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

[Joseph Gwinn]

On Thu, 07 Apr 2022 03:30:40 -0400, time-nuts-request at lists.febo.com 
wrote:
time-nuts Digest, Vol 216, Issue 11

> Message: 11
> Date: Thu, 07 Apr 2022 04:04:37 +0200
> From: ghf at hoffmann-hochfrequenz.de
> Subject: [time-nuts] Re: +1/f of transistors
> To: Discussion of precise time and frequency measurement
> 	<time-nuts at lists.febo.com>
> Cc: Joseph Gwinn <joegwinn at comcast.net>
> Message-ID:
> 	<d95bf46305ac13922e952df5977c9420 at hoffmann-hochfrequenz.de>
> Content-Type: multipart/mixed;
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> 
> 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

Interesting.  I would not have thought of this one.  It's big and 
slow, and made from modern very clean silicon, so it could have low 
1/f noise.  Its transition frequency (*= gain-BW product?) is 35 MHz, 
so it ought to work on switcher noise.


> 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.

While these chips are small, they are made from very clean material, 
so one wonders why so high. The circuit should be physically designed 
as if it were to be handling GHz signals, because it could be 
oscillating far above the capability of available instruments to 
detect.

High input path resistance in the GHz won't affect or cause 1/f noise 
near DC.

I would try a lossy ferrite bead on the input.  


> 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.

Regulators are pretty complicated, with many active devices, many in 
loops, and so are always noisier than a one-transistor capacitance 
multiplier with film capacitor.


> 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.

Try film capacitors.  The LIGO crowd did a study of 1/f noise of 
capacitors, and found the best.


> With a capacitance multiplier, one can use only film capacitors.

True.


> 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.

This is likely necessary.

 
>> 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.

Like AMOBEADS.  Or the usual suspects, Murata, Laird, and Wurth.


>> 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

Thanks.

Joe Gwinn