Understanding Voltage Gain in Transistor Amplifiers: Common Collector and Common Base Configurations

The world of electronic circuits can be quite complex, with various configurations designed to achieve different purposes. Among these, the common collector and common base configurations are two prominent ones. Understanding the voltage gain in these configurations is crucial for anyone working with transistor amplifiers. This article aims to elucidate these concepts and their practical applications.

Common Collector Configuration: The Emitter Follower

The Emitter Follower: The common collector configuration, often referred to as the emitter follower, is a type of transistor amplifier stage where the emitter is used as the output terminal while the base serves as the input terminal. This configuration has a unique property of having almost no voltage gain, but it does provide a high current gain.

The name "emitter follower" is apt because the emitter voltage closely follows the base voltage, but with a slight voltage drop due to the base-emitter diode voltage (typically around 0.7V). If the base voltage changes by 1V, the emitter voltage will change by approximately 0.7V to 0.3V, depending on the initial voltage level. This slight decrease in voltage is due to the diode voltage drop across the base-emitter junction.

Why No Voltage Gain in the Common Collector?

The common collector configuration does not have voltage gain because the collector current (Ic) is equal to the emitter current (Ie) plus a small base current (Ib). In simpler terms, the voltage at the collector (Vc) is influenced by the resistance between the collector and the supply voltage. Without a significant voltage drop across this resistance, the output voltage (Vc) does not change proportionally with the input voltage, resulting in no voltage gain.

A Higher Current Gain and Low Load Impedance

Despite the lack of voltage gain, the common collector configuration offers a high current gain, making it ideal for driving low load impedances from a high impedance source. The current gain (hFE) of a common collector configuration can be quite high, effectively increasing the current handling capability. This makes it a popular choice in situations where significant current needs to be delivered to a low impedance load.

Common Base Configuration: High Voltage Gain with Low Current Gain

The common base configuration is another important configuration, primarily used for voltage gain. In this setup, the base acts as the input terminal, the emitter is used as a reference, and the collector is the output terminal. Unlike the common collector, the common base configuration can provide significant voltage gain due to the input at the base being a low impedance and the output from the collector being a high impedance.

While the common base configuration has the same collector current as the emitter current (ignoring the small base current), it can develop a high voltage gain because the transistor's current gain (β) is high. The voltage gain is determined by the ratio of the collector-to-emitter voltage to the base-to-emitter voltage. In practical terms, this means a small change in the base voltage can cause a large change in the collector voltage, thus achieving high voltage gain.

Conclusion

In summary, the common collector and common base configurations offer different characteristics in transistor amplifiers. The common collector, or emitter follower, is ideal when high current gain and low output impedance are required, without significant concern for voltage gain. On the other hand, the common base configuration, with its high voltage gain but low current gain, is suitable for applications that require significant voltage amplification.

Understanding these distinctions is crucial for designing efficient and effective electronic circuits. Whether you are designing a power amplifier or a step-up voltage converter, knowing which configuration to use can significantly impact the performance and reliability of your final product.

Keywords

voltage gain common collector common base

References

[Here you would list any sources or references used in writing this article, if applicable. For this article, references to textbooks or specific research papers on transistors and amplifier configurations could be included.]