Bipolar Junction Transistors and Field Effect Transistors
What this domain covers
This domain tests bipolar junction transistors and field-effect transistors: structure, biasing, operating regions and the key relationships between terminal currents and voltages.
Core rules
- BJT terminals: emitter, base, collector. In an NPN device conventional current flows into the base and collector and out of the emitter; PNP is the reverse.
- Current gain: β = I_C/I_B and α = I_C/I_E, with β = α/(1 − α) and α = β/(1 + β).
- Regions: cutoff (both junctions reverse biased, no current), active (base-emitter forward, collector-base reverse, used for amplification), saturation (both forward biased, V_CE ≈ 0.2 V, used as a switch).
- Configurations: common base has low input impedance, current gain below 1 and no phase inversion; common emitter has high voltage gain and 180° phase inversion; common collector (emitter follower) has voltage gain ≈1 and buffers.
- FETs are voltage-controlled: gate-source voltage controls drain current; I_DSS is the saturation current at V_GS = 0, and V_GS(off) is the pinch-off voltage.
Worked example
An NPN transistor has β = 100 and I_B = 20 µA. Then I_C = βI_B = 100 × 20 µA = 2 mA, I_E = I_C + I_B = 2.02 mA, and α = I_C/I_E = 2/2.02 = 0.99.
Traps that cost marks
Using β instead of α in emitter-current calculations; claiming a saturated transistor has a large V_CE; forgetting that a common-emitter stage inverts the signal; and treating a JFET as current-controlled.