Transistor (NPN, CE)
I_C = β · I_B — tune V_BB through cutoff, active, and saturation regions.
Key Notes
A bipolar junction transistor (BJT) is a 3-terminal device with two p-n junctions: NPN or PNP. Terminals: Emitter (E), Base (B), Collector (C).
NPN transistor in 'active' mode: E-B junction forward-biased, C-B junction reverse-biased.
Emitter (heavily doped) injects electrons into Base (lightly doped, thin); most diffuse across to the Collector (moderately doped).
Small base current I_B controls large collector current I_C — transistor is a CURRENT AMPLIFIER.
Current gain β (or h_FE) = I_C/I_B; typically 50-300. α = I_C/I_E ≈ 0.99.
Three operating regions: (i) Cutoff (both junctions reverse, transistor OFF). (ii) Active (E-B forward, C-B reverse, amplifies). (iii) Saturation (both forward, transistor 'ON' as a switch).
Three common configurations: (i) Common-Emitter — high current AND voltage gain; (ii) Common-Base — voltage gain only; (iii) Common-Collector (emitter follower) — current gain, unity voltage gain, high input impedance.
Applications: amplifiers, switches, oscillators, logic gates. Modern MOSFETs (field-effect) dominate ICs.
Formulas
Current relations
Linked via β = α/(1−α). For α = 0.99: β = 99.
α-β relation
Small change in α → large change in β.
Voltage gain (CE)
Where g_m = I_C/V_T = I_C/(26 mV); negative sign = phase inversion.
Input vs output (CE)
Typical Si BJT values when ON / saturated.
Important Points
BJT current GAIN, voltage gain comes from passive loads.
Common-Emitter (CE) configuration is the most-used amplifier: high A_V, high A_I, but phase inverts.
Saturation: V_CE drops to ~0.2 V, transistor acts as closed switch. Used in digital logic.
Cutoff: I_C ≈ 0, transistor acts as open switch.
β varies widely across transistors of the same part number — design for β = β_min for reliability.
MOSFETs vs BJTs: MOSFETs are voltage-controlled, BJTs current-controlled. MOSFETs win in digital ICs; BJTs in analog low-noise.
Transistor (NPN, CE) notes from sciphylab (also known as SciPhy, SciPhy Lab, SciPhy Labs, Physics Lab). Class 12 physics revision for JEE Mains, JEE Advanced, NEET UG, AP Physics 1/2/C, SAT, and CUET-UG.