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

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On 14.04.22 15:53, Dan Kemppainen wrote:
> 
> 
> On 4/14/2022 3:30 AM, time-nuts-request at 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