Transistor Characteristics in Common-Emitter Configuration

Two families of curves. A transistor's practical, measurable behaviour in the common-emitter (CE) configuration -- the emitter terminal common to both the input (base) circuit and the output (collector) circuit -- is captured by two distinct families of experimentally plotted curves: the INPUT characteristic and the OUTPUT characteristic.

Input characteristic. The input characteristic plots the base current $I_B$ against the base-emitter voltage $V_{BE}$, with the collector-emitter voltage $V_{CE}$ held fixed. Because the base-emitter junction IS simply a forward-biased p-n junction (Section 9.19), this curve has exactly the same qualitative shape as an ordinary diode's forward characteristic (Section 9.10): $I_B$ stays close to zero for small $V_{BE}$, then rises steeply and nonlinearly once $V_{BE}$ passes a small knee voltage (around $0.6$-$0.7\ \text{V}$ for a silicon transistor). This curve is used to find the transistor's small INPUT RESISTANCE, the ratio of a small change in $V_{BE}$ to the resulting small change in $I_B$, a quantity used directly in the amplifier voltage-gain calculation of Section 9.22.

Output characteristic. The output characteristic plots the collector current $I_C$ against the collector-emitter voltage $V_{CE}$, plotted as a FAMILY of separate curves -- one curve for each of several fixed values of base current $I_B$. Each individual curve shows two distinct regions: for very small $V_{CE}$, $I_C$ rises steeply and almost linearly with $V_{CE}$ (this is the SATURATION region, described fully as one of the transistor's two switching states in Section 9.23); then, past a fairly small $V_{CE}$, each curve bends and runs nearly FLAT and HORIZONTAL over most of the remaining graph (the ACTIVE region), showing $I_C$ depends almost entirely on $I_B$ (a higher curve for a larger $I_B$) and only very weakly on $V_{CE}$ itself. …