this is my "scratch pad" simulation of a BJT with -2mv/C (-200mv/100C ?), and a diode with +50ppm/C (for vbr around 6.4v = + approx 32mv over +100C ?)
I think there should be a curve, but I do not know what the curve should look like, so I made it more linear
the BJT tempco parameter appears to be XTB (in my case, its -2.3, used on a 2N2222)
the diode tempco parameter are TRS1 and TRS2 (in my case, its TRS1 approx= -0.255, TRS2 1n)
TRS2 controls the quadratic curve fit.
TRS1 controls the linear slope fit.
the original BJT 2N2222 with XTB =1 default does not include a XTB parameter, it appear to fit around -1.5mv/C. if this is default, then all default BJT spice models are not "fitting" 2mv/C. maybe I am missing some other theory, else then, what I found here should be true? (I tried with a mmbt3904, it is nearer to 2mv/C than 2N2222 in fit, mmbt3904 XTB = 1.5 by default model, not 1)
the above require user to edit the dio and bjt files in "cmp" directory.
putting the 2 back into the same .op run from 25C to 125C, produces something that looks like -600ppm/C overall ref circuit, no heater of course. but was it suppose to be -600ppm? is this right?
by modifying the TRS1 to around -0.155. I could then get the bowed curve in the 2nd picture. the curve then becomes manipulated by a pre bias resistor {rt}. I think this part is discussed in the LTZ pdf, unheated compensation? I have seen some online docus talking about this, the curve is not suppose to be bowed but a "s" curve (^3) .
Im not sure if the BJT should be constant current fed or constant voltage
so maybe someone else can improve the model further with a known curve fit.