🚀 go-pugleaf

You forgot something in yuour analysis: THE SATURATION CURRENT Is IS
TEMPERATURE DEPENDENT. I have my copy of H&H (1st ed.) at home but I
checked in Sedra&Smith "Micr. Circuits" (4th ed.), page 126 where is
said that Is doubles for each 5 C degree rise in temperature.

Moreover, the change in Vbe (-2.1mV/degC) is supposed to occur at
CONSTANT emitter current, which is not the case. In the Common
emitter-grounded amplifier you have

Vbe=R_2 V_CC / (R_1 + R_2)

supposing that I_B<< than the current in the resistors. Now you have

I_C=I_S exp(Vbe/Vt)   (I_S and Vt known at 300 K)

if at 300 K you increase the temperature to, lets say, 305 K, the
value of Vt increases 1.67% relative to the 300 K value. However,
I_S DOUBLES (according to Sedra&Smith) and you have:

I_C = 2 I_S exp(Vbe/1.0167 Vt)  (I_S and Vt are the 300 K value)

The Vbe is CONSTANTLY BIASED if Ib << than the current in the
resistors and so you notice that, considering at 300 K that Vbe=0.65 V
and that Vt=25mV

 I_C (305 K) / I_C (300 K) = 1.3 

Raising in fact the Ic current. As you have seen, the key facts are
the large increase in Is and the fact that the Vbe value remains
approximately constant.


	J. Augusto