An emitter follower amplifier is a common-collector bipolar junction transistor (BJT) circuit that provides no voltage gain but acts as an impedance-matching buffer, delivering high current to a low-impedance load without loading down the preceding high-impedance signal source.
In practical circuit design, the emitter follower changes the current capacity and impedance profile of a signal, not its voltage amplitude. A high-impedance source—like a piezo sensor, a high-value voltage divider, or a microphone capsule—cannot drive a 50-ohm coaxial cable or an 8-ohm speaker directly. If you try, the source voltage collapses under the current demand. The emitter follower solves this by sensing the source voltage with its high-impedance base and reproducing it at the low-impedance emitter, acting as an electrical shock absorber.
Beginners frequently confuse the emitter follower with the common-emitter amplifier (which provides high voltage gain and inverts the signal phase) or the MOSFET source follower (which performs a similar buffering role but relies on gate-source threshold voltage rather than a fixed 0.7V base-emitter drop). When you hear 'amplifier,' do not automatically assume voltage multiplication; in this topology, we are amplifying current and power.
The Core Mechanics and Configuration Comparison
The defining characteristic of the common-collector topology is that the collector terminal is common to both the input (base) and output (emitter) AC signal paths, typically tied directly to the supply rail (Vcc) or AC ground. Because the output is taken from the emitter, the output voltage is always the base voltage minus the base-emitter diode drop (Vbe ≈ 0.7V for silicon BJTs). As the input signal swings up and down, the emitter 'follows' it exactly, offset by that 0.7V DC difference.
Think of the emitter follower like a mechanical gear shifter on a bicycle. A high gear gives you speed but low torque (high impedance, low current capability). A low gear sacrifices speed for massive torque (low impedance, high current capability). The emitter follower is the derailleur that translates the high-Z 'speed' of your signal source into the low-Z 'torque' needed to drive a heavy load.
BJT Amplifier Topologies at a Glance
To understand where the emitter follower fits in your design toolkit, compare it against the other two fundamental BJT configurations. This table should dictate which topology you reach for based on your input/output impedance requirements.
| Parameter | Common Emitter | Common Collector (Emitter Follower) | Common Base |
|---|---|---|---|
| Voltage Gain | High (10 to 500+) | Unity (≈ 1) | High (10 to 500+) |
| Current Gain | High (β) | High (β + 1) | Unity (≈ 1) |
| Input Impedance | Medium (1kΩ - 5kΩ) | High (10kΩ - 500kΩ+) | Low (10Ω - 100Ω) |
| Output Impedance | Medium/High (10kΩ+) | Low (1Ω - 50Ω) | High (100kΩ+) |
| Phase Shift | 180° (Inverting) | 0° (Non-inverting) | 0° (Non-inverting) |
As the table demonstrates, the emitter follower is the only configuration that offers both high input impedance and low output impedance, making it the undisputed champion for impedance buffering. For deeper theoretical derivations of these parameters, refer to standard semiconductor texts like the Electronics Tutorials Common Collector guide.
Worked Numeric Example: Biasing a 2N3904 Buffer Stage
Let's design a stable DC bias for an emitter follower using a ubiquitous 2N3904 NPN transistor. Our goal is to buffer an AC audio signal and drive a 100Ω load without clipping.
Design Parameters & Assumptions:
- Supply Voltage (Vcc): 12V DC
- Target Emitter Current (Ie): 5mA (sufficient for driving light audio loads)
- Transistor Beta (β / hFE): 150 (typical for a 2N3904 at 5mA)
- Base-Emitter Voltage (Vbe): 0.7V
- Thermal Voltage (Vt): ~25mV at room temperature
Step 1: Set the DC Operating Point (Q-Point)
To allow maximum symmetrical AC voltage swing, we set the DC emitter voltage (Ve) to half the supply rail.
- Ve = Vcc / 2 = 12V / 2 = 6V
- Calculate Emitter Resistor (Re): Re = Ve / Ie = 6V / 5mA = 1.2kΩ (Standard E12 value)
Step 2: Design the Base Voltage Divider
The base voltage (Vb) must be exactly one diode drop above the emitter voltage.
- Vb = Ve + Vbe = 6V + 0.7V = 6.7V
Next, we calculate the base current (Ib) to size our biasing resistors (R1 and R2). The divider current should be at least 10x the base current to prevent the base from pulling the divider voltage down.
