Chapter 9: Problem 35

Explain how an audio amplifier works, using the transistor concept.

Short Answer

Expert verified

An audio amplifier using a transistor works by amplifying an input audio signal through the control of current flow between its emitter and collector terminals in response to the input signal applied at the base-emitter junction. The input audio signal is fed to the base-emitter junction through a coupling capacitor, which blocks any DC component. Proper transistor biasing ensures it operates in the linear region for efficient amplification. The amplified signal at the collector is then processed and sent to a load, such as a speaker. Multiple transistor stages can be employed for higher amplification and better performance.

Step by step solution

Understanding Transistors

A transistor is a semiconductor device consisting of three layers of semiconductor material, called the emitter, base, and collector. Transistors can be either NPN (Negative-Positive-Negative) or PNP (Positive-Negative-Positive) type, depending on the arrangement of N-type and P-type materials. The primary function of a transistor is to control the flow of current between the emitter and the collector, through the base.

Transistor Operation

In order to control the current flow in a transistor, a small amount of current is applied to the base-emitter junction (input). When this input current is above a certain threshold, the transistor becomes 'ON', allowing a larger current to flow from the emitter to collector (output). This output current is proportional to the input current. Thus, a transistor essentially acts as a current amplifier with the ability to control the current flow between its terminals.

Amplification with Transistors

The ability of a transistor to control and amplify current makes it suitable for use in audio amplifiers. An audio amplifier increases the amplitude of a weak input audio signal (such as from a microphone or a music player) so that it can be heard through a speaker or headphones. When the weak input signal is applied to the base of a transistor, it causes a proportionately larger current to flow from the emitter to the collector. This amplified output signal can then be used to drive a load, such as a speaker.

Input Stage

In an audio amplifier using a transistor, the first stage is the input stage. The input audio signal (AC signal) is fed to the base-emitter junction of the transistor through a coupling capacitor. The coupling capacitor blocks any DC component of the input signal, thereby allowing only the AC component to be amplified.

Biasing

Proper biasing of the transistor is essential for efficient amplification. Biasing is the process of setting up a specific DC operating point for the transistor so that it can function in the active region and amplify the input signal effectively. This is achieved by providing a DC bias voltage to the base-emitter junction through a voltage divider network, ensuring that the transistor operates in its linear region.

Amplification

Once the input signal is applied to the base and the transistor is biased, the process of amplification occurs. As the input signal varies, the base current varies as well. This results in a proportional change in the emitter-to-collector current, thus amplifying the signal. The ratio of the change in output current to the change in input current is called the current gain (\(h_{\text{FE}}\)) or \(\beta \) of the transistor.

Output Stage

The amplified signal at the collector of the transistor is further processed to remove any DC component using another coupling capacitor. Finally, the amplified output signal is sent to a load such as a speaker or headphones, where it can be heard.

Multiple Stages

In practical audio amplifiers, multiple transistor stages are used to achieve higher amplification and better overall performance. These stages may include additional amplification, buffering, and impedance matching to suit the requirements of the specific application. In summary, an audio amplifier using a transistor amplifies an input audio signal by allowing a larger current to flow between its emitter and collector terminals in response to the input signal applied to its base-emitter junction. Through proper biasing and the use of multiple stages, an efficient and high-quality audio amplification can be achieved.

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Explain how an audio amplifier works, using the transistor concept.

Short Answer

Step by step solution

Understanding Transistors

Transistor Operation

Amplification with Transistors

Input Stage

Biasing

Amplification

Output Stage

Multiple Stages

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