Questions: Bipolar Junction Transistor (BJT) Fundamentals

Question 1

You analyze an NPN BJT circuit assuming active-region operation and find V_BE = 0.7 V and V_BC = +0.4 V. What does this result tell you?

• A. The assumption is confirmed — V_BE = 0.7 V is exactly right for the active region
• B. The assumption is wrong — a forward-biased base-collector junction indicates saturation, not active region
• C. The assumption is wrong — a positive V_BC means the transistor is in cutoff
• D. The assumption is confirmed as long as β > 50

Answer: (B)

In the active region, V_BE ≈ 0.7 V (forward biased) AND V_BC < 0 (reverse biased). A positive V_BC means the base-collector junction is forward biased, which is the defining condition for saturation. You must redo the analysis assuming saturation — set V_CE ≈ 0.2 V (saturation voltage) and solve for the currents from the external circuit, then verify I_B is sufficient to drive the transistor into saturation.

Question 2

In saturation, the collector current I_C equals β times the base current I_B.

• T. True
• F. False

Answer: False

I_C = β·I_B is valid only in the active region. In saturation, V_CE is clamped near 0.2 V by the circuit — the collector current is set by the external collector resistor via KVL (I_C = (V_CC − V_CE(sat)) / R_C), not by β. The base current is driven well beyond what β would require for active operation. If you mistakenly apply I_C = β·I_B in saturation, you will calculate a V_CE that is negative or unrealistically small, which is the signal that the active-region assumption is wrong.

Question 3

Why is voltage-divider bias preferred over simpler fixed-base (single-resistor) bias for BJT amplifier circuits?

Model answer: Fixed-base bias sets I_B directly through a resistor from V_CC, making I_C = β·I_B sensitive to β. Since β varies widely between transistors of the same type (e.g., 50–300) and drifts with temperature, the Q-point is unstable. Voltage-divider bias establishes V_B from a stiff resistor divider independent of β, so V_BE and I_E are stable. I_C is then determined by emitter current through R_E, not by β, making the Q-point robust to device variation.

The goal of biasing is a stable quiescent point (Q-point) that keeps the transistor in the active region across all expected operating conditions. Voltage-divider bias achieves this by making the base voltage β-independent: the divider current is chosen to be much larger than I_B, so the base voltage barely shifts when β changes. The emitter resistor R_E provides additional stability via negative feedback: if I_C tries to rise, V_E rises, reducing V_BE and pulling I_C back down.