In a self-bias (emitter-bias) BJT amplifier stage, what is the primary advantage provided by the self-bias network with respect to the operating point (Q-point)?

Difficulty: Easy

It provides a stable Q point over device and temperature variations
It guarantees very large voltage gain regardless of load
It ensures very high input impedance automatically
It forces a very high base current for strong conduction
It eliminates the need for collector resistors

Correct Answer: It provides a stable Q point over device and temperature variations

Explanation:

Introduction / Context:
Biasing a BJT for linear amplification requires choosing a Q-point that remains reasonably constant despite part-to-part variation and temperature changes. The self-bias (also called emitter-bias with emitter resistor and feedback) is a classic topology to stabilize that operating point using negative feedback from the emitter resistor.

Given Data / Assumptions:

BJT in common-emitter configuration with an emitter resistor providing DC feedback.
Base bias provided by a resistive divider or current source referenced to the emitter line.
Thermal effects increase collector current unless countered by feedback.

Concept / Approach:
As collector current increases (due to temperature or beta variation), the emitter current rises, increasing the voltage across the emitter resistor. This raises the emitter potential, reduces base-emitter voltage V_BE, and thereby counteracts the increase in current. The negative feedback stabilizes both I_C and V_CE, keeping the transistor within the desired linear region.

Step-by-Step Solution:

Assume I_C rises due to temperature.I_E ≈ I_C increases ⇒ V_E = I_E * R_E increases.V_BE = V_B − V_E drops, reducing conduction.New equilibrium Q-point established near the original value.

Verification / Alternative check:

Small-signal analysis shows DC feedback factor proportional to R_E; larger R_E increases stability but reduces gain unless bypassed for AC.

Why Other Options Are Wrong:

Large voltage gain is not guaranteed (R_E without bypass reduces gain).High input impedance depends on bias network and transistor; it is not guaranteed by self-bias alone.Forcing high base current is not an objective; in fact, the network seeks stable, not excessive, current.Collector resistors are still required for voltage gain and load line setting.

Common Pitfalls:

Confusing AC bypass (which restores gain) with DC stabilization; forgetting that temperature rise increases I_C unless counteracted.

Final Answer:

It provides a stable Q point over device and temperature variations

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In a self-bias (emitter-bias) BJT amplifier stage, what is the primary advantage provided by the self-bias network with respect to the operating point (Q-point)?

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