Common Collector (Emitter Follower) Calculator

Compute gain, input/output impedance of a common-collector BJT amplifier — the emitter follower buffer. Av ≈ 1, high Zin, low Zout.

Calculator Electronics Updated Apr 23, 2026
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How to Use
  1. Enter bias Ic, β, emitter resistor Re, and load RL.
  2. Tool computes gain (≈1), Zin (high), Zout (low).
Input
mA
Ω
Ω
Ω
Presets
Emitter Follower
Av
—
Zin
—
Zout
—
re intrinsic
—Ω

Show Work

Enter values.

Formulas

Av
Av = (Re||RL) / (re + Re||RL)
≈ 1 for Re||RL >> re.
Zin
Zin = rπ + (β+1)·(Re||RL)
High — load impedance boosted by β.
Zout
Zout = (Rs + rπ)/(β+1) || Re
Low — source Z divided by β.
re intrinsic
Vt / Ic
Small-signal emitter resistance.
V_out DC
V_in − Vbe
≈ V_in − 0.7 V
Current Gain
β+1
Emitter current = (β+1)·Ib

History of the Emitter Follower

The emitter follower entered textbooks in the 1950s as the third of the three standard BJT amplifier configurations (alongside common-emitter and common-base). Its near-unity voltage gain plus impedance transformation (beta+1 factor) made it the canonical output buffer for audio amps, instrument inputs, and driver stages. Op-amps with unity-gain voltage-follower wiring have largely replaced it in precision work, but emitter followers remain dominant in discrete Class-AB audio output stages and RF driver circuits.

About This Calculator

Enter Ic bias, β, emitter resistor Re, load RL, and source Rs (for Zout). The tool computes Av (close to 1), input impedance Zin (high), output impedance Zout (low).

For higher-current buffer capability without losing β: use Darlington pair. For precise unity gain: use op-amp voltage follower. Emitter follower shines when you need moderate current, simple biasing, and low cost. Everything runs client-side.

Frequently Asked Questions

Why emitter follower?

Unity-gain buffer: Av ≈ 1, Zin = β·(Re||RL) (high), Zout = (Rs + rπ)/(β+1) (low). Transforms a high-Z source into a low-Z drive without changing voltage.

Vs op-amp buffer?

Emitter follower is faster (no GBW limit at low current), simpler bias, and cheaper at high currents. Op-amp buffer has precisely A = 1, no voltage offset, and easier DC coupling.

Output DC offset?

V_out = V_in − Vbe ≈ V_in − 0.7 V. Must account for this in DC-coupled designs.

Common Use Cases

Microphone Input

High-Z electret mic driving ADC via emitter follower preserves signal amplitude.

Power Amp Driver

Second-to-last stage before output pair: low Zout to drive output BJTs hard.

Line Driver

Drive 50Ω coax or headphone load from a high-Z analog source.

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