Input and Output characteristics
Determination of
(a) Input resistance
(b) Current amplification factor
The input and output characteristics, of $\mathit{n}-\mathit{p}-\mathit{n}$ transistor, in its CE configuration, are as shown.

(a) Typical input characteristics Input resistance
${\mathit{r}}_{\mathit{i}}={\left(\frac{∆{\mathit{V}}_{\mathit{B}\mathit{E}}}{∆{\mathit{I}}_{\mathit{B}}}\right)}_{{\mathit{V}}_{\mathrm{CE}}}$

(b) Typical output characteristics
The relevant values can be read from the input characteristics.
(b) Current amplification factor
$\mathit{\beta }=\left(\frac{∆{\mathit{I}}_{\mathit{C}}}{∆{\mathit{I}}_{\mathit{B}}}\right)$
The relevant values can be read from the output characteristics, corresponding to a given value of ${\mathit{V}}_{\mathit{C}\mathit{E}}$.