44.1.1. Lecture – 44: Common Collector and Common Drain Amplifiers

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 it is Common Collector and Common Drain Amplifiers.

In this chunk, the lecturer introduces the topic of Common Collector and Common Drain Amplifiers. These amplifiers are integral components in analog electronic circuits used for signal amplification.

Think of an amplifier like a microphone that increases the sound level of a singer's voice so it can be heard in a large concert hall. Similarly, amplifiers boost weak electronic signals for processing.

We are in week-5 and we are discussing about the building blocks, specifically we are going to discuss as I said common collector amplifier and common drain amplifiers. We will discuss about the basic operation and biasing, and then also we will be discussing about circuit analysis to find its performance parameter expressions.

The instructor outlines the focus areas for the week, emphasizing a detailed understanding of two types of amplifiers: the Common Collector and the Common Drain. Students will learn about their basic functionalities, how to bias them, and how to analyze their performance metrics.

Imagine you're assembling a toy model. Each piece represents a concept: the operation of the amplifier, how to bias it correctly, and how to check if it works as expected. Just like ensuring each part fits together correctly makes the model function, understanding each concept is crucial for mastering electronic circuits.

So, the concepts we are going to cover in today’s discussion it is the following. 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 and then input capacitances.

The instructor mentions the motivation behind exploring new amplifier configurations. These configurations, namely the Common Collector and Common Drain, are chosen to address limitations observed in other types of amplifiers, particularly their performance in cascading stages.

Consider a team working on a project. They might shift strategies or use new tools (like switching to these amplifiers) to overcome challenges they faced with their previous approach. This adaptability is crucial for improving overall project success.

What we have seen here, it is suppose this is the small signal equivalent circuit and then small signal equivalent circuit of the second stage, and then if you directly connect it what we have observed that the input resistance and then output resistance of the previous stage, they were dividing the signal.

This chunk discusses the limitations encountered when cascading common emitter or common source amplifiers. Specifically, it notes that the input and output resistances can interfere with the input signal, degrading the overall performance.

If you think of passing a baton in a relay race, any miscommunication or physical obstruction can result in a slower time. Similarly, the input and output resistances can obstruct signal flow, leading to an overall drop in signal strength.

So, what is the solution for that? It is we can use a buffer in between these two circuits and if you have some specific buffer protecting the previous stage of the first stage from the loading effect coming from the second stage, then we can say that the overall gain of the system or overall the amplifier performance it remains intact even if you are cascading it.

This section introduces the concept of a buffer as a solution to the problems identified when cascading amplifiers. A buffer helps isolate stages, ensuring that the signal remains strong without being affected by the preceding stage's loading.

Consider a speaker using a microphone in a crowded room. A buffer acts like a sound mixer, balancing what is picked up by the microphone without letting it be overwhelmed by nearby noise, allowing for a clearer output.

And what are the basic requirements of this voltage amplifier? It is that the input resistance here it will be it should be as high as possible and on the other hand input capacitance would be as small as possible.

The lecturer emphasizes the key characteristics expected from a buffer amplifier: high input resistance, low output resistance, and small input capacitance. These traits collectively enhance the signal preservation during amplification.

Think of a high-quality earphone: it should easily receive sound from your phone (high input resistance), while preventing interference from surrounding noise (low capacitance). This ensures you hear the music clearly.

If it is BJT based circuit it is common collector configuration, if it is mass based circuit then it is common drain configuration.

The discussion transitions to the specifics of the Common Collector and Common Drain amplifier configurations, both designed to meet the requirements outlined previously.

Much like how different vehicles serve unique purposes—like a car for family trips versus a bicycle for short errands—different amplifier configurations are optimized for various electronic applications.

So, let me start with the common drain first say. So, here we do have the basic common drain configuration are listed here. So, we do have the MOSFET here, and at the input namely at the gate we are feeding the signal.

In this chunk, the focus shifts to the specifics of the Common Drain configuration using a MOSFET. The gate receives input signals, while the output is taken from the source.

Imagine a faucet where you control the flow of water at the top (the gate) and the water flows out from the bottom (the source). The water pressure and flow rate are analogous to the signals in the amplifier circuit.

So, what we like to say that the previous stage DC voltage, whatever the previous stage DC voltage may be there that may be good enough along with whatever the signal we do have.

This section explains how the Common Drain configuration adapts to fluctuations in the gate voltage while maintaining a stable current through the source, ensuring reliable operation.

Think of a thermostat: it adjusts the heating based on the current temperature but maintains a constant desired environment. The Common Drain amplifier similarly maintains a stable current irrespective of small changes in voltage.

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