61.5. Lecture – 61

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 introduction, Professor Pradip Mandal welcomes the students back to the course on Analog Electronic Circuits. He states that the discussion will focus on Multi-Transistor Amplifiers, particularly the Cascode Amplifier. This builds on previous discussions, indicating a structured curriculum where each lecture is interconnected, suggesting that knowledge from past lectures is vital for understanding the current topic.

Think of this structure like building blocks. Each block represents a concept from previous lectures, and Cascode Amplifiers are the blocks being added on top of a solid foundation. Just like how you wouldn’t build a tower on unstable ground, in electronics, understanding the basics is crucial before moving to more complex structures.

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. And we are going to discuss about Cascode Amplifiers...

In this segment, Professor Mandal summarizes the previous topics that have been covered in the course, including various amplifier configurations such as Common Emitter (CE), Common Collector (CC), Common Source (CS), and others. He explains that the upcoming lecture will specifically focus on Cascode Amplifiers, highlighting its significance in the context of the previously discussed configurations.

Imagine you’re preparing for an exam by reviewing previous chapters. Each chapter is important because it builds your understanding for the next one. In this case, the Cascode Amplifiers are like the final chapter in a book that ties all the concepts from earlier chapters together, making the learning journey cohesive.

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.

Here, the definition of a Cascode Amplifier is explained. It’s noted as a combination of two types of amplifiers: the Common Emitter (CE) and Common Base (CB) for BJTs, and similarly, the Common Source (CS) and Common Gate (CG) for MOSFETs. This information emphasizes the versatility and application of the cascode configuration in both BJT and MOSFET technologies.

Think of the Cascode Amplifier like a two-person relay team in a race. The first person (the CE stage) runs and hands off the baton (the signal) to the second person (the CB stage), who then continues to the finish line (the output). Just as a well-coordinated team can complete a race more efficiently, this amplifier configuration can enhance performance through its stages.

So, to start with you may recall we made a summary that a different performance matrices of CE amplifier... So, we need to basically revisit this important property of CE and CB to motivate ourselves that combining CE and CB it is giving us a better performance.

This chunk revisits the performance characteristics of the CE and CB amplifiers. The speaker notes that while CE amplifiers generally offer high gain, CB amplifiers excel in other properties, such as low input resistance. Understanding these metrics is crucial for appreciating how the combination of CE and CB in a Cascode Amplifier can lead to improved performance overall.

Consider two athletes with different strengths: one is a sprinter (CE with high gain) and the other is a marathon runner (CB with endurance). When they team up (forming the Cascode Amplifier), they leverage each other's strengths, resulting in better overall performance than either could achieve alone.

So, this is the corresponding circuit configuration common base circuit configuration and you may recall that it is voltage gain it is quite good, but we assume that the signal source resistance is 0...

This section describes the properties of the Common Base (CB) amplifier configuration. While the voltage gain is favorable, the input resistance is low, which makes it less suitable for direct voltage feeding. However, this configuration shines in applications requiring low input resistance but high output resistance, such as current buffer applications.

Think of the CB amplifier as a skilled bouncer at a nightclub. The bouncer allows high-energy guests (high voltage signal) to enter while being selective about who they let in (low input resistance). This ensures that the club maintains a high-energy atmosphere for those compatible with the environment (high output resistance).

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