Free BJT Emitter Follower Buffer Calculator

Calculate input impedance (Zin), output impedance (Zout), voltage gain (Av ≈ 1), and DC biasing for common-collector transistor buffers.

⚡ Circuit Biasing & Components

Supply Voltage (Vcc)

Transistor Gain (β / hFE)

Typical: 100 - 300 (2N3904)

Upper Base Resistor (R1)

kΩ

Lower Base Resistor (R2)

kΩ

Emitter Resistor (RE)

External AC Load (RL)

Source Internal Resistance (Rs)

Typical audio/sensor source: 600 Ω

📊 Buffer Performance & Impedances

AC Voltage Gain (Av)

0.994 V/V (-0.05 dB)

Non-inverting (0° phase shift buffer)

Input Impedance (Zin)

11.2 kΩ

High input load buffering

Output Impedance (Zout)

8.9 Ω

Ultra-low drive impedance

DC Emitter Current (I_E)

5.15 mA

r_e = 5.05 Ω (26mV / I_E)

Max Unclipped Swing

10.3 V_pp

Symmetrical Q-point centered

💡 The Impedance Transformer Magic

The emitter follower acts as an impedance step-down transformer: looking into the base, the emitter load is multiplied by β ($Z_{in} approx eta imes R_E$). Looking into the emitter, source resistance is divided by β ($Z_{out} approx R_S / eta$), allowing weak high-impedance sensors to drive heavy 50Ω or 600Ω cables with zero voltage loss.

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How the Common Collector (Emitter Follower) Works

In a BJT Common Collector amplifier, the collector is tied directly to the positive power supply rail ($V_{CC}$), acting as AC ground. The input signal enters the base, and the output is taken directly from the emitter. Because the emitter voltage tracks ("follows") the base voltage exactly 0.65V lower, this circuit is universally called an emitter follower.

1. AC Voltage Gain (Av ≈ 1)

Let $r_e = rac{26 ext{ mV}}{I_E}$ be the dynamic intrinsic emitter resistance of the forward-biased base-emitter junction, and $r_L' = R_E parallel R_L$ be the effective AC load:

A_v = r_L' / [r_e + r_L'] ≈ 0.98 to 0.995

Although the circuit provides no voltage gain ($A_v le 1$), it provides massive current gain ($A_i approx eta$) and high power gain.

2. Input and Output Impedance Transformations

Input Impedance ($Z_{in}$): Looking into the base, any resistance in the emitter leg appears multiplied by transistor gain:
Z_in = (R1 || R2) || [ β × (r_e + R_E || R_L) ]
Output Impedance ($Z_{out}$): Looking into the emitter, source resistance appears divided by transistor gain:
Z_out = R_E || [ r_e + (R1 || R2 || R_S) / (β + 1) ]

Frequently Asked Questions

Why does an emitter follower need bias resistors if gain is only 1?

The base-emitter junction requires a positive DC bias voltage (~0.65V) above the emitter to stay in the active forward-conduction region. Without R1 and R2 setting the base DC operating point near mid-supply, the negative half of any AC input signal would cut off the transistor, causing severe half-wave clipping.

Can an emitter follower drive low-impedance 8-ohm headphones?

While a standard single-transistor follower can drive a few milliamps, driving an 8-ohm speaker directly requires high peak currents that would pull the transistor out of class-A bias. For low-impedance audio loads, use a push-pull complementary NPN/PNP pair (Class AB buffer).

How does the emitter follower compare to an op-amp voltage follower?

An op-amp follower has virtually infinite input impedance and lower output impedance due to massive negative feedback. However, a discrete BJT emitter follower is vastly faster, has no slew-rate limiting, operates easily into RF frequencies (100+ MHz), and introduces zero op-amp phase lag or stability ringing.

What is the "bootstrapping" technique in emitter followers?

Bootstrapping connects a small capacitor from the emitter back to the midpoint of a split base-bias resistor network. Because the emitter AC voltage is nearly identical to the base, almost zero AC current flows through the bias resistors, raising input impedance from 10 kΩ up to several megaohms.