JFet minimum and maximum transconductance curves - Page 1
Forum section: electronics / beginners

author: robsims
author_rank: Frequent Contributor
timestamp: June 05, 2020, 08:32:15 pm
content: Hi, i'm self studying electronics and what i can't understand is how a jfet can have a maximum and a minimum transconductance curves. I understand you can have either Vgs at zero and Id at max or Vgs(off) and Id at zero. But how come there can be a Vgs min and Vgs max and likewise Id min and Id max. For example a 2N5459 has a Vgs that varies between -2 to -8 and id varies from 4 to 16 mA.

author: magic
author_rank: Super Contributor
timestamp: June 05, 2020, 09:15:04 pm
content: It's variation between units - each one will have Vgs(off) and Idss within the specified limits, but they aren't all identical.

author: robsims
author_rank: Frequent Contributor
timestamp: June 05, 2020, 09:56:38 pm
content: Many thanks Magic.So what you are saying is that every 2N5459 has it's own Vgs(off) and Idss within the range of -2 to -8 for Vgs(off) and 4 to 16 mA for Idss.If you want to design a circuit do you take the average values of Vgs and Idss?

author: robsims
author_rank: Frequent Contributor
timestamp: June 05, 2020, 10:31:20 pm
content: I think i got it. It's like BJT's HFE which varies between units of the same BJT model

author: David Hess
author_rank: Super Contributor
timestamp: June 05, 2020, 11:18:03 pm
content: Quote from: robsims on June 05, 2020, 10:31:20 pmThat is right but JFET characteristics vary even more. It is very common for JFETs to be graded into 3 different part numbers based on transconductance.

author: robsims
author_rank: Frequent Contributor
timestamp: June 05, 2020, 11:48:34 pm
content: Many thanks for the information David.

author: Mr Evil
author_rank: Regular Contributor
timestamp: June 06, 2020, 11:30:54 am
content: Quote from: robsims on June 05, 2020, 09:56:38 pmYou have to ensure that the circuit will work with both the minimumandmaximum values.

author: David Hess
author_rank: Super Contributor
timestamp: June 06, 2020, 12:29:00 pm
content: Quote from: David Hess on June 05, 2020, 11:18:03 pmI should have included that while the hfe for bipolar transistors varies significantly, it is not a critical parameter in design. Hfe is more comparable to gate leakage in a JFET which also varies significantly.Transconductance in bipolar transistors is both higher than in FETs and very predictable. The same applies when comparing Vbe to Vgs(th).

author: Kleinstein
author_rank: Super Contributor
timestamp: June 06, 2020, 12:43:51 pm
content: With JFETs there is quite some variation between units. There are some times additional graded ones available with a smaller range or pairs with Vgs(th) relatively close together.Together with Vgs(th) other parameters also change, like the on resistance Idss and trans-conductance at a given voltage. The parameters are usually high correlated, so that if parts are matched for similar Vgs(th) other parameters will also be quite close.If possible one designs a circuit in a way that it can tolerate large variations in Vgs(th). Things like the trans-conductance show less variation if used at the same current. So a defined current can help.

author: robsims
author_rank: Frequent Contributor
timestamp: June 06, 2020, 02:11:43 pm
content: Many thanks guys for the information. The book i am studying JFET's out of is "Electronic Principles" of Albert Malvino and David Bates. Please let me know if you have other/better studybooks

author: TimFox
author_rank: Super Contributor
timestamp: June 06, 2020, 03:10:54 pm
content: In my experience with Idss-graded JFETs, the transconductance from part-to-part is roughly a function of the drain current, but the Idss grade tells you how much drain current can be expected, since you do not want to forward-bias the gate diode.