Output Characteristics

The output characteristic plots the collector current $I_C$ against the collector-to-emitter voltage $V_{CE}$, at several fixed values of base current $I_B$: $I_B$ is first set to a chosen value, then $V_{CE}$ is stepped up and the resulting $I_C$ recorded, and the whole sweep is repeated for several different values of $I_B$, giving a family of curves. Four important regions are read off this family. (i) SATURATION region: as $V_{CE}$ rises above 0 V, $I_C$ rises rapidly to an almost-$I_B$-independent value in a nearly ohmic region (labelled OA on the curve) up to the so-called knee voltage; transistors are always operated ABOVE this knee voltage in normal (non-switch) use. (ii) CUT-OFF region: even after $I_B$ is reduced all the way to zero, a small collector current still exists, due to minority carriers crossing the collector-base junction plus a surface leakage current $I_{CEO}$ -- this is the cut-off region, so called because the main (base-current-driven) collector current is effectively cut off. (iii) ACTIVE region: here the emitter-base junction is forward biased and collector-base junction reverse biased (exactly the forward-active bias condition of 9.4.2), and $I_C$ is nearly flat/independent of $V_{CE}$ for a given $I_B$ -- this is the region used for voltage, current and power amplification. (iv) BREAKDOWN region: if $V_{CE}$ is pushed beyond the manufacturer's rated value, $I_C$ rises enormously and causes junction breakdown, potentially damaging the transistor. The output impedance $r_o$ is defined as the ratio of a small change in $V_{CE}$ to the resulting small change in $I_C$, at fixed $I_B$: $r_o=(\Delta V_{CE}/\Delta I_C)_{I_B}$; unlike the input impedance, the output impedance for a common-emitter transistor is very LOW (the active-region curves are nearly flat, i.e. large $\Delta I_C$ for small $\Delta V_{CE}$ is NOT what low output impedance means -- rather it is that the curve is nearly flat, so $\Delta I_C$ is small even for a sizeable swing in $V_{CE}$, giving a large $\Delta V_{CE}/\Delta I_C$... note the text states the output impedance is low, consistent with treating the active-region slope $\Delta I_C/\Delta V_{CE}$ itself a …