# How to calculate output resistance for a transistor in common base configuration?

This question more related to my physics practical than a theory question. How do i calculate output resistance if i have the corresponding table for it? I know Ro=deltaVcb/deltaIc. As i can see in the picture after some time deltaIc=0. So does the resistance become infinity? I am also providing my recorded practical data for reference.

In general the current in the collector does NOT depend on the voltage of the collector.This happens because the CB junction reverse biased -> no current should flow.You are asking how transistors work and why collector voltage does NOT(it does change it a little bit but not for the reason you think)change the collector current.

How do i calculate output resistance if i have the corresponding table for it?

There are two output resistances here: 1)total AC, and 2)differential (dynamic) output resistances. The second one causes the incremental variation of the first.

AC output resistance is the total instantaneous resistance for a given output voltage Vcb. Althu this is the parallel equivalent of three components, Rc || Rc-b junction || Rload, the Rc component usually dominates, i.e., AC Ro ~=Rc.

The differential resistance, is the incremental resistance to the AC/total resistance. Meaning, the AC resistance would vary if Vcb is varied, and that variation is accounted by the change in the differential resistance. The eqn you give is for the dynamic differential Ro = delta Vcb / delta Ic.

As i can see in the picture after some time delta Ic=0. So does the resistance become infinity?

For your 2nd question, yes the dynamic output resistance Ro nears infinity as you increases the reverse biasing voltage Vcb. Its effect can be seen from your eqn >> the contribution of differential Ro to delta Ic approaches zero. Or from the current eqn, Ie = Ic + Ib, continuously increasing Vcb would result in a smaller and smaller increment of Ic (and decreasing Ib) but which most classroom lectures would simplify it as flat curve.

Hence, as a more accurate graph (above figure) would show, the output collector current Ic is approaching the value of the input emitter current Ie. That's why, a common base configuration is a current follower amplifier.