The Common Emitter (CE) Characteristic

Using the circuit of Fig. 16.22, two standard characteristic curves are measured for a transistor in the common emitter configuration. The INPUT CHARACTERISTIC (Fig. 16.23) plots the base current $I_B$ against the base-emitter voltage $V_{BE}$, with the collector-emitter voltage $V_{CE}$ held fixed throughout the measurement. As expected for what is, after all, a forward-biased p-n junction, the curve stays close to zero current until $V_{BE}$ exceeds the junction's barrier potential, after which $I_B$ rises steeply, essentially reproducing the shape of an ordinary forward diode characteristic. From this curve, the INPUT DYNAMIC RESISTANCE is defined as $r_i=\left(\dfrac{\Delta V_{BE}}{\Delta I_B}\right)_{V_{CE}\,\text{constant}}$ -- the (small-signal) resistance the base-emitter junction presents to a changing input signal.\n\nThe OUTPUT CHARACTERISTIC (Fig. 16.24) plots the collector current $I_C$ against the collector-emitter voltage $V_{CE}$, this time with the base current $I_B$ held fixed for each individual curve -- different values of $I_B$ give a whole FAMILY of such curves, with larger $I_B$ producing correspondingly larger $I_C$. Within each curve, $I_C$ rises very sharply for small $V_{CE}$ near the origin, but then quickly flattens out into a nearly HORIZONTAL line for larger $V_{CE}$: as long as the collector-base junction stays reverse biased, $I_C$ is essentially INDEPENDENT of $V_{CE}$, and depends instead almost entirely on $I_B$ (via $I_C\approx\beta_{DC}I_B$). From this curve, the OUTPUT DYNAMIC RESISTANCE is defined as $r_o=\left(\dfrac{\Delta V_{CE}}{\Delta I_C}\right)_{I_B\,\text{constant}}$ -- since the output curve is nearly flat, $\Delta I_C$ is small for a given $\Delta V_{CE}$, so $r_o$ works out to …