# Large signal current gain vs small signal current gain

1. ### hhi.rabbit

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large signal current gain hFE = (IC - ICBO)/(IB + ICBO)

and small signal current gain hfe = IC/IB

1. what is the difference between these two.
2. why cant 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

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

Last edited: Aug 17, 2013

### Staff: Mentor

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.

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.

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

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thanks