operating principles

operating principles

1. What Operating Principles Mean (BJT Operation in Three Modes)

Operating principles describe carrier flow for each bias combination:

Mode	EB Junction	CB Junction	Behaviour
Active (amplifier)	Forward	Reverse	$I_C = β I_B$, controlled amplification
Saturation (switch ON)	Forward	Forward	$V_{CE,sat}≈0.2V$, both junctions forward, $I_C$ max, $β$ collapses
Cutoff (switch OFF)	Reverse	Reverse	$I_C≈0$ (only leakage $I_{CEO}$)
Reverse active (inverted)	Reverse	Forward	$β$ low, rarely used

2. Active Mode Detailed (NPN)

• EB forward: $V_{BE}≈0.7V$ lowers barrier, electrons injected from emitter (n+) into base (p). Since $N_E≫N_B$, injection dominated by electrons.
• In base: injected electrons are minority in p-base, diffuse toward collector. Base thin and lightly doped ⇒ most (≈99%) reach collector junction without recombining (which would create $I_B$ recombination component).
• CB reverse: collector-base depletion field (positive $V_{CB}$) sweeps arriving electrons into collector ⇒ collector current $I_C$ flows, essentially independent of $V_{CE}$ (output resistance high). Small fraction recombines in base ⇒ base current $I_B$ (needed to supply holes for recombination).
• Relation:

PNP: holes from emitter p to base n, same but currents reversed.

3. Why $β$ Large

with

where $γ$ = emitter injection efficiency (~0.99), $α_T$ = base transport (~0.99), $M≈1$ ⇒ $α≈0.98$ ⇒ $β≈50$.

4. Worked Checks

(a) NPN active: V_BE=0.7V, I_B=20µA, β=100 ⇒ I_C=2mA, I_E=2.02mA ✓

(b) Need to saturate? Increase I_B until V_CE≈0.2V, both junctions forward ⇒ I_C no longer =β I_B (limited by RC).

(c) Cutoff: V_BE<0.5V ⇒ EB reverse, CB reverse ⇒ I_C≈ I_CBO (nA) ✓

(d) Why base current small? Only recombination + back-injection ⇒ few µA.

5. Exam Points

• List four modes (active/saturation/cutoff/reverse) with bias — 5 marks, very frequent.
• Explain active: EB forward injection, base diffusion, CB reverse sweep — 7-10 marks, very frequent (the most common operating principles question).
• Show $I_C=β I_B$, $I_E=I_C+I_B$ — 3-5 marks, frequent.
• Mention $V_{BE}≈0.7V$, $V_{CE,sat}≈0.2V$ — 3 marks.