- Ib = Ie / (β + 1) = 5mA / 151 ≈ 33µA
- Target Divider Current = 10 × Ib = 330µA (0.33mA)
- R2 = Vb / 0.33mA = 6.7V / 0.33mA ≈ 20.3kΩ → Use 20kΩ
- R1 = (Vcc - Vb) / 0.33mA = (12V - 6.7V) / 0.33mA ≈ 16.06kΩ → Use 16kΩ
Step 3: Calculate AC Impedances
Now we verify the buffering capability. First, find the internal AC emitter resistance (re):
- re = 25mV / Ie = 25mV / 5mA = 5Ω
The input impedance looking directly into the base is β multiplied by the total AC resistance in the emitter leg. Assuming the 100Ω load is AC-coupled in parallel with the 1.2kΩ Re:
- AC Emitter Load (R_ac) = 1.2kΩ || 100Ω ≈ 92Ω
- Zin(base) = β × (re + R_ac) = 150 × (5Ω + 92Ω) = 150 × 97Ω = 14.55kΩ
The total circuit input impedance includes the biasing resistors in parallel:
- Zin(total) = R1 || R2 || Zin(base) = 16k || 20k || 14.55k ≈ 6.1kΩ
While 6.1kΩ isn't infinite, it is vastly higher than the 100Ω load, successfully preventing the source from being dragged down. If you need higher input impedance, increase R1/R2 values and use a Darlington pair configuration to multiply β.
Where You Meet the Emitter Follower in Practice
You will rarely see a discrete emitter follower used just to pass a DC voltage; its real value emerges in specific subsystems where current delivery and impedance isolation are critical.
Audio Power Output Stages
In almost every analog audio amplifier, the final output stage driving the speaker is a push-pull emitter follower (often using complementary NPN/PNP pairs like the MJL21193/MJL21194). The preceding voltage-amplification stages cannot source the amps of current required by a 4-ohm speaker. The emitter followers provide the current gain while preserving the voltage waveform generated by the driver stage. Class-AB biasing is applied to the bases to eliminate the 0.7V crossover distortion inherent to the B-E junction.
Linear Voltage Regulators
Look inside the schematic of a classic LM317 or a discrete bench power supply. The 'pass transistor' that handles the heavy load current is wired as an emitter follower. An error amplifier (op-amp) drives the base, and the emitter follows that control voltage minus Vbe. The op-amp only needs to supply the base current (milliamps), while the emitter follower delivers the load current (amps).
Oscilloscope Probe Buffers
Active oscilloscope probes (like the Tektronix P6245) use FET or BJT emitter/source followers right at the probe tip. This isolates the high-impedance probe tip (often 100kΩ to 1MΩ) from the 50-ohm coaxial cable feeding the scope, preventing the cable's capacitance from rolling off high-frequency signals.
Emitter Follower Amplifier FAQ and Troubleshooting
Why is my output clipping asymmetrically on the positive half-cycle?
This happens when the input signal swings so high that the base voltage exceeds the collector voltage (Vcc). When Vb > Vc, the base-collector junction becomes forward-biased, and the transistor enters saturation. The emitter can never exceed Vcc - Vce(sat). To fix this, ensure your input AC signal is capacitively coupled and that the DC bias leaves enough headroom between Vb and Vcc.
How do I eliminate the 0.7V DC offset at the output?
The 0.7V offset is a fundamental property of the silicon B-E junction. In AC applications, simply place a coupling capacitor in series with the output to block the DC component. In DC-coupled applications (like precision voltage buffers), you must use an op-amp with negative feedback taken from the emitter back to the inverting input; the op-amp will automatically add 0.7V to its own output to force the emitter to exactly match the non-inverting input voltage.
Can I use an emitter follower to drive a relay coil?
Yes, but with a caveat. A relay coil is a highly inductive load. When the transistor turns off, the collapsing magnetic field generates a massive reverse voltage spike that can punch through the B-E junction (which typically has a low reverse breakdown voltage, Vebo, around 6V for a 2N3904). You must place a flyback diode (like a 1N4148 or 1N4007) in reverse parallel across the relay coil to clamp the inductive kickback and protect your buffer transistor.
