Transistor as an Amplifier: Common-Emitter Configuration

The common-emitter configuration. In a common-emitter (CE) amplifier, the emitter terminal is connected to a common reference point (ground) shared by BOTH the input signal loop (applied between base and emitter) and the output signal loop (taken between collector and emitter) -- the configuration from which Section 9.20's characteristic curves, and Section 9.21's $\beta$, are both defined, and by far the most widely used of the three possible transistor amplifier configurations, mainly because it provides substantial gain in BOTH current and voltage simultaneously.

Circuit and biasing. The transistor is first biased, using a resistor network and the supply $V_{CC}$, so that it sits at a suitable fixed operating point somewhere in the FLAT, active region of its output characteristic (Section 9.20) -- never near cut-off or saturation, both of which would badly distort the amplified signal. The small AC input signal $v_i$ is then superimposed on this fixed DC base bias (typically via a coupling capacitor, so the AC signal passes through but the DC bias point is undisturbed), and the amplified AC output $v_o$ is taken across the collector load resistor $R_C$, again via a coupling capacitor.

Why the output is amplified. A small AC rise in the input signal $v_i$ produces a corresponding small rise in the base current $I_B$ (following the input characteristic of Section 9.20); because $I_C = \beta I_B$ in the active region, this produces a MUCH LARGER swing in the collector current $I_C$ (scaled up by the current-gain factor $\beta$, often tens to hundreds); and this larger current swing, flowing through the (comparatively large) collector load resistor $R_C$, produces a correspondingly large swing in the VOLTAGE developed across $R_C$ -- the amplifier's voltage gain (Section 9.23's Numerical worked examples show this explicitly) comes from converting a current gain into a voltage swing across a chosen load resistance. …