On 22/02/2021 18:01, Jim Wagner wrote:
I wonder if there is a way to estimate any of the first order bjt parameters, such has dc beta or Ft for a model? Yes, a test bench could be created but I was wondering if one could read the model and do some basic mental arithmetic that would estimate how the model behaves. I am thinking, for example, of the bog-common 2N3904 which has a huge range of beta in real life. Knowing what beta the model maker chose for the model would help in doing the correlation between simulation behavior and bench behavior.
The two parameters (beta and Ft) you've chosen have no fixed value; they both vary with Ic, Vce and temperature as well as sample to sample. If you mean BF and not beta, then that's a different matter. Beta is the real-world value of Ic/Ib, which varies as described above. BF, on the other hand is a model parameter and fixed, but that doesn't necessarily make it equal to beta, as the "simulated" beta is affected by a host of other model parameters, like IKF, NK, IS and NF that modify how "beta" varies under different conditions. BF might equal the peak value of beta for some devices, but it might not for others. There is no one model parameter that defines Ft, although it is affected by TF. However, it is also affected by Ic and Cbe, which are also not model parameters but a physical ones.
But are you asking how model creators decide on what constitutes a typical device sample to base the model on? Or are you saying you want to minimally modify the default BJT to make it have a passing resemblance to a particular real device? It sounds a bit "hand-wavy".
--
Regards,
Tony
Tony Casey