Re: JFET preamp, battling 1/f noise



Phil Allison wrote:

<elephantcelebes@xxxxxxxxxxxxx


Summarizing what I think that I have learned so far: The JFET itself is
likely to be the only palpable source of 1/f noise, assuming a
reasonably designed circuit. For newbies like me (mere physics PhD),
1/f noise is something that's just there empirically, for which a
deeper understanding than a simple parametric model is of little
practical use.

I found one device, LSK170, with an impressive 1/f corner around 10 Hz.
Notably, it is an IC.

** Notably, it is *not* an IC.

Beware of press releases with purple prose.

It appears that JFET op amp chips also have lower
1/f corners than mainstream discrete JFETs. Any ideas as to the reason?

** Their noise level at high frequencies is higher.

BTW

Why are you so concerned with this nonsense in an audio band pre-amp ?

http://www.jensen-transformers.com/as/as098.pdf

Since 95 % of the bandwidth is *above 1kHz* , that is where the audible
noise is.

And a device with 1nV / sqrt Hz is essentially noiseless compared to a guitar
pickup !

Graham

.

Relevant Pages

  • Re: Capacitor-feedback for low noise
    ... >> issue we'll address here is voltage noise. ... >> feedback path to the source of the JFET, ... I must not have been very clear, this is the JFET configuration I was ... To get a wide 75MHz bandwidth Jeroen specified a gain of 10x, ...
    (sci.electronics.design)
  • Re: Capacitor-feedback for low noise
    ... >> You can also take the feedback from the output of a second stage. ... > consider the value for R2, and the 0.65nV spot noise of Jeroen's JFETs. ... > R2's noise adds to the JFET noise. ... > stage gain is low, yet full output may be desired. ...
    (sci.electronics.design)
  • Re: Long-tailed JFET pair question
    ... The idea is to keep the gain of the input stage just high enough to "hide" the voltage noise of the OpAmp (if the JFET has, say, 4nV/rtHz and the OpAmp has 16, a gain of 5 should be enough). ...
    (sci.electronics.design)
  • Re: JFET preamp, battling 1/f noise
    ... It's an RF type in a tiny SMD pack. ... Summarizing what I think that I have learned so far: The JFET itself is ... reasonably designed circuit. ... 1/f noise is something that's just there empirically, ...
    (sci.electronics.design)
  • Re: low noise amplifier for high impedance source
    ... Nature, as well as Murphy's Law, because the Philips bf862 ... JFET is far, far better than it should be! ... Its noise isn't appreciable higher at 1 to 10kHz. ... I wouldn't put anything past Philips ...
    (sci.electronics.design)