Emitter Follower Calculator

Design a common-collector (emitter follower) buffer amplifier. This topology provides near-unity gain with high input impedance and low output impedance - ideal for driving low-impedance loads without loading the source.

Schematic

Calculated Values

Component values rounded to standard E24 series.

R1 (upper bias):
R2 (lower bias):
Re (emitter):
Cin (input):
Cout (output):

Buffer Performance

The emitter follower excels at impedance transformation - high Zin, low Zout. Gain is slightly less than 1 due to the voltage divider formed by re' and Re.

Input impedance (Zin):
Output impedance (Zout):
Voltage gain:
Input attenuation:
Effective gain:
Expected output:

Input / Output Waveforms

The emitter follower has no phase inversion - output follows input (hence the name). Clipping occurs if output exceeds the available swing.

DC Load Line

The load line shows the operating path. For an emitter follower, the DC load is just Re (collector connects directly to Vcc).

Frequency Response

Low frequencies are limited by coupling capacitors (Cin, Cout). High frequencies are limited by the transistor's β rolloff at fβ = fT/β. The emitter follower has minimal Miller effect since voltage gain is near unity.

Design Formulas

Click to show step-by-step design procedure

Step 1: Choose Emitter Current

Higher current = lower output impedance, but more power dissipation:


                    I_E chosen based on load requirements

                    Rule of thumb: I_E > 10 × (V_out / R_load)

Step 2: Calculate Emitter Resistor

Set Ve at roughly half of Vcc for maximum swing:


                    V_E ≈ V_CC / 2

                    R_E = V_E / I_E

Step 3: Calculate Intrinsic Emitter Resistance


                    r_e' = 26mV / I_E = 0.026 / I_E

Step 4: Calculate Bias Resistors


                    V_B = V_E + 0.7V

                    I_B = I_E / β

                    I_divider = 10 × I_B

                    R_total = V_CC / I_divider

                    R₂ = (V_B / V_CC) × R_total

                    R₁ = R_total - R₂

Step 5: Calculate Voltage Gain

Gain is slightly less than 1:


                    A_v = (R_E ∥ R_load) / (r_e' + R_E ∥ R_load)

                    A_v ≈ 0.95 to 0.99 typically

Step 6: Calculate Input Impedance


                    Z_in(base) = β × (r_e' + R_E ∥ R_load)

                    Z_in = R₁ ∥ R₂ ∥ Z_in(base)

Step 7: Calculate Output Impedance


                    Z_out = (r_e' + R_source/β) ∥ R_E

                    For low R_source: Z_out ≈ r_e' ∥ R_E ≈ r_e'

Step 8: Calculate Coupling Capacitors


                    C_in = 1 / (2π × f_low × Z_in)

                    C_out = 1 / (2π × f_low × R_load)

Key Characteristics

Voltage gain: A_v ≈ 1 (unity gain buffer)
Current gain: A_i ≈ β (high)
Power gain: A_p ≈ β (significant)
Input impedance: High (β × R_E)
Output impedance: Low (≈ r_e')
Phase shift: 0° (no inversion)

Vcc:	V
Input (peak):	V
Source impedance:	Ω
Low frequency (-3dB):	Hz
Load resistance:	Ω
Emitter current (Ie):	mA

Transistor:
β (hFE):
fT:	MHz