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

[Dan Kemppainen]

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?

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