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

Thu Apr 7 02:04:37 UTC 2022

Am 2022-04-07 1:37, schrieb Joseph Gwinn:

>> Lately, I've been seeing papers using various microwave pHEMTs but, by
>> the time you find and read the paper, the part is no longer available.
>> And, of course, just like low noise MMICs (PGA-103, GALI-74) you have 
>> to
>> measure them yourself to find out - because the mfr only measures from
>> 50 MHz and up.
> 
> It's a big problem.  Nor do they specify DC parameters all that well.
> 
> 
>> A good example is the 2018 paper by Chen, et al. which references the
>> ATF54143 - a 3 year old paper, and the part isn't available any more.
>> The 2SK117 shows up a lot in some older articles and app notes (e.g.
>> from Wenzel) - it's discontinued, but potentially available from some
>> surplus/obsolete dealers.

>> There is a list in The Art of Electronics, but some of them won't be
>> available.   Some datasheets do have the curve - the JFE150 from TI 
>> has
>> its voltage noise curve right on the front page.
> 
> What those folk are currently using for capacitance multipliers and
> the like (where low 1/f noise is also essential) are SiGe transistors
> like the following:  BFP640H (Infineon), BFP780, SAV541
> (MiniCircuits), and 2S2114K (NPN, beta 1200) for high current

and 2SD2704 from ROHM, even more beta

Those SiGe transistors have wonderful low Rbb of just a few Ohms,
which results in nice low voltage noise, but some have 1/f corners
of 50 MHz or more; that kills my application completely.

My application was a base band low noise amplifier and I wanted
a C-multiplier to suppress the noise on Vcc that is fed into
the input Cascode. That noise comes from a LT3042 regulator:
2nV/rtHz in the flat part, but it rises worse than 1/f until
you get the full broadside of Rset = 13K7 here. Increasing the capacitor
over Rset to 47uF or even 100 uF tantalum removed that for
my requirements. The amplifier stays below 1nV/rtHz until
at least <5 Hz.

The amplifier has 16 CPH3910 FETs in parallel (On Semi), the purple
line is the noise of a 60 Ohm resistor or 1 nV/rtHz. The horizontal
part of the purple line should be at -180 dBV, gain was not yet tuned
when the picture was made.

The line with the input shorted is 10 dB lower, so the amplifier's
own voltage noise is abt. 320 pV/rtHz. I don't think that JFETs
can do much better, including the overpriced JFE150.

Having large organic electrolytics across Vcc (Panasonic SEPF 1000u/16)
seems to worsen things at low frequencies. (orange trace)
I had that result also with other amplifiers. I think that from time
to time, a bunch of electrons defects through the capacitor, creating
some popcorn/telegraph noise.

Seeing the effect of a 60 Ohm resistor at the input of a low noise
amplifier also makes it clear why 17 dBm+ mixers give bad results
when used as phase detectors. The mixer literature shows that most
17 dBm+ mixers have a resistor in series to each diode to produce
back bias for the other branch, only bypassed for RF.
Probably an array of low level mixers with power dividers and the
outputs summed up fares better as low noise phase detector.

The NIST 2N2222 ring mixer does not have these resistors, contrarily
the transistors are used as switches and not as real diodes, which
keeps their impedance low. Ok, the diode noise is only half-thermal.
But the resistor noise is not.

I think I'll have to set up a series of 1/f noise measurements for AF
and RF parts. The pre-amp puts me in a position to see it in the base
band directly.

> applications.  And the best choices do keep going obsolete.  These
> have GHz gain-bandwidth products, and want to oscillate, so some base
> or gate resistance (often a ferrite bead) is necessary.
> 
> There would be two applications in a low-noise oscillator.  One would
> be as a capacitance multiplier to filter the Vcc provided to the
> oscillator circuit, giving considerable PSRR.  The other would be in
> the oscillator circuit itself.

+1

cheers,
Gerhard
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