# Why is alpha dc when calculated from beta dfifferent from alpha calculated from Ic&Ie

1. Nov 10, 2012

### zee824

i have been asked in my lab report to calculate alpha in a bjt common emmitter circuit using 2 different formulas and explain why one is misleading.

β=Ic/Ib= 537.1*10^-6/6.743*10^-6≈85
α= Ic/Ie=573.1*10^-6/566.357*^-6≈1.011
α=β/β +1≈0.988
i have worked out the two values of alpha where our beta is roughly 85 however im not sure why alpha when calculated from Ic/Ie is missleading.

i have tried to search on the internet however i have found nothing yet and same with my lecture notes.
any help is very much appreciated and i applogise if this is in the wrong section.
thanks
zee

Last edited: Nov 10, 2012
2. Nov 10, 2012

### tiny-tim

welcome to pf!

hi zee! welcome to pf!
it's asking why (Ic/Ib)/((Ic/Ib)+1) = Ic/(Ic + Ib) is not equal to Ic/Ie

3. Nov 10, 2012

### zee824

Re: why is alpha dc when calculated from beta dfifferent from alpha calculated from I

hi thanks for the quick reply tiny-tim however in this case they are infact equal since ic+ib=ie but im still getting different values. i feel it is something to do with the capacitors or the configuration of the resistors that is causing a slight error but im not sure and im even less sure about why this is misleading

Last edited: Nov 10, 2012