On 4/14/2022 3:30 AM, [email protected] 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?
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.)
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.
Dan
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