[time-nuts] Re: +1/f of transistors

[Lux, Jim]

On 4/14/22 3:00 PM, ed breya wrote:
> It depends on context and your definition of low noise. I was 
> picturing ones commensurate with the 1/f discussion, trying to get 
> down toward DC. Many are spec'd for so many nV or uV p-p over 0.1 to 
> 10 Hz. The OP-07 is a good example of this class.
>
> A lot of opamps are spec'd ok for their intended use, but you have to 
> look fully at the charts and numbers for your actual use. For 
> instance, the ADA4817 looks great as a HF/RF amp, with 4 nV/rt-Hz, but 
> that's at 100 kHz. Similar for the OPA656. At 10 Hz, it looks like 
> it's well over 100 nV/rt-Hz.
>
> The JFET types are fine for high-Z amplifiers and such, but can't 
> reach the near-DC region in the few nV/rt-Hz (at 10 Hz, say).
>
> The CFAs may be OK too - they're built for low-Z, so may tend to be 
> operating the + inputs at fairly high bias currents, like the low 
> noise opamps.
>
> The good part overall, is that to get near the nV/rt-Hz range, the 
> source resistance has to be very low anyway, so at least the bias 
> currents are manageable, although zero would be nicer. 


of course, if your low noise frequencies of interest start at 100kHz, 
then whether the 1/f crossover is at 100Hz or 10kHz is less important :)

My recreational complaining gripe is that there's a lot of interesting 
low noise parts intended for the microwave market, but they don't even 
bother giving data below 50 MHz.  So it becomes a matter of tribal lore 
- and you have the risk of "we got a really good batch" or "we don't 
make those anymore".

Vishay Siliconix *used* to make JFETs (e.g. JANTXV2N4858) with good 
noise properties (3 nV/sqrt(Hz) at 1 kHz) - but they're not in the JFET 
business any more.  They even put the noise on the datasheet, although 
with a "not guaranteed JEDEC spec", footnote. And they still sell them, 
maybe (It says "contact factory for JFET products" - factory stock, I 
suppose). Or you can scrounge around and find 4 or 8 from some dealer.  
And you can buy plenty of 2n4858s from other people, but *they* don't 
say anything about the noise.  And how do you find parts like this? Word 
of mouth from someone who spent a bunch of time looking at data sheets, 
bought a bunch of different kinds, and tested them.

I suppose that's why this is a recreational complaint - I (or anyone) 
who needed low noise FETs basically has to suck it up and go through 
datasheets, but parts, and test them.  There's no "easy fix"

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