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

[Lux, Jim]

On 9/26/22 5:28 AM, Attila Kinali via time-nuts wrote:
> 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.


What has been the list's experience with noise models in SPICE for ICs 
(e.g. Analog Devices provides SPICE models for low noise op-amps)

We've been hand calculating using the datasheet values, and then 
measuring in the circuit, and as expected, we get comparable 
measurements (perhaps a bit better than data sheet performance).