Large signal current gain vs small signal current gain

[hhi.rabbit]

large signal current gain h_FE = (I_C - I_CBO)/(I_B + I_CBO)

and small signal current gain h_fe = I_C/I_B

1. what is the difference between these two.
2. why can't we use large signal current gain in small signal currents??
3. i have these notes
on page 57 (page 1 on pdf) paragraph 2 of these (please go through the link)

Notes

A comparison of the simple model
with the iv characteristics of the
BJT shows that our simple largesignal model is very crude and is
not accurate for AC analysis.
The simple model cannot be used to calculate these changes
(It assume vBE is constant!).

how come we have V_BE constant in large signal models??

 

[NascentOxygen]

The large signal model is what you use for biasing arrangement, it is non-linear. The small signal model is a linearized approximation around an operating point, and being linear it is much easier to analyze. Mostly, it gives results which agree closely with an actual transistor, so that justifies the model.

2. why can't we use large signal current gain in small signal currents??

Because the calculations won't agree with what you measure on the actual transistor. The closer the model agrees with reality, the better it is and the more accurately we can design circuits.

how come we have VBE constant in large signal models??

I think you are referring to V_BE = 0.6v, well, it is a pn junction voltage drop!

 

[hhi.rabbit]

thanks