BJT npn transistor capacitor

[intensenex]

i need help in calculating charging time for the capacitor.

From what i know the general equation for capacitor charging time is:
V=Vs(1- exp (t/RC) )
so T= RC-ln(1-(V/Vs))

now i need V = 0.989V and i think Vs =1.7-0.7=1 V
but multisim says about T= 1.15 ms (when using oscilloscope across capacitor)
and i get about 40 ms.

i have
ln(1-(V/Vs)) =4.605
i get Ie=11mA
and R=(8900+(0.7/Ie))= 8900+62.5

i dont understand how i am not gettin it right

If R is about 300 ohms i get close to the 1.3 ms but according to common emitter transistor model how can i get that low resistance

 

[Ratchit]

intensenex,

From what i know the general equation for capacitor charging time is:
V=Vs(1- exp (t/RC) )

So if t is a long time, the voltage will approach a very high negative value in the above equation?

Ratch

 

[crutschow]

You missed a minus sign in front of the t.

 

[intensenex]

Srry like crutschow siad i missed the negative sign infront of T so exp(of negative infinity) is 0.

V=Vs(1- exp (t/RC) )