transistor the jfet is shorten ? , like Q304 in HP3458 or Q458 in HP34420

author: Gyro
author_rank: Super Contributor
timestamp: October 14, 2019, 07:21:44 pm
content: They are being used as very low leakage diodes. (Some) JFETS are very good in this respect.

author: SiliconWizard
author_rank: Super Contributor
timestamp: October 14, 2019, 08:52:12 pm
content: Yep!You can find a typical application in this note, along with some values:http://www.ti.com/lit/an/sboa058/sboa058.pdf

author: jaromir
author_rank: Unknown
timestamp: October 14, 2019, 09:05:24 pm
content: Also, when selecting proper JFET you may find this document usefulhttps://www.onsemi.com/pub/Collateral/AN-6609.pdf.pdf

author: namster
author_rank: Regular Contributor
timestamp: October 15, 2019, 04:38:30 pm
content: it is now much clearer , to avoid a leakage current from base of transistor , je jfet is used as a low leakage diode , i don't know if i can ask a question about RL used after diode ( L300 R319 ) or (L451 R466)

author: Kleinstein
author_rank: Super Contributor
timestamp: October 15, 2019, 05:05:58 pm
content: The LC combination can have 3 purposes:1. some protection / damping of sort pulses (e.g. ESD)2. reducing EMI3. help the Ohms current source with stability : the resistor parallel to the inductor may be for this reason, as a current source does not like a pure inductor.JFETs are sometimes used as low leakage diodes, as some types (e.g. 1N4117-9) are tested for low leakage and similar tested diodes are rare and tend to be more expensive. Diodes that are typical low leakage are plenty the difficulty is getting some that are actually individually tested.

author: Gyro
author_rank: Super Contributor
timestamp: October 15, 2019, 05:57:30 pm
content: JFETS already being in light proof packages is another helpful factor for low leakage. Many diodes are in glass packages where photocurrent can become a big issue, some low leakage applications sleeve them - but then you need to worry about sleeve material leakage (heatshrink is poor), trapped contaminants under the sleeving, etc.

author: dom0
author_rank: Super Contributor
timestamp: October 15, 2019, 06:06:42 pm
content: Quote from: Gyro on October 15, 2019, 05:57:30 pmAnd then you have things like the 1N4148 - gold doped; glass package. Practically a photodiode.FWIW it may be possible to replace these JFETs with the reversed B-E junction of a 08/15 random transistor, which are very low leakage (typically). However, if that particular FET is blown, the PMOS output of the source is almost certainly blown as well.

author: namster
author_rank: Regular Contributor
timestamp: October 16, 2019, 04:43:19 pm
content: so to reduce the effect of antenna caused by wire length , a RL circuit is used in series ? isn't that ? , and instead of using a diode wich have a leakage current of about µV a jfet is connected as a low leakage current wich don't alter reference current , I thank you all for these clarifications