JFET rds-on and noise - Page 1
Forum section: electronics / beginners

author: fonograph
author_rank: Frequent Contributor
timestamp: May 27, 2017, 01:40:51 am
content: Generaly speaking,do JFETs with lower on resistance have lower 1/f noise corner freqency? I read somewhere that small transistors with higher on resistance have higher 1/f noise while big transistors with lower on resistance have lower 1/f noise,is that true?

author: Zero999
author_rank: Super Contributor
timestamp: May 27, 2017, 08:24:32 am
content: It makes sense. Generally, larger transistors are less noisy because they're roughly equivalent to many small transistors connected in parallel, which also reduces the noise.Noise sources are uncorrelated waveforms, so adding noise sources together doesn't generate double the noise voltage, as only the parts in phase add, the other parts will be anti-phase so will subtract. If you take a signal, put it through two amplifiers, each adding equal noise to the system, then take the average output of both of the amplifiers, the result will be less noisy, than if you'd only used one amplifier.Here's a link what Linear Technology say on the matter:http://www.linear.com/solutions/5657

author: David Hess
author_rank: Super Contributor
timestamp: May 27, 2017, 01:11:18 pm
content: The broadband noise roughly depends on the channel resistance so lower channel resistance actually *increases* the 1/f corner but this does not reflect higher 1/f noise; it just means that the corner frequency where the 1/f noise and white noise are equal moved up in frequency.So the 1/f noise is only related to channel resistance in the sense that larger transistors have lower 1/f noise and also have lower channel resistance. Geometry and process purity determine 1/f noise; high W/L ratio and high area yield lower 1/f noise.

author: danadak
author_rank: Super Contributor
timestamp: May 27, 2017, 01:37:32 pm
content: I looked at a number of papers, seems consensus is 1/f strongly dependenton process. The defining equations seem to indicate Noise 1/f strongly dependenton 1/L2, W not a factor. So longer channel = lower 1/f, but rapidlydiminishing returns.Rdson strongly dependent on W / L. Longer the channel, yields lower Rdson.Jumping off edge of cliff looks like lower Rdson has lower 1/f.Corner f also proportional to 1/L2.Maybe an expert can bail me out here......http://bwrcs.eecs.berkeley.edu/Classes/icdesign/ee240_s09/lectures/Lecture05_Electronic_Noise_6up.pdfhttp://www.silvaco.com/content/kbase/noise_modeling.pdfRegards, Dana.

author: rfeecs
author_rank: Frequent Contributor
timestamp: May 27, 2017, 06:45:37 pm
content: Those references refer to MOSFETs. JFET is a different animal.Here's one for JFET:https://www.fairchildsemi.com/application-notes/AN/AN-6602.pdfQuote

author: David Hess
author_rank: Super Contributor
timestamp: May 27, 2017, 07:39:24 pm
content: Quote from: rfeecs on May 27, 2017, 06:45:37 pmAs far as I know, the mechanism for 1/f noise in MOSFETs and JFETs is the same, charge trapping in the gate which shifts the gate threshold voltage. In MOSFETs the charge gets trapped in the gate dielectric and I guess in JFETs it gets trapped in the depletion zone formed in the gate's PN junction.Probably not coincidentally MOSFET and JFET gate threshold voltages are not as stable as bipolar transistor Vbe while the former have higher 1/f noise and the later has lower 1/f noise and the same pattern appears comparing MOSFETs to JFETs.Quote from: danadak on May 27, 2017, 01:37:32 pmUm, isn't this backwards? The channel *current* is proportional to W/L so a longer L yields a higher resistance.

author: rfeecs
author_rank: Frequent Contributor
timestamp: May 27, 2017, 11:49:22 pm
content: Quote from: David Hess on May 27, 2017, 07:39:24 pmSounds like two different animals to me.It depends on whether the noise is from the surface or the bulk. The JFET depletion region is nice high quality bulk material without many traps.A few more references:http://www.eng.auburn.edu/~wilambm/pap/2011/K10147_C011.pdfQuotehttp://pessina.mib.infn.it/WOLTE2014/van%20der%20ziel/1.90201.pdfQuote