BJT Emitter Bias Current

Formula

Description

In the standard voltage divider bias configuration, the collector current is approximately determined by the base voltage (set by the divider) minus the base-emitter junction voltage (≈ 0.7 V for silicon), divided by the emitter resistor. This assumes β is large enough that Ic ≈ Ie, and the divider is stiff (divider current >> base current). This bias technique provides excellent stability against β variation and temperature changes, which is why it is the most common BJT bias configuration in discrete transistor circuits.

Variables

• Ic — Collector current (A)
• V_B — Base voltage from voltage divider (V)
• R_E — Emitter resistor (Ω)

Practical Notes

V_BE varies with temperature at about −2 mV/°C, so Ic will increase slightly with temperature. For germanium transistors, V_BE ≈ 0.3 V. For the bias to be stable, the voltage divider should be designed so that the divider current is at least 10× the base current: (R1‖R2) < 0.1 × β × RE. The collector resistor RC determines the voltage gain: Av ≈ −RC/RE (with emitter bypass cap: Av ≈ −RC×gm).