When it comes to amplifiers, the common-base (CB) amplifier is a unique configuration that offers several advantages, especially in specific applications. In this article, we’ll explore the common-base amplifier’s functionality, its key characteristics, and why it’s less commonly used compared to other amplifier configurations like the common-emitter and common-collector.
What is a Common-Base Amplifier?
The common-base amplifier is a type of bipolar junction transistor (BJT) amplifier where the input signal is applied at the emitter , and the output is taken from the collector circuit. This configuration is illustrated in the circuit diagram below. The base terminal is common to both the input and output circuits, hence the name "common-base."
Key Characteristics of the Common-Base Amplifier
Low Input Impedance : One of the defining features of the common-base amplifier is its low input impedance , typically ranging between 50 to 75 Ω . While this makes it suitable for certain applications, such as matching with 50-Ω antenna inputs in receivers, it also limits its use in many other scenarios where higher input impedance is preferred.
High Output Impedance : On the flip side, the common-base amplifier boasts a high output impedance , which can be advantageous in certain power amplification applications. The power amplification is governed by the formula , making it effective in delivering power to high-impedance loads.
Voltage Amplification : The common-base amplifier is often used as a voltage amplifier , particularly in low input impedance circuits. However, its current gain is always slightly less than unity, meaning it doesn’t amplify current as effectively as voltage.
Superior Temperature Stability and Linearity : Another advantage of the common-base amplifier is its superior temperature stability and linearity . These qualities make it ideal for high-frequency operations, where maintaining signal integrity is crucial.
High-Frequency Performance : The CB amplifier can operate effortlessly at very high frequencies , making it a go-to choice for applications like Class C high-frequency amplifiers.
Why Isn’t the Common-Base Amplifier More Popular?
Despite its strengths, the common-base amplifier isn’t as widely used as the common-emitter BJT amplifier or common-collector configurations. The primary reason for this is its low input impedance , which can be a limiting factor in many circuits. However, it does find niche applications, such as in 50-Ω antenna inputs of receivers or as Class C high-frequency amplifiers.
The JFET Equivalent: The Common-Gate Amplifier
In the world of field-effect transistors (FETs), the equivalent of the BJT’s common-base amplifier is the common-gate amplifier . This configuration is often found in the intermediate frequency (IF) stages of some receivers, as shown below.
Components and Their Roles in a Common-Gate Amplifier
C2, C3, R2, and RFC (Radio Frequency Choke) : These components are used for decoupling , ensuring that unwanted signals are filtered out.
C4 and C6 : These capacitors are responsible for RF coupling , allowing the RF signal to pass through while blocking DC components.
C5 : This capacitor can be adjusted to achieve a flatter frequency response across the amplifier’s passband, ensuring consistent performance.
T1 : This transformer is used for impedance matching , which is crucial for optimizing the amplifier’s performance, especially given its low input impedance.
Conclusion
While the common-base amplifier may not be as popular as its counterparts, it offers unique advantages that make it indispensable in specific applications. Its low input impedance , high output impedance , and excellent high-frequency performance make it a valuable tool in the designer’s arsenal, particularly in RF and high-frequency circuits.
Whether you’re working on a receiver’s antenna input or designing a high-frequency amplifier, understanding the common-base amplifier’s strengths and limitations can help you make informed decisions in your circuit design. And for those working with FETs, the common-gate amplifier serves as a reliable alternative with similar characteristics.
By leveraging the right amplifier configuration for your specific needs, you can ensure optimal performance and efficiency in your electronic designs.
