Understanding the Emitter Follower Configuration

The emitter follower, also known as a common collector amplifier, is a type of bipolar junction transistor (BJT) circuit configuration. In this configuration, the input signal is applied to the base terminal, and the output signal is taken from the emitter terminal. The collector terminal is common to both the input and output circuits, usually connected to the power supply (AC ground).

Why Emitter Follower is Used for Impedance Matching

The primary characteristic that makes the emitter follower widely used is its impedance transformation property. It exhibits a very high input impedance and a very low output impedance. This makes it ideal for connecting a circuit with high output impedance (like a sensor or a previous amplifier stage) to a circuit with low input impedance (like a speaker or a cable). Without impedance matching, connecting a high-impedance source directly to a low-impedance load can lead to significant signal loss or distortion due to voltage division.

Here's why the impedance characteristics are like this:

• High Input Impedance: Looking into the base, the input impedance is approximately given by \(Z_{in} \approx \beta \times (r_e + R_E)\), where \(\beta\) is the transistor's current gain, \(r_e\) is the internal emitter resistance, and \(R_E\) is the external emitter resistor. Since \(\beta\) is typically large (e.g., 100 or more), the input impedance becomes quite high.
• Low Output Impedance: Looking into the emitter, the output impedance is approximately given by \(Z_{out} \approx r_e + \frac{R_{signal}}{\beta}\), where \(R_{signal}\) is the impedance of the source driving the base. This formula shows that the output impedance is relatively low, often just a few ohms or tens of ohms, especially if \(R_{signal}\) is not excessively large.

This significant difference between input and output impedances allows the emitter follower to act as a buffer, effectively isolating the source from the load and ensuring maximum power transfer by bridging the impedance gap.

Analyzing Other Potential Uses

Let's consider the other options provided and why they are not the primary use for an emitter follower:

• Current gain: The emitter follower does provide significant current gain, approximately equal to the transistor's \(\beta\). This is necessary for it to drive a low-impedance load. However, providing current gain is a feature shared with other configurations (like common emitter), and it's the impedance matching capability that distinguishes the emitter follower's primary application.
• Voltage gain: The voltage gain of an emitter follower is slightly less than unity (approximately 1). Specifically, \(A_v = \frac{R_E}{r_e + R_E}\), which is always < 1. It reproduces the input voltage at the output with almost no amplification. Therefore, it is not used for voltage amplification.
• Power gain: The emitter follower provides power gain. Since it has current gain (\(A_i > 1\)) and approximately unity voltage gain (\(A_v \approx 1\)), the power gain \(A_p = A_v \times A_i \approx 1 \times \beta = \beta\), which is greater than 1. However, like current gain, power gain is a consequence of its operation, and impedance matching is its defining characteristic and primary application purpose.

Therefore, while an emitter follower exhibits current and power gain and has near-unity voltage gain, its most valuable and primary application is impedance matching between a high-impedance source and a low-impedance load.

Revision Table: Emitter Follower Characteristics

Additional Information: Transistor Configurations

Transistors can be used in different configurations, each offering distinct characteristics suited for various applications:

• Common Emitter (CE):
  • Input at Base, Output at Collector, Emitter Common.
  • Provides both voltage and current gain (high power gain).
  • Input impedance is moderate, output impedance is moderate.
  • Output is 180 degrees out of phase with the input.
  • Often used as a voltage amplifier stage.
• Common Collector (CC) - Emitter Follower:
  • Input at Base, Output at Emitter, Collector Common.
  • Voltage gain < 1, high current gain, high power gain.
  • High input impedance, low output impedance.
  • Output is in phase with the input (0 degrees phase shift).
  • Primarily used for impedance matching and buffering.
• Common Base (CB):
  • Input at Emitter, Output at Collector, Base Common.
  • Provides voltage gain, current gain < 1.
  • Low input impedance, high output impedance.
  • Output is in phase with the input (0 degrees phase shift).
  • Often used for high-frequency applications or as a current buffer.

Understanding the unique impedance and gain characteristics of each configuration helps in selecting the right one for a specific electronic circuit design requirement.