Transistor as an Amplifier

A transistor biased in its active region can amplify a weak input signal -- amplification means increasing a signal's amplitude/strength. Where large amplification is needed, several transistor stages are cascaded together with coupling elements (resistors, capacitors, transformers) between them, called a multistage amplifier; the chapter develops the physics using a single-stage common-emitter amplifier. In such a circuit, the Q-point is first fixed near the middle of the DC load line so the output can swing maximally in either direction without distortion (9.4.5); a load resistor $R_C$ in the collector circuit converts changes in $I_C$ into an output voltage; capacitor $C_1$ couples the AC input signal $V_S$ onto the base while blocking any DC bias from the source; the emitter bypass capacitor $C_E$ gives the amplified AC signal a low-reactance path around the emitter resistor (so the AC gain isn't degraded by DC bias-stabilising resistance); and the coupling capacitor $C_C$ passes the amplified AC signal on to the next stage in a multistage design while again blocking DC. The relations $I_C=\beta I_B$ and (by Kirchhoff's voltage law in the output loop) $V_{CE}=V_{CC}-I_CR_C$ govern the DC operating conditions exactly as in 9.4.5. During the POSITIVE half cycle of the input signal $V_S$, the forward voltage across the emitter-base junction increases, so $I_B$ increases, which increases $I_C$ by a factor $\beta$; the larger $I_C$ increases the voltage drop $I_CR_C$ across the collector resistor, which DECREASES $V_{CE}$ -- so a positive-going input produces a negative-going (amplified) output. During the NEGATIVE half cycle, $V_S$ decreases the forward voltage, so $I_B$ (and $I_C$) decrease, the drop across $R_C$ decreases, and $V_{CE}$ …