Understanding the Emitter Follower in a Common Collector Circuit

The use of a common collector circuit, often referred to as an emitter follower, is widespread due to its unique characteristics and applications. This article aims to elucidate the underlying principles, workings, and advantages of this circuit, particularly focusing on why it is known as an emitter follower.

Overview of the Common Collector Circuit

At its core, the common collector circuit involves applying a signal to the base of a transistor, with the output taken from the emitter. The design of this circuit places the collector terminal in common with the power supply, hence the name common collector.

Basic Circuit Configuration

In the simplest form, the collector is connected to a DC power source, and the emitter is linked to the "return" of the power source through a resistor. The quiescent base voltage is adjusted to bias the transistor appropriately.

Signal Behavior and Amplification

The input signal causes a proportional current to flow into the base. The key property of the transistor is that the collector current will be β (beta) times the base current, where β is the transistor's amplification factor. Consequently, the current in the emitter will be approximately β times the base current. Given that β is typically much greater than 1, the emitter current is effectively β times the base current.

Considering the forwarded-biased transistor, the base voltage remains relatively stable, around 0.7V for a silicon transistor and 0.3V for a germanium transistor. As the applied voltage to the base changes, the base current accordingly changes. The current through the emitter resistor changes, leading to a proportional change in the emitter voltage. The voltage across the emitter resistor is determined by the current through the resistor times its value. As a result, the emitter voltage follows the input voltage, differing by approximately 0.7V (0.3V for germanium).

Why is a Common Collector Circuit Known as an Emitter Follower?

Due to the feedback loop created by this configuration, the emitter voltage closely follows the input voltage. This is the fundamental reason why this circuit is referred to as an "emitter follower." The output maintains the same peak-to-peak amplitude as the input, ensuring a direct and efficient transmission of the signal.

Practical Applications and Implications

The common collector circuit has several practical advantages. One of the most notable is its ability to maintain a high input impedance, making it ideal for signal generation or as a buffer in circuits. Additionally, the near 1:1 relationship between the input and output voltages makes it particularly suitable for impedance transformation, voltage buffering, and as a stable voltage reference.

Examples of Input and Output Voltages

For a more concrete understanding, let's consider some numerical examples:

At the base input of 5V, the output at the emitter would be approximately 4.7V (5V - 0.3V). At the base input of 10V, the output at the emitter would be around 9.7V (10V - 0.3V).

In both cases, it is evident that the emitter voltage closely follows the base voltage, underscoring the circuit's role as an emitter follower.

Conclusion

The common collector circuit, or emitter follower, is a fundamental component in circuit design. It is known as an emitter follower primarily due to its unique characteristic of following the input voltage closely without amplifying it. Understanding this concept is crucial for both theoretical and practical applications in electronics and electrical engineering. Whether in signal processing or as a buffer stage, the emitter follower remains a vital tool in the designer's arsenal.