Q7Analog Electronics

An NPN transistor has two N-type semiconductor regions (emitter and collector) sandwiching a thin P-type base. For amplification, the emitter-base junction is forward biased and the collector-base junction is reverse biased (active region operation). The emitter injects electrons into the base; since the base is very thin and lightly doped, most electrons (95–99%) diffuse through and are collected by the reverse-biased collector junction. A small base current $I_B$ controls a much larger collector current $I_C$:

A small AC signal applied to the base causes large variations in $I_C$, which when passed through a collector resistance $R_C$ produces a large voltage swing. This constitutes voltage amplification.

The input characteristics of CE configuration plot base current $I_B$ versus base-emitter voltage $V_{BE}$, keeping $V_{CE}$ constant. Key observations:

• The curve resembles a forward-biased diode characteristic.
• Cut-in voltage for Si transistor: $V_{BE}\approx 0.6\text{V}$.
• As $V_{CE}$ increases (for same $V_{BE}$), the depletion width of CB junction widens, reducing effective base width (Early effect), causing slight left-shift of the curve.
• Input resistance: $r_{be}=\Delta V_{BE}/\Delta I_B$ at constant $V_{CE}$.

The output characteristics plot $I_C$ versus $V_{CE}$ for different values of $I_B$. Three distinct regions are visible:

• Saturation region ($V_{CE}<V_{CE(sat)}\approx 0.2\text{V}$): Both junctions forward biased. $I_C$ rises steeply with $V_{CE}$.
• Active region ($V_{CE}>V_{CE(sat)}$): CB junction reverse biased, EB forward biased. $I_C\approx \beta I_B$ is nearly constant — this is the amplification region.
• Cutoff region ($I_B=0$, $V_{CE}$ large): Both junctions reverse biased; only leakage current $I_{CEO}$ flows.