Common-Collector (Emitter Follower) Amplifier

In electronic circuit design, connecting a weak signal source to a demanding load is a classic problem. The source might have high internal impedance, causing its signal to be attenuated or distorted when driving a low-impedance load like a speaker or another transistor stage. This is where the common-collector amplifier, universally known as the emitter follower, becomes indispensable. It acts as a nearly perfect buffer, faithfully transferring a voltage signal from its input to its output while performing a critical impedance transformation: presenting a high impedance to the source and a low impedance to the load, thereby preventing signal degradation.

Basic Configuration and Voltage Gain

The defining characteristic of an emitter follower is its topology. The input signal is applied to the base, the output is taken from the emitter, and the collector is connected directly to the power supply, making it common to both input and output signals (hence "common-collector"). The resistor from the emitter to ground sets the quiescent (DC) operating point. For AC signal analysis, this resistor is effectively in parallel with any external load connected to the output.

When an input voltage $V_{in}$ increases, it increases the base-emitter voltage ($V_{BE}$), which increases the emitter current. This increased current raises the voltage across the emitter resistor, causing the output voltage $V_{out}$ (at the emitter) to rise. Crucially, the output voltage "follows" the input voltage, minus the approximately 0.7V DC drop across the base-emitter junction. For AC signals, this means the voltage gain $A_v$ is very close to, but slightly less than, one. The gain is given by:

$$A_v=\frac{V_{out}}{V_{in}}\approx \frac{R_E}{R_E+r_e}$$

Here, $r_e$ is the transistor's small-signal emitter resistance, approximately equal to $25mV/I_E$, where $I_E$ is the DC emitter current. Because $R_E$ is typically much larger than $r_e$, the fraction approaches 1. For example, if $I_E=2mA$, then $r_e\approx 12.5\Omega$. With $R_E=1k\Omega$, the gain $A_v\approx 0.988$. This unity voltage gain means the circuit provides no voltage amplification. Furthermore, because the output at the emitter increases when the input at the base increases, there is no phase inversion between input and output, unlike in common-emitter amplifiers.

Input and Output Impedance: The Core Buffering Action

The emitter follower’s true value lies not in voltage gain, but in its impedance characteristics. Its input impedance ($Z_{in}$) is very high. From the base looking in, the impedance is effectively $\beta \times (R_E||R_L)$, where $\beta$ is the transistor's current gain. This is because the emitter resistor $R_E$ (in parallel with the load $R_L$) is "reflected" back to the base, multiplied by $\beta$. If $\beta =100$ and $R_E||R_L=500\Omega$, the input impedance seen from the base is roughly $100\times 500\Omega =50k\Omega$. This high input impedance means the follower draws very little current from the signal source, preventing it from being "loaded down."

Conversely, the output impedance ($Z_{out}$) is very low. From the emitter looking back into the circuit, the output impedance is approximately the resistance in the base circuit (including the source impedance $R_S$) divided by $\beta$, plus the small $r_e$. The formula is:

$$Z_{out}\approx \frac{R_S}{\beta }+r_e$$

If the source impedance $R_S=10k\Omega$ and $\beta =100$, the first term becomes only $100\Omega$. Added to a small $r_e$, the total output impedance can be just a few hundred ohms or less. This low output impedance allows the emitter follower to deliver current efficiently to a low-impedance load without a significant drop in output voltage.

Current Gain and Power Transfer

While the voltage gain is near unity, the current gain of the emitter follower is high, approximately equal to the transistor's $\beta$. The circuit takes a small input base current and delivers a much larger output emitter current to the load. This ability to supply current is what makes it an excellent buffer. It can accept a signal from a high-impedance source that cannot supply much current (like a microphone or sensor) and use the transistor's action to deliver a replica of that voltage signal to a load requiring significant current (like a motor or speaker). In essence, it provides power gain (current × voltage) while maintaining voltage fidelity, enabling efficient power transfer from source to load.

Practical Applications and Biasing Considerations

The emitter follower's role as an impedance transformation buffer is its most common application. You will find it as the output stage of a multi-stage amplifier to drive a speaker, as a buffer between different filter stages to prevent interaction, or at the input of a measuring instrument to present a high impedance to the circuit under test.

A practical circuit must be properly biased. A simple single-resistor base bias is rarely used due to poor stability. A voltage divider bias network ($R1$ and $R2$) at the base is standard, providing a stable DC base voltage. The input capacitor blocks DC from the source, and an output capacitor may be used to block the DC emitter voltage from the load. In these configurations, the DC biasing resistors ($R1$ and $R2$) appear in parallel with the high input impedance from the transistor's perspective, slightly lowering the overall circuit input impedance. For the highest input impedance, a bootstrapping technique can be employed, where feedback is used to make the biasing network appear as a much higher impedance at the signal frequency.

Common Pitfalls

1. Ignoring the Load on Biasing: When calculating the DC operating point ($I_E$, $V_E$), you must consider the effect of the external load resistor $R_L$. It is in parallel with the emitter resistor $R_E$ for DC if no output capacitor is used. If an output capacitor is present, $R_L$ only affects the AC signal. Failing to account for this can lead to incorrect quiescent point calculations and signal clipping.

2. Overestimating the Voltage Swing: The output voltage can only swing between a lower limit (near ground, limited by transistor saturation) and an upper limit (the supply voltage $V_{CC}$ minus the voltage drop across $R_E$). A common mistake is to assume the output can reach $V_{CC}$. In reality, the emitter voltage is always about 0.7V below the base voltage. If the base is driven to $V_{CC}$, the emitter can only go to $V_{CC}-0.7V$.

3. Confusing Impedance Formulas: A frequent error is using the simplified input impedance formula $Z_{in}\approx \beta R_E$ without considering the biasing resistor network ($R1||R2$) or the source impedance. The total circuit input impedance is the parallel combination of the bias resistors and the transistor's own high input impedance ($\beta (R_E||R_L)$). Forgetting this parallel effect leads to an overestimation of the circuit's true input impedance.

4. Neglecting $r_e$ in Gain Calculations: While $A_v\approx 1$ is a good estimate, for precise design—especially with small emitter resistors or large currents—the $r_e$ term must be included. Assuming a perfect gain of 1 can lead to errors in calculating signal levels through a chain of buffers.

Summary

• The common-collector (emitter follower) amplifier provides a voltage gain very close to one (unity) with no phase inversion between input and output signals.
• Its primary function is impedance buffering: it features very high input impedance, minimizing loading on the preceding signal source, and very low output impedance, enabling it to drive heavy (low-impedance) loads effectively.
• While voltage gain is near unity, current gain is high (approximately $\beta$), allowing the circuit to deliver substantial power to the load.
• It is fundamentally an impedance transformation buffer, essential for matching high-impedance sources to low-impedance loads without signal loss.
• Proper design must account for biasing network loading, realistic voltage swing limits, and the small-signal emitter resistance $r_e$ for accurate analysis.