64.1.2. Department of Electronics and Electrical Communication Engineering

Prof. Pradip Mandal Department of Electronics and Electrical Communication Engineering Indian Institute of Technology, Kharagpur Lecture – 64 Multi-Transistor Amplifiers: Cascode Amplifier (Contd.)-Numerical Examples (Part B)

This section introduces Professor Pradip Mandal from the Department of Electronics and Electrical Communication Engineering at IIT Kharagpur. It sets the stage for Lecture 64, which continues the discussion on Multi-Transistor Amplifiers with a focus on the Cascode Amplifier and provides numerical examples to illustrate these concepts.

Imagine a university lecture series where each session builds upon the previous one. By introducing specific topics and examples related to cascode amplifiers, students are drawn into the practical applications of electronic circuits, similar to how they might engage in a series of cooking classes that gradually introduce new recipes and techniques.

Before I going to the next topic I must see here that this calculation of the C I did a small mistake hear, it should be 135 because C = C + in in π C (1 + 204) and so, here we do have 5 and also here we do have 10. So, I miss this 10 µ part. So, 10 + 1025.

In this passage, Professor Mandal corrects a previous calculation regarding capacitance, where he mistakenly omitted a value about a capacitance component (C). He emphasizes the need to be meticulous in calculations, saying that the correct value should be derived from a specific formula that considers additional elements of the circuit.

This can be likened to cooking, where missing an ingredient—like forgetting to add salt—can completely change the taste of a dish. Just as a cook must carefully measure and include every ingredient, engineers must meticulously calculate each component of a circuit to ensure it functions correctly.

So far what we have discussed that the advantage of cascode amplifier with respect to standard CE amplifier and namely what you have seen is that in case if you are retaining this passive element for both the cases, then gain wise we do not get much advantage. But then what we have seen that in case if we have the input resistance and the source resistance together forming a significant significantly low frequency pole due to this large value of the C, then we have seen that the cascode amplifier it is giving some advantage.

This chunk discusses how the cascode amplifier offers certain advantages over the conventional Common Emitter (CE) amplifier. While the gain does not show significant improvement with passive elements, the cascode amplifier excels in specific situations, such as when a low-frequency pole occurs due to the combination of input and source resistances.

Think of a highway that allows cars (current) to travel faster and further (gain) at higher speeds but slows them down at lower speeds. The cascode amplifier acts like the highway, optimizing performance based on the conditions, similar to how cars will handle certain terrains better than others.

The cascode amplifier it is having two types of advantages; one is extending the bandwidth as we just now we have discussed particularly in presents of significantly large value of the source resistance, the other advantage which is commonly used is that the increasing the gain drastically.

The cascode amplifier provides two main advantages: it can extend the bandwidth—especially when source resistance is large—and significantly increase the gain. This is particularly useful in applications that require high amplification without sacrificing performance across varying frequencies.

This can be compared to a multi-speed bicycle. When you shift to a higher gear (cascode amplifier), you can go faster (higher gain) on flat terrains, and it allows you to maintain efficiency over longer rides (extended bandwidth), just as an advanced amplifier can manage various signals effectively.

In fact, cascode amplifier it is having two types of advantages; one is extending the bandwidth as we just now we have discussed particularly in presents of significantly large value of the source resistance, the other advantage which is commonly used is that the increasing the gain drastically.

This discussion focuses on the practical use of the cascode amplifier in various circuit configurations. By using a higher resistance in the circuit (like 10 MΩ), the professor notes that the cascode amplifier’s ability to enhance gain becomes much more evident, demonstrating its utility in real-world applications.

Imagine a sports team that improves their performance by training differently (using a cascode configuration). The increase in resistance (like advanced practice techniques) allows them to maximize their strengths (gain) while maintaining endurance (bandwidth), yielding better results in competitions.

But of course, we have to keep in mind that while we have increase this one the gain, though the miller factor affecting the C increases the C, but the corresponding upper cutoff frequency is the it is not limiting factor for defining the bandwidth of the circuit rather the problem it is elsewhere.

While the cascode amplifier increases gain, it can also increase the Miller capacitance, which may influence the upper cutoff frequency of the circuit. The implication here is that while gain may improve, one must be cautious about how this adjustment may inadvertently restrict bandwidth. It is essential to find a balance between gain and frequency response for optimal performance.

Think of it as increasing the dimensions of a room (gain) to fit more furniture (signals), but doing so may reduce the air circulation (bandwidth). Like balancing space and comfort in interior design, engineers must carefully manage the trade-offs between gain and bandwidth.

So, that is about the cascode amplifier. So, depending on our application if you are looking for very high gain, but then we can if in case if we are we can tolerate with lower bandwidth, then we will be going for cascode amplifier.

In conclusion, the cascode amplifier is a powerful tool in electronics, especially useful when high gain is prioritized over bandwidth. It highlights the importance of choosing the right configuration based on specific needs in circuit design and applications.

This is similar to choosing a high-performance vehicle (cascode amplifier) designed for speed (gain) rather than one suited for fuel efficiency (bandwidth). Depending on your needs (such as racing vs. everyday driving), you would select the vehicle that best meets those criteria.

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