A circuit with a fixed emitter current is called

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Emitter bias

Explanation: An emitter bias circuit is a BJT biasing configuration that utilizes both positive and negative supply voltages to maintain a nearly constant emitter current. By placing a large resistor in the emitter leg, the circuit provides negative feedback, which stabilizes the operating point ($Q$-point) against variations in transistor parameters.

Working Principle: The principle relies on Kirchhoff's Voltage Law applied to the base-emitter loop. Since the emitter voltage is primarily dictated by the negative supply ($-V_{EE}$) and the emitter resistor ($R_E$), the current $I_E$ remains relatively independent of the transistor's current gain $\beta$. This negative feedback mechanism makes the circuit significantly more stable than simple base bias.

Key Formulas: $I_E \approx \frac{V_{EE} - V_{BE}}{R_E}$ — Emitter current equation; $I_C \approx I_E = \beta I_B$ — Collector current relation

Key Points: Uses a dual-power supply to provide bias stability.; The emitter resistor $R_E$ introduces negative feedback to stabilize $I_E$.; Independent of $\beta$ variations to a large extent.; Commonly used in differential amplifier stages.

Advantages: High thermal stability of the $Q$-point; Independent of the transistor's $\beta$ parameter; Simple circuit configuration requiring few components

Disadvantages: Requires a dual-polarity power supply ($+V_{CC}$ and $-V_{EE}$); Higher power dissipation across the emitter resistor

Applications: Differential amplifiers; Current source circuits; DC biasing in discrete amplifier stages

Additional Info: In a fixed emitter current circuit, the emitter resistor $R_E$ serves to swamp out the effect of the base-emitter voltage drop $V_{BE}$.. Option A is incorrect because base bias is highly dependent on $\beta$. Option C is too generic, and Option D refers to the dual supply requirement, but the naming convention for the topology is 'Emitter bias'.

Answer: B is correct — An emitter bias circuit utilizes an emitter resistor and dual supplies to ensure the emitter current is held at a fixed, stable value regardless of BJT parameter fluctuations.

Exam Tip: When analyzing bias circuits, always check the emitter leg; if there is a resistor present, it almost certainly indicates a design intended to stabilize current against $\beta$ variations.

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