Bipolar Junction Transistors and Biasing
This domain tests BJT structure, current relationships, operating regions and the bias circuits that hold the Q-point steady.
Core concepts
- Two junctions give three regions. The emitter is heavily doped, the base is thin and lightly doped, the collector is physically large to dissipate heat.
- Current relations: I_E = I_B + I_C; α = I_C/I_E; β = I_C/I_B; β = α/(1 − α).
- Active region (BE forward, BC reverse) is used for amplification; saturation (both forward, V_CE ≈ 0.2 V) and cut-off (both reverse) are used for switching.
- I_CBO roughly doubles for every 10 °C rise, so fixed bias is thermally unstable. Voltage-divider bias with an emitter resistor is preferred because the base voltage is set by the resistor ratio, largely independent of β.
- An emitter bypass capacitor restores AC gain without disturbing the DC Q-point.
Worked example
V_CC = 12 V, R1 = 10 kΩ, R2 = 2.2 kΩ, R_E = 1 kΩ, R_C = 2.2 kΩ, large β. V_B = 12 × 2.2/12.2 = 2.16 V. V_E = 2.16 − 0.7 = 1.46 V. I_E = 1.46/1 kΩ = 1.46 mA ≈ I_C. Then V_CE = 12 − 1.46 mA × (1 kΩ + 2.2 kΩ) = 12 − 4.67 = 7.33 V, comfortably in the active region.
Traps
Writing I_C = I_E; using the wrong α–β rearrangement; applying 0.7 V blindly to a germanium device; forgetting that saturation needs a base current larger than I_C/β.