The common base configuration is a fundamental transistor amplifier configuration often employed in various electronic circuits. While it may seem less popular compared to the common emitter and common collector configurations, the common base configuration offers distinct advantages, making it the ideal choice for specific applications. In this article, we delve into the intricacies of the common base configuration, exploring its characteristics, advantages, and key applications, ultimately answering the question: When are BJTs used in common base configuration?
Understanding the Common Base Configuration
The common base configuration derives its name from the fact that the base terminal is common to both the input and output circuits. In this configuration, the input signal is applied to the emitter, while the output signal is taken from the collector. The base terminal serves as a reference point for both the input and output signals.
Key Characteristics of the Common Base Configuration
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High Input Impedance: One of the most notable features of the common base configuration is its high input impedance. This impedance is typically in the range of a few hundred ohms to a few kilohms. This characteristic makes the common base configuration ideal for applications where a low input current is required.
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Low Output Impedance: The output impedance of a common base configuration is relatively low, typically ranging from a few tens of ohms to a few hundred ohms. This low output impedance makes the common base configuration suitable for driving low-impedance loads.
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Current Gain (α): The current gain (α) of a common base configuration is always less than 1. This means that the collector current (Ic) is slightly less than the emitter current (Ie). The current gain is typically in the range of 0.95 to 0.99.
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Voltage Gain: The voltage gain of a common base configuration is typically greater than 1, which means that the output voltage can be higher than the input voltage. The voltage gain is determined by the ratio of the collector resistance to the emitter resistance.
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Phase Shift: The output signal in a common base configuration is in phase with the input signal.
Advantages of the Common Base Configuration
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High Input Impedance: This characteristic allows the common base configuration to amplify signals with low input currents, making it suitable for applications where a low current source is used.
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Low Output Impedance: This characteristic ensures efficient power transfer to low-impedance loads, such as speakers and other audio devices.
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Wide Bandwidth: The common base configuration exhibits a wide bandwidth, making it suitable for high-frequency applications.
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High Current Gain: Although the current gain is slightly less than 1, the common base configuration can still provide significant current amplification for specific applications.
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Low Noise: The common base configuration produces less noise compared to other configurations, making it suitable for applications requiring low noise levels.
When are BJTs Used in Common Base Configuration?
The common base configuration finds application in various electronic circuits, particularly when specific characteristics are desired. Here are some key scenarios where BJTs are used in common base configuration:
1. High-Frequency Amplifiers: The wide bandwidth and low output impedance of the common base configuration make it ideal for amplifying high-frequency signals. For example, in radio frequency (RF) amplifiers, the common base configuration is frequently used to amplify signals in the VHF and UHF bands.
2. Current Buffer: The common base configuration can act as a current buffer, providing a high current output while maintaining a low output impedance. This makes it useful in circuits where a high current source is required to drive a low-impedance load, such as in power amplifiers.
3. Impedance Matching: Due to its high input impedance and low output impedance, the common base configuration can effectively match impedances between different circuits. This is particularly useful in applications where signal transmission between high and low impedance circuits is required, such as in audio amplifiers.
4. Low Noise Amplifiers: The low noise performance of the common base configuration makes it suitable for applications requiring low noise levels, such as in preamplifiers and instrumentation amplifiers.
5. Transistor Switches: The common base configuration can also be used as a transistor switch. The base terminal can be used to control the flow of current from the emitter to the collector. This application is particularly useful in circuits where a high-speed switch is required.
Conclusion
The common base configuration is a versatile BJT amplifier configuration with unique characteristics that make it suitable for a wide range of applications. While it may not be as widely used as the common emitter or common collector configurations, its high input impedance, low output impedance, wide bandwidth, and low noise performance make it an ideal choice for high-frequency amplifiers, current buffers, impedance matching circuits, low noise amplifiers, and transistor switches. By understanding the advantages and applications of the common base configuration, engineers can leverage its unique capabilities to design efficient and reliable electronic circuits for various purposes.