Bipolar Junction Transistors and Amplifiers
4. Bipolar Junction Transistors and Amplifiers
This domain tests the structure and operation of NPN and PNP transistors, the current relations, biasing, and the action of a common-emitter amplifier.
Core concepts
- Two junctions give three regions: emitter (heavily doped), base (thin, lightly doped), collector (largest).
- In active mode the emitter-base junction is forward biased and the collector-base junction reverse biased.
- Current relations: I_E = I_C + I_B; α = I_C/I_E (about 0.95–0.99); β = I_C/I_B (typically 50–300); β = α/(1−α) and α = β/(β+1).
- Configurations: common base (voltage gain, no current gain), common emitter (current and voltage gain, phase inversion), common collector or emitter follower (voltage gain ≈1, high input impedance).
- The DC load line joins V_CE = V_CC (cut-off) to I_C = V_CC/R_C (saturation); the Q point should sit near the middle for class A operation.
- Coupling capacitors pass AC and block DC; a bypass capacitor raises AC gain.
Worked example
A transistor has β = 100 and base current 20 µA. I_C = βI_B = 100 × 20 µA = 2 mA. I_E = I_C + I_B = 2.02 mA. α = I_C/I_E = 2/2.02 = 0.990.
Traps
- Interchanging α and β, or treating β as fixed when it varies with temperature and current.
- Forgetting that a CE stage inverts the signal (180° phase shift).
- Assuming the transistor amplifies when it is saturated or cut off - it then acts as a switch.
- Placing the Q point so the output clips on one half-cycle.