44.1. Analog Electronic Circuits

So, dear students welcome back to our NPTEL online certification course. The course title it is Analog Electronic Circuit, and now myself Pradip Mandal from E and EC Department of IIT, Kharagpur. Today’s topic of discussion is Common Collector and Common Drain Amplifiers. Based on our overall flow, let us see where we stand. We are in week-5 and we are discussing the building blocks, specifically we are going to discuss common collector amplifier and common drain amplifiers.

This introduction sets the stage for discussing two important types of amplifiers used in analog electronic circuits: the common collector and common drain amplifiers. The speaker, Professor Pradip Mandal, emphasizes his role and the context of the course, specifying that they are in the fifth week of content, focusing on amplifier building blocks. Understanding the structure of amplifiers is crucial as it impacts their performance in circuits.

Think of an amplifier like a water pipe system. Just as pipes can direct and manage water flow, amplifiers control and enhance electrical signals. In this course, we will explore how to build and optimize these 'pipes' for better 'water' flow in our electronic circuits.

We shall start with the motivation of going for this new configuration namely common collector and common drain amplifiers, and then basic operation biasing, and then analysis for specifically for voltage gain, input and output impedance of those amplifiers.

The motivation for using common collector (BJT) and common drain (MOSFET) configurations arises from limitations experienced with previous amplification stages, such as common emitter and common source amplifiers. These limitations often include signal degradation and issues with bandwidth when connecting multiple stages of amplifiers together. By using common collector and common drain configurations, one can achieve high input impedance, low output impedance, and a voltage gain of approximately one, which prevents significant signal loss during cascading stages.

Imagine you have multiple speakers linked together to amplify sound. If one speaker is inefficient, it can muddy the sound for all others. By using a better amplifier configuration, much like a clearer speaker setup, we ensure each part of our system works harmoniously without losing quality.

What we have seen is that when cascading two stages by connecting the output of one CE amplifier to the input of the next CE amplifier, the input resistance and output resistance of the previous stage divide the signal. As a result, the signal arriving to the input of the second amplifier is not the same as the signal obtained in unloaded condition.

In this chunk, the speaker describes the issues that arise when connecting multiple stages of amplifiers (common emitter amplifiers in this case). The interaction between the output resistance of the first amplifier and the input resistance of the second causes signal attenuation. Thus, the second amplifier does not receive the full signal strength that it would have in isolation, leading to degraded performance.

This can be likened to a game of telephone. If the first person whispers a message to the next and the next person is far away or mishears it, the message can become garbled. To ensure the message remains clear, we need to strengthen the communication method, similar to how an effective amplifier configuration maintains signal integrity.

The solution is to use a buffer between these two circuits. If we have some specific buffer protecting the previous stage from the loading effects coming from the second stage, then we can say that the overall performance remains intact even if we are cascading it.

The concept of a 'buffer' is introduced as a practical solution to the issues faced when cascading amplifiers. A buffer acts as an intermediary stage that prevents the load from affecting the previous amplifier's performance. This allows the full signal to be delivered to the next stage without degradation.

Consider a relay race where each runner passes a baton. If the next runner is too slow or too weak, the baton might drop, slowing down the whole team. A buffer here would be like a strong athlete who ensures that the baton is passed seamlessly, maintaining the team's overall speed and performance.

The basic requirements of this voltage amplifier are that the input resistance should be as high as possible, and the output resistance should be as small as possible. The input capacitance would also be as small as possible.

To function effectively, buffer circuits must meet certain criteria: high input resistance to avoid signal loss, low output resistance to efficiently drive the next stage, and low input capacitance to enhance bandwidth and speed. Each of these attributes ensures that the buffer can perform its role without interfering with the signal.

Think of a strong fan pushing air into a room. The fan's ability to draw in air (high input resistance) without letting it escape inefficiently (low output resistance) and with minimal blockage (low input capacitance) determines how well it circulates air, similar to how a good buffer enhances signal strength in electronics.

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