61.4. Indian Institute of Technology, Kharagpur

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. 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 this should be CE, CE followed by CB and CS followed by CG.

In this chunk, the lecturer welcomes students and introduces the topic of cascode amplifiers. It is emphasized that this discussion follows previous topics covered in the course, including various transistor amplifier configurations. The lecture's focus is on cascode amplifiers that combine different types of transistor amplifiers (common emitter followed by common base and common source followed by common gate). This helps set the context for understanding how these amplifiers work together to improve performance, especially in terms of gain.

Think of amplifiers like layers of a cake. Each layer represents a different type of amplifier configuration. Just like a cake is more enjoyable with all its layers combined, using multiple transistor configurations together can enhance overall performance, leading to a better amplification 'treat.'

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 and qualitatively we said that some of them are high, some of them are not so high or whatever it is and in fact, even though R it is high it is not good for voltage amplifier.

This section discusses the performance metrics of common emitter (CE) amplifiers. It highlights the importance of understanding various performance parameters, such as voltage gain, input resistance, and output resistance. The lecture notes that while a CE amplifier can have a high gain, it may not always be suitable for voltage amplification due to limitations in input or output resistance. Recognizing these metrics helps design better amplifiers that effectively meet specific application needs.

Imagine you are trying to amplify the sound of a music player using a speaker. If the speaker (analogous to the amplifier) is too powerful for the music player (source), it may distort the sound instead of enhancing it. Similarly, in amplifiers, having the right balance of performance metrics ensures amplifying the signal effectively without distortion.

So likewise we also have discussion related to a common base, common collector, this part we already have discussed so we will not be covering now. 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 touches upon other amplifier configurations, specifically common base (CB) and common collector (CC) amplifiers. The understanding of these configurations is essential as they each possess unique properties that can complement the performance of a common emitter amplifier when combined. The idea is that by using a combination of amplifier types, one can enhance overall performance, particularly in terms of gain and input/output impedance characteristics.

Consider a team working on a group project. Different team members have distinct strengths: one may excel in planning (common emitter), another in executing tasks efficiently (common base), and yet another in communicating outcomes (common collector). By leveraging each member’s strengths together, the team achieves better results than any individual could achieve alone.

So, I should say that cascode circuit, it is essentially CE-CB, but I should say it is simplified or modified version. So, here we do have the CE-CB amplifier whereas, if you see here this Q in fact, whatever the DC voltage you do have here that may be useful, I should not say DC voltage rather I should say the current.

The chunk introduces the concept of the cascode amplifier as a modified version of the CE-CB configuration. It points out how the design is tailored to ensure that the DC voltage and currents in the circuit enhance performance without affecting the operating point of the transistors involved. This modified connection is crucial since it helps maintain the stability and performance of both the CE and CB stages in conjunction.

Think of a cascode amplifier like a sturdy bridge that supports two roads (the CE and CB stages) over a river. The bridge’s design must ensure that neither road is affected by the river’s current (DC voltage/current) beneath it, allowing both roads to function smoothly without interference.

So, what we said here it is DC current of Q it is supplying require DC current of Q , on the other hand we are feeding the signal at the base of Q and we are expecting that this signal it will be producing signal current here.

This portion discusses how the biasing of the cascode amplifier works. The current from one transistor supports the bias needs of another, leading to a more efficient setup. This current sharing helps maintain the operating points of both transistors while allowing for effective signal amplification. The proper functioning of this arrangement is essential for optimizing performance and avoiding saturation of either transistor.

Imagine two friends planning a trip where one is responsible for driving (Q1) and the other for navigation (Q2). If the driver runs low on fuel (bias current), the navigator cannot effectively map out the route. However, if they support each other—one refueling while the other checks directions—they can ensure the trip goes smoothly and efficiently.

So, we may require additional circuit we may require really a clever circuit here which offers high value of this R. Assuming that it is possible to get that then we will be getting very good gain out of good voltage gain out of this one.

This portion outlines how to achieve high voltage gain with the cascode amplifier configuration. The lecturer indicates that an additional clever circuit may be necessary for increasing performance further by ensuring high values of specific resistances. The relationship between resistance, current, and voltage gain is critical to understanding how to achieve the desired amplifier characteristics.

Picture a factory producing toys. If the assembly line is designed efficiently (high resistance), it can handle larger quantities more quickly (gains in voltage). If adjusted correctly, the factory produces a larger number of toys per hour, akin to achieving a higher gain in the amplifier, illustrating the principle that properly designed systems can lead to enhanced output.

So, this is the output port and R which is R coming in parallel with whatever the resistance coming out of this entire circuit. So, if I want to know what will be the resistance of this circuit the circle circuit what it can say that this r and r , they are coming in parallel.

The final chunk discusses the output impedance of the cascode amplifier, emphasizing its importance and how it can be mathematically derived. Knowing the output impedance helps designers understand how efficiently the amplifier can drive loads without performance degradation. The relationship between the different resistances in parallel affects how the output impedance can be optimized for various applications.

Think of output impedance like the diameter of a water pipe. A wider pipe allows water (signal) to flow more freely (good performance). By analyzing how different pipe sections (resistances) work together in parallel, one can optimize the system for improved efficiency and effectiveness, just as adjusting output impedance can enhance an amplifier's performance.

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