Small-Signal Emitter Resistance

re = VT / Ie

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Result
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Formula

re = VT / Ie (≈ 25 mV / Ie)

Description

The intrinsic small-signal emitter resistance of a bipolar transistor is the thermal voltage divided by the DC emitter current. It is not a physical resistor but the dynamic resistance of the base–emitter junction, and it sets the maximum voltage gain of a common-emitter stage (Av = −Rc/re). Higher bias current lowers re and raises gain but also raises power.

Variables

  • re — Small-signal emitter resistance (Ω)
  • VT — Thermal voltage (≈25–26 mV)
  • Ie — DC emitter current (A)

Practical Notes

At 1 mA, re ≈ 25 Ω. Because re depends on bias current, an undegenerated common-emitter stage has gain that varies with operating point and temperature; an external emitter resistor stabilises it.

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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