Question

Most commonly used bias in BJT amplifier circuit is

• A. fixed bias
• B. self bias
• C. collector to base bias
• D. collector feedback bias

Hint:

Voltage divider bias = most stable and widely used.

Answer 1

The Correct Option is B

Step 1: Explain the self-correcting mechanism of self bias.
Self bias uses an emitter resistor $R_E$. If temperature rises and $I_C$ increases, then $I_E$ increases, so $V_E = I_E R_E$ rises. Since $V_B$ is held fixed by the voltage divider, $V_{BE} = V_B - V_E$ decreases, reducing $I_B$ and pulling $I_C$ back down. This is automatic negative feedback.

Step 2: Compare stability factors.
Fixed bias has $S = 1+\beta$ (worst). Self bias with a stiff divider has $S \approx 1$ when $\beta R_E \gg R_{th}$. This is the best achievable stability with a simple resistor network.

Step 3: State the practical conclusion.
Because self bias provides a stable Q-point that is nearly independent of $\beta$ and temperature, it is adopted in virtually all practical BJT amplifier circuits. \[ \boxed{\text{Self bias}} \]