Need help with the process for finding input resistance of a

[jalbers@bsu.edu]

I have been trying to follow along with a books calculation for the
input resistance of a transistor which they define as Rin = delta
(Vbe) / delta (Ib) or Rin would be 1 / (slope of the diode curve) or
1 / (derivitive of the diode curve) . Doing the math and assuming
that KT/Q = 26 mv and assuming that (Is) the reverse saturation
current is negligible, I get the desired Rin = 26 mv / Ib.

My question is why is Rin = delta (Vbe) / delta (Ib) and not just
Vbe / Ib . My some how flawed reasoning is that if you look at ohms
law E = IR, then R = I/E and therfore Rin should be Vbe / Ib . Why
is Rin equat to the change in current divided by the change in
voltage?

Any help would be greatly appreciated. Thanks

 

[John Popelish]

jalbers@bsu.edu wrote:

My question is why is Rin = delta (Vbe) / delta (Ib) and not just
Vbe / Ib . My some how flawed reasoning is that if you look at ohms
law E = IR, then R = I/E and therfore Rin should be Vbe / Ib . Why
is Rin equat to the change in current divided by the change in
voltage?

Because the resistance is not ohmic. Ohmic resistances hold
a constant ratio of voltage to current, regardless of the
magnitude of voltage or current. Diode junctions do not.
So resistance can be defined for such devices only
incrementally (over tiny ranges of voltage and current).


--
Regards,

John Popelish