6.1 Structure and Physical Operation

• For amplifier applications, the BJT is operated in the active mode
• Switching applications make use of the cutoff and saturation modes

Modes of Operation

• Depending on the bias conditions on its two junctions, the BJT can operate in one of three possible modes:
  1. Cutoff - both junctions reverse biased
  2. Active - EBJ forward biased and CBJ reverse biased
  3. Saturation - both junctions forward biased
• If the base-emitter voltage ($V_{BE}$) is greater than the base-emitter turn-on voltage (typically about 0.6 to 0.7 V for silicon transistors), the EBJ is forward-biased.
  • If the base-emitter voltage ($V_{BE}$) is less than the turn-on voltage, the EBJ is reverse-biased.
• If the base-collector voltage ($V_{BC}$) is greater than the base-collector turn-on voltage (also about 0.6 to 0.7 V for silicon transistors when forward-biased), the CBJ is forward-biased.
  • If the base-collector voltage ($V_{BC}$) is negative or less than the turn-on voltage, the CBJ is reverse-biased

6.2 IV Characteristics

• A BJT operating in the active mode provides a collector current $i_C = I_se^{\frac{|V_{BE}|}{V_T}}$
• The base current $i_B = i_C’ \beta$
• Emitter current $i_E = i_C + i_B$
• $i_C = \alpha i_E$, and $\beta = \frac{\alpha}{1-\alpha}$ and $\alpha = \frac{\beta}{\beta - 1}$

6.3 Circuits at DC

• To ensure operation in the active mode, the collector voltage of an NPN transistor must be kept higher than approximately 0.4V below the base voltage
• For a PNP transistor, the collector voltage must be lower than approximately 0.4V above the base voltage
• Otherwise, CBJ becomes forward biased, and the transistor enters the saturation region
• At a constant collector current, the magnitude of the base-emitter voltage decreases by about 2mV for every 1 degree C rise in temperature