Difficulty: Easy
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:
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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