40.1. Frequency Response Of CE/ CS Amplifiers Considering High Frequency Models Of BJT And MOSFET (Part A)

So, dear students so, we will come back to our NPTEL online certification course on Analog Electronic Circuits, myself Pradip Mandal from E and EC Department of IIT Kharagpur. Today’s topic of discussion it is Frequency Response of CE, CS Amplifiers Common Emitter and Common Source Amplifiers Considering High Frequency Model of BJT and MOSFET.

In this introductory segment, the professor sets the context for the lecture, focusing on the frequency response of two types of amplifiers: Common Emitter (CE) and Common Source (CS) amplifiers, specifically considering high-frequency effects. The aim is to understand how high-frequency models of Bipolar Junction Transistors (BJTs) and Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs) impact their performance.

Think of the frequency response as how a music speaker responds to different notes (frequencies). Just as a speaker might sound clearer at some notes than others due to its design, amplifiers can perform differently depending on their frequency response characteristics.

In fact, we already have started about this frequency response of CE amplifier and CS amplifiers, but there we did not consider capacitances associated with the MOS transistor itself. So, today’s discussion it is a we will see what will be the impact of the capacitances associated with the devices the transistors on its frequency response particularly for common emitter and common source amplifier.

Here, the professor revisits previous discussions on the frequency response of CE and CS amplifiers, noting that earlier analyses excluded the capacitances of the MOS transistors. The focus of today's lecture will be to explore how these capacitances affect the amplifiers' frequency response, which is crucial for high-frequency applications.

Imagine tuning an old radio. Initially, you might hear a lot of static (like not considering the capacitances), but once you adjust the antenna (akin to including capacitances), the sound becomes clearer and more refined, just as the frequency response improves when you account for those capacitances.

So, the concepts we are planning to cover today is the following. First of all we like to; we like to highlight the points that the impact of the high frequency response on the frequency response of CE and CS amplifiers and then we will see that there is a need of some theory, proposed by Miller called Miller’s theorem.

This segment outlines the plan for the lecture, emphasizing the importance of understanding how high-frequency response impacts both CE and CS amplifiers. The professor also introduces Miller's theorem, which will be used to simplify the analysis of the capacitative effects in the circuits. This sets the stage for a deeper understanding of circuit behavior at high frequencies.

Consider Miller's theorem as a shortcut or a recipe in a cooking class. Just like a recipe helps simplify complex dishes into manageable steps, Miller's theorem helps simplify the analysis of complex circuit behaviors, making it easier to understand their responses.

So, now our task is to find the frequency response of this equivalent amplifier representing both common emitter and common source amplifier. So, to analyze this circuit let we try to see that, what the additional things we have to do are.

The lecture transitions into practical circuit analysis, where the goal is to understand the frequency response of an equivalent amplifier model for both CE and CS amplifiers. The professor indicates that applying Miller's theorem will involve additional steps in the analysis, allowing the students to translate complex capacitance effects into simpler terms.

Think of this as solving a puzzle. At first glance, the pieces (the circuit elements) seem confusing. But by using concepts like Miller's theorem (your guiding image), you can start to see how those pieces fit together to reveal the bigger picture—the frequency response.

Now, we also consider since so, this C and C effectively they are providing input capacitance, input parallel capacitance called say C to see its effect it is also we consider the source resistance r .

In this part, the professor discusses the role of capacitors within the amplifier circuits, specifically C and C, which contribute to the input capacitance. Additionally, he emphasizes the influence of source resistance on the overall frequency response, indicating that without acknowledging these resistances, the analysis would lack accuracy.

Imagine trying to fill a swimming pool (the input capacitance) with a garden hose (the source resistance). If the hose is too small, it affects how quickly water fills the pool. Similarly, the source resistance can significantly influence how well signals are processed through the amplifier.

Now, if I consider generalized model and here we do have the generalized model of the two amplifiers namely common emitter amplifier and common source amplifier.

The discussion introduces a generalized model that encapsulates both CE and CS amplifiers, allowing for a unified analysis. By comparing these models, the professor highlights how capacitances influence input and output ports within the amplifier, which is pivotal for understanding their performance in real-world applications.

Think of a universal remote control designed to work with various devices. Just as this remote can manage different electronic equipment (like a TV or DVD player), the generalized model allows for the analysis of different amplifier types under similar principles.

Now, I must say that this splitting of this C capacitance a one for input port another is for output port, it is normally done by a theory proposed by Miller which is commonly known as Miller’s theorem.

This chunk focuses on Miller's theorem, which outlines how to effectively split the bridging capacitance from the input port to the output port into equivalent components. This technique simplifies analysis and calculation when evaluating frequency responses, especially in cases where capacitors have a significant impact.

Consider a large distillation tower in a factory, where the vapor travels both upwards and downwards at the same time. By dividing the vapor’s path into two manageable sections (like the input and output in Miller’s theorem), the operations can be monitored and optimized more effectively.

So, the in the next slide we consider a special case of this Z and we consider this C and then we will try to find what will be the corresponding equivalent capacitance, we do have a here and here.

The professor prepares to delve into a specific case application of Miller's theorem, applying the concepts to determine equivalent capacitances. This becomes relevant for practical circuit analysis and aids in understanding how adjustments in capacitance can impact overall amplifier behavior.

Imagine adjusting a pair of binoculars to focus clearly on a distant object. Just as you manipulate the lenses (the capacitances) to provide a clearer view (the frequency response), applying Miller's theorem can help enhance our understanding of how amplifiers respond under different conditions.

--