# Emitter follower augmented input current

Hello, I have this emitter follower augmented ( I think that's his name in english) but I don't know from where is the beta+1 on the Iin ecuation?

• You're asking why $I_{B3} = \frac{I_{E3}}{\beta+1}$? And "Q3 is in forward active mode" is not enough explanation? Sep 28 '17 at 17:31
• Yes that's what I'm asking. Edit: I looked into that and it explains why. Sep 28 '17 at 17:33
• Your circuit is a current-mirror with beta helper not an emitter follower Sep 28 '17 at 17:40

Q3 is in forward active mode because the base-emitter junction is forward biased and the base-collector junction is reverse biased.

In this mode we have the very well known characteristic $$I_c = \beta I_b .$$

Since the current coming out of the emitter is (by KCL) the sum of the base and collector currents,

$$I_e = I_b + I_c = I_b + \beta I_b$$

and thus

$$I_b = \frac{I_e}{1+\beta}.$$

(Note: circuit theory purists would normally take $I_e$ to mean the current going in to the emitter, so there'd be some sign changes in the equations above)