BJT beta model in LTspice

[bio_man]

Hi,

Beta of BJT is Low when Ic is low. Then, it increases as Ic increases but after some Ic value, Beta starts to fall down. why is that?

I tried to simulate this in LTspice using 2N3904 BJT by ploting Beta vs Ic, it seems Beta always decreases!!
Any thoughts? it came to my mind that the model of this BJT is not correct, I tried two others but the same curve is found.

 

[dick_freebird]

Low current rolloff of hFE vs Ic can be fitted by NE or the C2
(ratio of ISE to IS, dialect dependent) model params. I prefer
the C2 because NE is very "touchy" especially when stacked
on top of Monte Carlo variation there.

High current rolloff is IK, knee current.

Run some .modif loops across ranges and pick the best fit by
eyeball, is my approach. Edit the model files to add scaling
multipliers to the model params that matter, loop and pick.

 

[bio_man]

would you please elaborate more on C2 and NE?

what do you mean by .modif loops? how can I implement them in Ltspice?

and many thanks for your reply