Input Stage of Multistage Amplifier

The input stage of a multistage amplifier is responsible for receiving the input signal and providing amplification. It is the first stage in the amplification process and plays a crucial role in determining the overall performance of the amplifier. There are different configurations that can be used as the input stage, including the common base, common emitter, and common collector configurations.

Common Base Configuration
The common base configuration is one of the possible choices for the input stage of a multistage amplifier. In this configuration, the base terminal of the transistor is the input terminal, the collector terminal is the output terminal, and the emitter terminal is common to both input and output. Some key points about the common base configuration are:

- The input impedance of this configuration is relatively low, allowing for effective impedance matching with the signal source.
- It provides a high voltage gain, making it suitable for applications where a high voltage amplification is required.
- The output impedance is relatively high, which may lead to a mismatch with the subsequent stages of the amplifier.

Common Emitter Configuration
The common emitter configuration is another option for the input stage of a multistage amplifier. In this configuration, the emitter terminal is the input terminal, the collector terminal is the output terminal, and the base terminal is common to both input and output. Some key points about the common emitter configuration are:

- The input impedance is relatively high, which makes it suitable for driving the subsequent stages of the amplifier.
- It provides a high voltage gain and a relatively high power gain, making it suitable for applications where both voltage and power amplification are required.
- The output impedance is relatively low, ensuring good impedance matching with the subsequent stages of the amplifier.

Common Collector Configuration
The common collector configuration, also known as the emitter follower configuration, can also be used as the input stage of a multistage amplifier. In this configuration, the collector terminal is the input terminal, the emitter terminal is the output terminal, and the base terminal is common to both input and output. Some key points about the common collector configuration are:

- The input impedance is relatively high, making it suitable for driving the subsequent stages of the amplifier.
- It provides a voltage gain slightly less than unity (less than 1), but it has a high current gain.
- The output impedance is relatively low, ensuring good impedance matching with the subsequent stages of the amplifier.

Conclusion
While all three configurations have their advantages and disadvantages, the common base configuration is often preferred as the input stage of a multistage amplifier. It provides a low input impedance for effective impedance matching with the signal source and offers a high voltage gain. However, the specific choice of configuration depends on the requirements of the application and the desired performance characteristics of the amplifier.