Why the Common Base Amplifier Is Not Commonly Used as an Audio Amplifier

The common base amplifier is not commonly used as an audio amplifier for several reasons. This article delves into these reasons and explains why other configurations are generally better suited for audio amplification.

Input Impedance

The common base configuration has a low input impedance, typically much lower than that of common emitter or common collector configurations. This makes it less compatible with high-impedance sources which are common in audio applications. For example, a silicon transistor biased at 1 mA collector current has an input impedance of about 26 Ohms in its emitter. Doubling the collector current to 2 mA reduces the input impedance to about 13 Ohms. Most modern audio sources, such as microphones or line level inputs, do not have impedances as low as 26 Ohms. Only speakers used as microphones, such as those in intercom systems, might have impedances this low. If several audio sources need to be fed into the amplifier, a low input impedance in a common base amplifier could cause impedance-related issues.

Voltage Gain

While the common base amplifier can provide decent voltage gain, it generally does not offer as much gain as the common emitter configuration. Audio applications often require higher gain to drive speakers or other components effectively. Common emitter amplifiers are specifically designed to provide this high gain, making them a preferred choice for audio applications.

Phase Shift

The common base amplifier does not invert the signal; it maintains the same phase. This can be a disadvantage in certain audio applications where phase inversion is beneficial or required for circuit design. Other configurations, like the common emitter, allow for easier phase inversion which can be crucial in audio processing.

Limited Bandwidth and Frequency Response

Although common base amplifiers can have good high-frequency response, they may not perform as well at lower frequencies which are crucial for audio signals. The bandwidth and frequency response of audio applications typically require a broader range, especially for bass and mid-range frequencies. Other configurations such as the common emitter or common collector can offer better overall frequency response in audio applications.

Complexity and Size

In practical applications, the common base amplifier may require additional components to match impedance and improve performance. This can make the overall design more complex compared to other configurations. For instance, designing with operational amplifiers can simplify the circuit as they offer higher open-loop gain and better isolation between audio sources. This simplicity leads to lower distortion and better performance.

Common base amplifiers are more often used in RF (radio frequency) applications or in situations where low noise and high frequency response are critical. While they excel in these applications, they lack the necessary features to function optimally in audio environments.

Usage Context

Common base amplifiers are invaluable in RF applications and scenarios where low noise and high frequency performance are paramount. However, for audio amplification, other configurations are generally more suitable. This is due to their better performance in terms of input impedance matching, voltage gain, phase handling, bandwidth, and overall complexity.

Conclusion

While the common base amplifier has its advantages in specific applications, other amplifier configurations typically outperform it in audio applications. These alternatives offer better input impedance, higher gain, and more consistent phase response, making them more appropriate for driving speakers and other audio components effectively.

Note: The information provided in this article is based on industry standards and general principles of electronics design. Specific applications may require tailored solutions based on the exact requirements.