A Class B amplifier is a type of electronic amplifier used to amplify audio or radio frequency signals. It's specifically designed to improve the efficiency of signal amplification by minimizing power wastage during the amplification process. Class B amplifiers are commonly used in audio applications such as power amplifiers for speakers and in radio frequency applications.
The defining characteristic of a Class B amplifier is that it operates in a push-pull configuration, where two complementary active devices (usually transistors) work together to amplify the signal. These devices are configured so that each handles one half of the input waveform. One device amplifies the positive half of the waveform, while the other amplifies the negative half.
Key features of a Class B amplifier:
Push-Pull Configuration: As mentioned, Class B amplifiers use a push-pull configuration, where the two active devices work alternately to amplify both halves of the input waveform. This helps reduce distortion and improve efficiency.
Cutoff and Conduction: The push-pull arrangement in Class B amplifiers leads to a specific operating characteristic. Each transistor is biased to turn off when there is no input signal. They only conduct (allow current to flow through) when there is an input signal present.
Reduced Biasing: Since the transistors are biased to operate only when conducting, this configuration reduces the power dissipation during idle or no-signal periods. This results in higher efficiency compared to other amplifier classes, such as Class A.
Efficiency in Signal Amplification:
Efficiency in signal amplification refers to the ratio of useful output power to the total input power (which includes both the useful output power and any power wasted as heat). For Class B amplifiers, efficiency is relatively high due to the transistors' on/off operation and the push-pull configuration. When the input signal is small or absent, both transistors are mostly off, leading to minimal power consumption.
Theoretically, a Class B amplifier can achieve a maximum theoretical efficiency of 78.5%. However, due to practical limitations such as biasing requirements, crossover distortion (distortion at the point where the two halves of the waveform switch), and other non-ideal behaviors of real-world components, the actual efficiency may be somewhat lower.
To reduce crossover distortion and enhance the performance of Class B amplifiers, Class AB amplifiers were developed. Class AB amplifiers incorporate a small bias current to allow for a smooth transition between the positive and negative halves of the waveform, reducing distortion while still maintaining relatively high efficiency compared to Class A amplifiers.
In summary, a Class B amplifier is designed for efficiency in signal amplification by operating its active devices in a push-pull configuration, conducting only when necessary, and minimizing power wastage during idle periods.