61.6. Multi-Transistor Amplifiers: Cascode Amplifier (Part A)

Dear, students welcome back to our NPTEL online certification course on Analog Electronic Circuits, myself Pradip Mandal from E and ECE department of IIT Kharagpur. And today’s topic of discussion, it is Multi - Transistor Amplifiers in fact, this is continuation of our previous discussion.

In this part of the lecture, the instructor welcomes the students and introduces the topic of Multi-Transistor Amplifiers, specifically focusing on Cascode Amplifiers. He emphasizes that this topic is an extension of previous discussions related to different amplifier configurations.

Imagine you're learning about different types of vehicles – cars, trucks, and buses. Each serves a different purpose, much like different amplifier configurations each have unique characteristics and applications.

Now the plan overall plan if you see according to our weekly plan so far we have covered CE – CC; CS - CD and CC - CC; Darlington pair etcetera both theory as well as numerical examples.

The instructor reviews previous lessons where different amplifier configurations like Common Emitter (CE), Common Collector (CC), Common Source (CS), and others were discussed. This sets the context for the introduction of the cascode amplifier.

Think of a chef who is learning various cooking techniques – grilling, boiling, and frying. Each technique builds on what was learned earlier, preparing for more complex recipes, just as each amplifier builds on previous knowledge.

So, I should say this is combination of common emitter amplifier with common base. So, this is BJT version and this one is a MOS version common source followed by common gate.

A cascode amplifier is formed by cascading two different types of amplifiers, specifically a Common Emitter (CE) followed by a Common Base (CB) for BJTs and a Common Source (CS) followed by a Common Gate (CG) for MOSFETs. This configuration enhances performance, particularly gain.

Think of a relay race, where one runner passes the baton to the next. The CE stage is like the first runner, while the CB stage is the next, working together to improve the overall performance of the relay.

But prior to that since it is essentially coming from a common source followed by common gate. So, we shall start with common source followed by common gate configuration and then we will simplify to conclude to cascode amplifier using MOSFET.

Before diving into the specifics of the cascode amplifier, the instructor mentions that the principles will be introduced by first discussing the common source and common gate configurations of MOSFETs, leading to an understanding of how they come together to form a cascode amplifier.

This is akin to building a house: starting with the foundation (common source), adding walls (common gate), and finally creating a complete structure (cascode amplifier) that is much stronger than any individual component.

I like to say that you might have observed that if we consider simply common emitter amplifier, its gain is typically quite high more than 100. On the other hand if I consider common source amplifier it is gain it is not so high.

The instructor points out that while the common emitter amplifier has a high gain (more than 100), the gain for a common source amplifier tends to be lower. The significance of using a cascode amplifier arises from the need for higher gain in the MOSFET configuration, which can be addressed by this unique arrangement.

Imagine trying to lift a heavy object: a single person may struggle to lift it alone, but if two people (representing different stages of amplification) work together, they can manage much more efficiently.

So, to start with you may recall we made a summary that a different performance matrices of CE amplifier. So this is the basic CE amplifier configuration and then we also have different performance matrices and their expressions.

The discussion shifts to the performance characteristics of the CE amplifier, which serves as a reference point for understanding how the cascode amplifier improves on certain metrics. Understanding these characteristics allows students to appreciate the rationale behind using the cascode configuration.

Consider a student preparing for an exam: they first review their previous test scores (performance metrics) to identify weaknesses, which informs how they study for the next exam (improving their performance through the new configuration).

So, if we put say performances of CE and CB together to construct a new configuration called CE-CB, then let us see what kind of performance we do expect.

Combining the strengths of the Common Emitter (CE) and Common Base (CB) amplifiers culminates in a new configuration that benefits from the performance metrics of both, resulting in enhanced voltage gain. This section emphasizes the importance of understanding how different configurations can work synergistically.

It's like combining two different genres of music: the rhythm of one (CE) complements the melody of another (CB), creating a new, harmonious piece that is greater than the sum of its parts.

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