55.1.4. Lecture – 55

Yeah, dear students welcome back to the online certification course of Analog Electronic Circuit. Myself Pradip Mandal from E and EC Department of IIT, Kharagpur, today’s discussion it is Multi Transistor Amplifiers. So far we have discussed about different amplifier configurations, so where one transistor primarily you know single transistor it was doing the amplification and other activities. So, the transistor it was either BJT or MOS transistor and for different configurations we have seen the merits and demerits of the different configurations.

This chunk introduces the topic of multi-transistor amplifiers, which are more complex setups that utilize more than one transistor. The lecturer, Pradip Mandal, is coming from a background in electronics and is guiding the course. Previously discussed were simpler amplifier configurations, which mainly used a single transistor for amplification (like BJT or MOSFET). Each configuration has its own advantages and disadvantages.

Think of a single transistor amplifier like a stand-alone speaker in a room, amplifying sound by itself. Multi-transistor amplifiers, on the other hand, can be likened to a symphony orchestra where multiple instruments (transistors) work together to produce a richer sound experience.

Based on that merits and demerits we shall try to combine different configurations together, so that we can get overall better performance. So, let us see what is the overall plan in the next slide.

The lecturer emphasizes the goal of combining different transistor configurations to improve overall amplifier performance. This combination is based on the strengths and weaknesses discussed earlier. The approach aims to achieve better characteristics than what can be obtained from a single configuration.

Imagine a cooking recipe where you combine various ingredients; just as combining certain items can enhance flavors, combining different amplifier configurations can enhance signal amplification capabilities.

But, before going to the overall plan let me see our alignment with our course layout. We are in week-6 and we are in between of the block level and module level. In fact, we are will be moving back and forth here. And, so, philosophically we can say there these are building blocks or they may be modules and to be precise whatever the topic we will be discussing here it is common emitter, common collector cascaded together. Then may be common source and common drain configurations cascaded together. Then common collector and common collector cascaded together and so and so.

This segment explains the current progression in the course outline. The course is designed to systematically build knowledge, and the lecturer notes that this segment focuses on combining different configurations like common emitter (CE) and common collector (CC) to enhance functionality. The course layout is structured to facilitate a gradual understanding.

Consider learning to build a house: you first lay down the foundation (understanding basic concepts), then construct the walls (adding complexity with configurations), and finally, you would add roofs and interiors (excel in combining them for optimal performance).

So, the topic we will be covering today it is enlisted here. We shall start with the motivation of going for mixing different configuration together and then we basically we will summarize whatever the earlier discussion we are having.

In this portion, the motivation behind mixing different transistor configurations is introduced. This helps in overcoming the limitations inherent in single configurations, allowing a better functional outcome. The discussion is expected to summarize previous topics while setting the stage for understanding the benefits of these combinations.

This is similar to how athletes often cross-train in different sports; by improving skills in one sport (configuration), they can enhance their performance in another. Combining techniques yields stronger, more versatile outcomes.

So, if you see the common emitter followed by a common collector, its purpose it is to decrease the output impedance. So, we can say that conceptually we can decrease the output impedance of existing amplifier by simply cascading one common collector stage.

This part elaborates on how the arrangement of a common emitter stage followed by a common collector stage can effectively lower output impedance. Output impedance is critical for good amplifier performance because lower output impedance can enhance power transfer to the load.

Think of this like a water pipeline: if the pipe diameter (impedance) is reduced, the flow of water (signal) becomes less restricted, leading to more efficient delivery to the end point (the load).

So, likewise if we have say common collector amplifier already and then whatever the output impedance is coming from the given common collector amplifier, if you want to further decrease it is output impedance you can cascade with another common collector stage, so that the overall output impedance it will be even lower than that.

This chunk discusses the implications of cascading additional stages, specifically using a common collector amplifier to reduce the output impedance even further. Each amplifier stage can contribute to an overall reduction effect when designed correctly.

• Chunk Title: Increasing Input Impedance
• Chunk Text: So, conceptually again we can increase the input impedance of an existing amplifier by connecting one common collector stage at the input side.
• Detailed Explanation: Here, the aim is to increase the input impedance of an amplifier by using a common collector stage at the input. This is particularly significant in ensuring minimal loading effect on the previous stage, thus maintaining signal integrity.

This can be likened to a very sensitive microphone that requires a lightweight protective case; the protective case (the common collector stage) allows sound to be captured without interfering with the microphone's operation.

So, these are the discussion it is primarily on BJT circuit. In fact, we do also have a special popular configuration called Darlington pair we shall see.

This chunk indicates that most of the discussion focuses on BJT circuits and introduces the Darlington pair, a configuration that can achieve high current gain very efficiently. It showcases the importance of BJTs in functionality and performance enhancement.

Just as two people working together can achieve a greater task than if they worked alone (like a team in a relay race), the Darlington pair allows two transistors to work together for improved performance.

And, then we do have the most counterpart namely we can decrease the output impedance of say one common source amplifier existing common source amplifier by cascading; by cascading with a common drain amplifier.

This segment explains how similar principles apply in MOSFET circuits where cascading a common drain configuration can also reduce output impedance, thereby enhancing overall performance.

Think about traffic flow in a multi-lane highway; when additional lanes are added after a constriction, the overall flow improves. Similarly, cascading configurations can smooth out impedance issues.

Note that the common source amplifier since its gate impedance is very high we need not to precede the common source amplifier by another see common drain configuration which means that in BJT both we do have scopes to in in change or improve the output impedance as well as the input impedance.

In this portion, a distinction is made between BJT and MOSFET configurations. Specifically, it states that the high gate impedance of a common source amplifier allows for straightforward cascading without needing additional configurations, highlighting differing approaches in improving amplifier performance.

This can be like having a high-performance water filter; it doesn't always require multiple pre-filters to function optimally, showcasing efficiency in setup.

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