[time-nuts] Re: Collector current that minimizes BJT noise

[Attila Kinali]

Moin, moin,

On Sun, 18 Sep 2022 12:28:36 -0700
Matt Huszagh via time-nuts <time-nuts at lists.febo.com> wrote:

> This raises the more general question: how do people find the minimum
> noise operating conditions for BJTs? Do you go straight to measurement,
> or do you first attempt to estimate it from datasheet and SPICE values?
> If so, can you shed some light on this process? I've attempted, for
> instance, to estimate rbb from SPICE models (using the equations above)
> and have had worse than bad success equating these to various measured
> results (from Art of Electronics and various papers). It's worth
> mentioning that measured results are also wildly different between
> sources, so this whole thing may be an exercise in futility anyway.

I would like to expand here a bit on what other people wrote:

Spice models, especially of older transistors, were designed to
meet the datasheet. If you are lucky, someone measured the
four-quadrant graph paramters of the transistor (instead of looking
at the datasheet) and adjusted the model to match those values. 
This means that the Spice model does not match the physical realities
of the transistor but rather matches the measured parameters. That
works well for DC simulations, but only approximately for AC and
(fast) transient simulations. Noise simulations are derived from
those DC parameters, but because nobody measures noise parameters,
the spice model is not checked for accurate noise modeling. 
I.e. it usually does not match the device's actual behaviour.

Which means, if you want to use Spice for noise modeling, then you
have to measure the transistor parameters yourself and adjust the
model until it matches those. 

This also gets compounded by the fact that the noise simulation
system in Spice is rather crude and ignores many factors that come
into play.

Thus it's usually more accurate to take some measured noise parameters
of transistors (e.g. Art of Electronics 3rd ed) and then calculate
the noise through the system by hand.

I would also recommend to look at [1] instead of Motchenbacher, as it
is not only more up to date, but quite a bit more extensive in its
coverage of noise modeling of devices.

Additionally, in my experience, the intrinsic noise of the transistor
can be ignored in most applications where a low noise architecture/device
is being used. The up and down-conversion of noise due to non-linearities
are much more of a problem than shot noise and Rbb noise, which is usually
lower than the input noise in our type of applications. In particular flicker
noise performance is, due to above mentioned up/down conversion, directly
connected to the amplifier's IP2 performance. Which gets especially important
when you are using multi-stage amplifiers.

If flicker noise is a limiting factor in your application, then the way to
go is to use a system that actively stabilizes the collector current.
Using a push-pull architecture where even harmonics cancel out (gives usually
between 6 and 20dB of even harmonic cancelation, more when transistors are
matched) also minimizes flicker noise.

If you are using a differntial pair or similar architecture, ensure that
the base voltage bias is adjusted for each branch individually. A good
substitute for measuring collector current is the output DC bias, which
should be zero for two identical transistors. I.e. you can use that to
steer the base voltage of one transistor to minimize output DC bias
and thus even mode harmonics.

And last but not least: Don't trust any model you have not verified.
I.e. after you've done your calulations and simulations, build the
device and see whether it matches what you expect. Then build a second
prototype with slightly different choices and verify that it still
matches the model/calculation and is indeed worse than your first prototype.
That is, unless your first prototype is good enough and you don't
want to optimize it further.

				Attila Kinali


[1] "Electronic Noise and Interfering Signals", by Vasilescu, 1st ed, 2005

-- 
In science if you know what you are doing you should not be doing it.
In engineering if you do not know what you are doing you should not be doing it.
        -- Richard W. Hamming, The Art of Doing Science and Engineering