When you need to couple a high-impedance signal source to a low-impedance load without losing signal integrity, you use a common collector amplifier. Also known as an emitter follower, this BJT configuration features a voltage gain of approximately unity, high input impedance, and low output impedance. It's the workhorse circuit for impedance transformation, signal buffering, and power amplification stages in analog and mixed-signal systems.
Unlike common emitter circuits designed for massive voltage multiplication, the common collector setup routes the output signal from the emitter terminal rather than the collector. This architectural shift trades away voltage gain to achieve powerful current amplification and isolation benefits. Engineers rely on this topology to prevent load impedance from dragging down sensitive upstream circuitry.
Circuit Configuration and Working Principle
In a standard NPN common collector circuit, the collector connects directly to the positive DC supply rail ($V_{CC}$). The input signal enters through the base terminal, while the output is extracted from the emitter terminal via an emitter resistor ($R_E$). Because the collector ties straight to the AC ground of the power supply rail, the collector acts as the common node for both input and output loops, which gives the configuration its name.
The DC bias network typically uses a voltage divider at the base to set a stable quiescent operating point. As the input voltage at the base rises, the base-emitter junction forward-biases slightly, pulling the emitter voltage up with it. The emitter voltage closely tracks the base voltage minus the standard silicon diode drop ($V_{BE} \approx 0.7\text{ V}$). This tight tracking behavior creates the characteristic "follower" response where the output faithfully mimics the input waveform's voltage swings while delivering substantial current to downstream components.
To understand the core electrical behavior, you can review technical transistor guidelines provided by Texas Instruments.
Key Performance Characteristics
Analyzing an emitter follower requires evaluating four primary parameters: voltage gain, current gain, input impedance, and output impedance.
Voltage Gain
The voltage gain ($A_v$) of a common collector amplifier is close to 1, expressed mathematically as:
$$A_v = \frac{v_{out}}{v_{in}} \approx 1$$
It's technically slightly less than unity (often $0.98$ to $0.99$) because small fluctuations in base current alter the base-emitter voltage drop. The output never exceeds the input, meaning this circuit doesn't amplify voltage amplitudes.
Current Gain
While voltage gain stays flat, current gain ($A_i$) is remarkably high. The relationship depends directly on the transistor's forward current transfer ratio ($\beta$ or $h_{FE}$):
$$A_i \approx \beta + 1$$
If you use a transistor with a $\beta$ of 150, the circuit multiplies the input base current by roughly 151 at the emitter load. This massive current boost enables the circuit to drive heavy resistive or reactive loads.
Input and Output Impedance
The defining superpower of the emitter follower lies in its terminal impedances.
- Input Impedance ($Z_{in}$): Reflected through the base, the input impedance becomes very high because the load resistance is multiplied by the transistor's beta factor ($Z_{in} \approx \beta \times R_E$). This high impedance prevents the amplifier from loading down weak sensor outputs or preceding gain stages.
- Output Impedance ($Z_{out}$): Conversely, the output impedance measured looking back into the emitter is exceptionally low. This allows the circuit to act as an ideal voltage source that maintains stable output voltage levels even when driving variable or low-resistance loads.
Practical Design Steps
Building a working common collector stage requires balancing DC biasing with AC coupling requirements. Here is how engineers design a functional emitter follower step by step.
- Determine the supply voltage ($V_{CC}$): Choose a standard power rail, such as $+12\text{ V}$ or $+15\text{ V}$, based on your system requirements and maximum expected signal swing.
- Select the emitter resistor ($R_E$): Calculate $R_E$ to set the desired quiescent DC emitter current ($I_E$). For instance, if you want $2\text{ mA}$ of quiescent current across a $12\text{ V}$ split or single supply, set $R_E$ to place the quiescent emitter voltage near half the supply ($6\text{ V}$).
- Choose biasing resistors ($R_1$ and $R_2$): Set up a voltage divider at the base to establish a DC base voltage equal to your target emitter voltage plus $0.7\text{ V}$. Ensure the divider current is at least 10 times larger than the maximum expected base current to keep bias points stable.
- Add coupling capacitors: Place input and output capacitors ($C_{in}$ and $C_{out}$) to block DC bias voltages from bleeding into preceding or succeeding stages while letting AC signals pass freely. Select capacitance values large enough to maintain a low reactance at your lowest operating frequency.
Common Design Mistakes
Even though the circuit is structurally simple, novice designers often run into predictable pitfalls that degrade performance.
- Neglecting thermal runaway: Transistor characteristics shift with temperature. Without proper emitter degeneration or stable base biasing, rising temperatures increase collector current, generating more heat and potentially destroying the device.
- Ignoring output clipping: Because the output voltage swings between ground and $V_{CC}$, failing to center your DC bias point clips the AC waveform prematurely on positive or negative peaks.
- Overlooking capacitive loading: Driving heavy capacitive loads directly from the emitter can introduce phase shifts, leading to ringing or high-frequency oscillations in wideband circuits.
- Mismatched coupling capacitor values: Using undersized capacitors restricts low-frequency response, rolling off bass frequencies in audio setups or attenuating control signals in instrumentation hardware.
Frequently Asked Questions
Why is a common collector amplifier called an emitter follower?
It earns the name because the output voltage taken at the emitter closely tracks, or "follows," the input voltage applied at the base, maintaining a nearly identical waveform shape with an offset of about $0.7\text{ V}$.
Can a common collector amplifier provide voltage gain?
No. Its voltage gain is strictly less than 1 (unity). It trades voltage amplification entirely for current gain, impedance buffering, and signal isolation.
Where are emitter followers used in real-world circuits?
They appear frequently as buffer stages between high-impedance sensors and analog-to-digital converters, as output stages in audio power amplifiers to drive low-impedance speakers, and as voltage regulators in power supplies.
How does input impedance stay so high in this configuration?
The collector current feedback through the emitter resistor bootstraps the input circuit. This feedback makes the base-emitter junction appear much larger to the source, multiplying the load resistance by the transistor's $\beta$ value.
