Input Characteristics

The input characteristic plots the relationship between base current $I_B$ and base-to-emitter voltage $V_{BE}$, measured at several fixed values of collector-to-emitter voltage $V_{CE}$: $V_{CE}$ is first set to a chosen value (above about 0.7 V, to keep the collector-base junction reverse biased), then $V_{BE}$ is stepped up and the resulting $I_B$ recorded at each step, and the whole sweep is repeated for other values of $V_{CE}$. Two observations follow from the resulting family of curves: first, the curve resembles the forward characteristic of an ordinary p-n junction diode (unsurprising, since the base-emitter junction IS a forward-biased diode), showing a threshold/knee voltage $V_K$ (0.7 V silicon, 0.3 V germanium) below which $I_B$ is very small, beyond which $I_B$ rises rapidly with $V_{BE}$; second, increasing $V_{CE}$ shifts the whole curve slightly OUTWARD (i.e. a given $I_B$ now needs a slightly larger $V_{BE}$), because a larger $V_{CE}$ widens the collector-base depletion region, which in turn narrows the effective base width and reduces the base current for the same $V_{BE}$. The input impedance $r_i$ is defined as the ratio of a small change in $V_{BE}$ to the resulting small change in $I_B$, at fixed $V_{CE}$: $r …