Q.Sketch the static characteristics of a common emitter transistor and bring out the essence of input and output characteristics.

Step 1 (Test circuit). All three curves are obtained from one circuit: bias supplies $V_{BB}$ (base loop) and $V_{CC}$ (collector loop) with rheostats to independently vary $I_B$ and $V_{CE}$/$I_C$, and meters to read $I_B$, $I_C$, $V_{BE}$ and $V_{CE}$.

Step 2 (Input characteristics). Plotting $I_B$ against $V_{BE}$ at several fixed $V_{CE}$ gives a curve resembling an ordinary forward diode characteristic, with a knee voltage near 0.7 V (silicon) / 0.3 V (germanium); increasing $V_{CE}$ shifts the curve slightly outward (narrower effective base width reduces $I_B$ for the same $V_{BE}$). Input impedance $r_i=(\Delta V_{BE}/\Delta I_B)_{V_{CE}}$ is HIGH.

Step 3 (Output characteristics). Plotting $I_C$ against $V_{CE}$ at several fixed $I_B$ reveals four regions: SATURATION (near-ohmic rise near $V_{CE}=0$), CUT-OFF ($I_B=0$, small residual leakage $I_{CEO}$), ACTIVE (nearly flat $I_C$, used for amplification, EB forward/CB reverse), and BREAKDOWN (beyond the rated $V_{CE}$, $I_C$ rises destructively). Output impedance $r_o=(\Delta V_{CE}/\Delta I_C)_{I_B}$ is LOW.

Step 4 (Transfer characteristics). Plotting $I_C$ against $I_B$ at fixed $V_{CE}$ gives an approximately straight line through a small intercept ($I_{CEO}$); its slope, $\beta=(\Delta I_C/\Delta I_B)_{V_{CE}}$, is the forward current gain, typically 50-200. …

Describe each of the three characteristic curves, what is held fixed, what …

• Mixing up which quantity is held constant for each curve (input: $V_{CE}$ fixed; output: $I_B$ fixed; transfer: $V_{CE}$ fixed). …