BJT Emitter-Bias Current

Ic ≈ (Vb − Vbe) / Re

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Result
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Formula

Ic ≈ Ie = (Vb − Vbe) / Re

Description

In a voltage-divider-biased transistor stage, the emitter current is set by the base voltage minus the base-emitter drop, divided by the emitter resistor. Because this depends on resistor ratios and the stable 0.7 V junction drop rather than on the poorly-controlled beta, it gives a predictable, temperature-stable operating point—the standard way to bias discrete amplifiers.

Variables

  • Ic — Collector (≈ emitter) current (A)
  • Vb — Voltage at the base node (V)
  • Vbe — Base-emitter drop (~0.7 V)
  • Re — Emitter resistor (Ω)

Practical Notes

Set Vb with the R1/R2 divider, then the emitter resistor fixes the current. This negative feedback through Re makes the bias point nearly independent of β, the key to a robust design.

Related Concepts

All Discrete Amplifiers formulas →
Thermal Voltage Small-Signal Emitter Resistance Transistor Transconductance Diode Dynamic Resistance Common-Emitter Voltage Gain BJT Alpha–Beta Relationship Darlington Pair Current Gain Emitter-Follower Input Impedance

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