61.2. Prof. Pradip Mandal

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 which are essentially I should say CE, CE followed by CB and CS followed by CG.

In this introduction, Prof. Pradip Mandal outlines the weekly course plan and previous topics covered related to transistor amplifiers. The focus will now shift to Cascode Amplifiers, a combination of Common Emitter (CE) and Common Base (CB) configurations for BJTs, and Common Source (CS) followed by Common Gate (CG) configurations for MOSFETs. This shows how different transistor configurations contribute to amplifying signals effectively.

Think of a cascode amplifier like a relay race in sports. Just as each runner passes the baton to the next runner to complete the course more quickly, the cascode amplifier passes the signal efficiently from one stage to the next for better performance.

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 its gain is not so high. So, we must be having some alternative for particularly for MOSFET version otherwise that circuit may not be really much of an use.

Here, Prof. Mandal compares the gains of Common Emitter (CE) amplifiers and Common Source (CS) amplifiers. While CE amplifiers can provide a high gain (often greater than 100), traditional CS amplifiers may not, prompting the need for configurations like the cascode to enhance the gain in MOSFET applications.

Imagine you are trying to get a message across a crowded room. A megaphone (CE amplifier) helps you shout your message loudly, while speaking normally (CS amplifier) might not be enough to be heard by everyone. To ensure that more people hear your message, you might use a relay system (cascode amplifier) where one person's loud voice helps amplify the message to the next, who then passes it along further.

So, two configurations common collector and common base they do have their dual property and they are essentially used as voltage mode and current mode buffer respectively.

Prof. Mandal explains that the Common Emitter (CE) and Common Base (CB) configurations serve different purposes in amplifier circuits. CE is often used as a voltage amplifier, while CB is used as a current amplifier. By combining these configurations, one can significantly improve amplifier performance, as each configuration complements the other.

Picture a multi-step cooking process. The first step (CE) might involve boiling vegetables for flavor (voltage gain), while the second step (CB) involves pureeing them to make a sauce (current gain). Both steps are necessary and, when combined, they enhance the dish's overall outcome.

Now we will see that in fact, the condition of the of the DC operating point of CE and the CB stage is in this connection they are remaining isolated, but then of course, the signal it is going from the first stage to the to the second stage at its input.

In this segment, Prof. Mandal indicates that while the DC operating points of the CE and CB stages remain isolated to avoid any interference, the AC signal can nonetheless pass from one stage to the next. This isolation helps maintain circuit stability while allowing signal processing to continue unhindered.

This is similar to organizing a two-roomed party where a DJ in one room plays music (CE) while a dancing floor in another room awaits (CB). While the DJ is loud, the rooms are separated by a wall (isolation), keeping the sound manageable while allowing partygoers to enjoy the music in both spaces.

So, I should say that cascode circuit, it is essentially CE-CB, but I should say it is simplified or modified version.

In this conclusion, Prof. Mandal sums up the cascode amplifier as a streamlined version of the combined CE and CB configurations. By adjusting and modifying these setups, the cascode configuration maximizes performance, particularly in achieving higher gain and better stability in amplifying signals, especially in MOSFET designs.

Think of the cascode configuration like an improved bicycle design—while a standard bike (CE-CB) can get you around well, an upgraded model (cascode) with better gears and streamlined structure allows you to climb hills faster with less effort, symbolically representing better performance in the amplifier context.

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