# Why is the input impedance of an emitter follower defined as $\Delta V_{B}/\Delta I_{B}$, as opposed to $V_{B}/I_{B}$?

In The Art of Electronics, chapter $$\2\$$:Transistors, page $$\65\$$, the author defined the input impedance $$\r_{in}\$$ of the emitter follower below

as $$\r_{in}=\Delta V_{B}/\Delta I_{B}\$$. Why did he use variations of $$\V_{B}\$$ and $$\I_{B}\$$?

I thought the input impedance was defined as the input voltage $$\V_{in}\$$over the input current $$\I_{in}\$$, what am I missing?

• What if 'v' is an ac signal ..... Nov 27 '19 at 16:59

Often the differential impedance ($$\\frac{dV}{dI}\$$) is more important than the crude resistance ($$\V/I\$$).