On 14.04.22 15:53, Dan Kemppainen wrote:
On 4/14/2022 3:30 AM, time-nuts-requ...@lists.febo.com wrote:
I wouldn't go as far as saying that we KNOW EXACTLY how to reduce 1/f
noise or all the other unwanted side effects (RTN, leakage currents,
you name it). We do observe however, that in many cases these effect
are somewhat correlated with each other and with a variety of defects.
In many cases, these defects can be annealed (mostly through high
temperatures), but there's you get into conflict with other physical
properties you want to achive. One example: for modern three-digit GHz
fT SiGe HBTs (and also for double-digit GHz devices...), you'll need a
very thin base and a sharp doping profile. Application of high
temperatures for extended periods not only anneals the defects, but
also enhances diffusion of the dopant atoms so the can move along
their concentration gradient. Such spreading of the doping profiles
and germanium contents however is detrimental to their RF performance,
as a wider base and/or less steep doping profiles correlate to lower
fT and fMAX.
Following the discussion on low 1/f noise transistors, has me wondering
how this applies to low 1/f opamps. Obviously, they need to be made with
transistors. There are some opamps which are significantly lower in very
low frequency 1/f noise, at least based on some testing done and
reported on EEVBLOG forum.
Are these opamps designed intentionally to be low 1/f noise (as reported
in the datasheets), or does this just happen due to the nature of making
low drift opamps?
In other words, is the low frequency low 1/f parameter an effect of
making a low drift opamp, or is this an intentional design criteria?
I'd vote for intentional. Ofcourse, once a manufacturer learns that a
specific kind of product out of a specific plant is especially good in
terms of 1/f noise, and reproducibly so, at some point somebody will
come to the conclusion that one could exploit that specific property as
best they can, no matter what the original design intent was. After all
ultralow noise also lends itself as marketing figure. And this
subsequent design inevitably will follow intentional design criteria.
How much of the 1/f is driven by thermal issues? That is, on the order
of a second to a few seconds part temperature can change bias points,
which in turn could look like noise. Is this a big consideration in
transistors with Vbe drops (tempco ~2mV/C)? (I'm assuming most of the
discussion on transistors the 1/f corner is above a few Hz for microwave
parts.)
Until now I only actually measured low frequency noise twice where I
intentionally swayed from nominal temperature of 300K. In both cases, -
again talking SiGe RF HBTs - the only part of the noise that
significantly changed with temperature was the white noise, 1/f noise
stayed solidly locked. Due to the withe noise increasing with increasing
temperature, of course, the corner frequency between white and 1/f noise
did change accordingly, but not due to thermally influenced variation of
1/f noise. As HBTs are current-controlled devices, you will have to make
sure to not compare apples with oranges. So you have to choose bias
points with the same currents, which can be a bit tricky unless you
actually force the currents which takes the tempco out of consideration.
If you look really closely, you'll notice that even with constant
current biasing, bias conditions are not identical at two different
temperatures, as also other parameters change, but having the same
collector current usually is close enough. Unless you directly measure
base current noise, then you should aim for identical base current of
course.
In practical application I was chasing a low frequency (Sub Hz) 1/f
noise issue in the tuning voltage control of a microwave VCO. By
switching to very low noise regulators and 1/f opamps in that chain the
issue was greatly reduced. Not having spent a lot of time with low 1/f
noise parts I was quite surprised how much difference there was in the
sub Hz region between different parts. It has me wondering why there's
so much variation between models of opamp.
Maybe I'm mistaken, but I'd think for sub-Hz noise issues, chopper
opamps would be the first thing that comes to mind.
I would suspect one of the main reasons for the huge variation between
parts you observed is due to a combination of process properties and
design choices. If 1/f noise is not considered important for a specific
product, it will not be taken into account during the design, and
definitely will not be tested for in production. With sheer luck you
might end up with a low noise part, but as you're not looking for it you
probably will never know...
Bests,
Florian
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