What is the formula for Emitter-Follower Input Impedance?

The Emitter-Follower Input Impedance formula is Zin ≈ β × RE (plus β·re), where Zin—Input impedance looking into the base (Ω); β—Current gain; RE—Emitter resistance / load (Ω).

What does Zin mean in the Emitter-Follower Input Impedance formula?

Zin is Input impedance looking into the base (Ω).

Emitter-Follower Input Impedance

Q: How do you calculate Emitter-Follower Input Impedance?

Use the formula Zin ≈ β × RE (plus β·re). An emitter follower (common-collector stage) presents a high input impedance because the emitter resistor is reflected to the base multiplied by the current gain. Enter the values you know in the calculator above and it solves for the remaining variable.

Zin ≈ β · RE

Calculator

Input Impedance

Current Gain β

Emitter Resistor

Result

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Formula

Zin ≈ β × RE (plus β·re)

Description

An emitter follower (common-collector stage) presents a high input impedance because the emitter resistor is reflected to the base multiplied by the current gain. This lets the stage buffer a high-impedance source to a low-impedance load with near-unity voltage gain. It is the classic impedance-transforming output buffer.

Variables

Zin — Input impedance looking into the base (Ω)
β — Current gain
RE — Emitter resistance / load (Ω)

Practical Notes

The full expression is Zin = β·(re + RE). The output impedance is correspondingly low: Zout ≈ re + Rsource/β. This is why followers drive heavy or capacitive loads well.

Related Concepts

All Discrete Amplifiers formulas →

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