28.2. Common Emitter Amplifier (Contd.) Numerical Examples (Part A)

Dear students, welcome back to NPTEL online course on Analog Electronic Circuit. Myself Pradip Mandal from E and EC Department of IIT, Kharagpur. So, this is a continuation of our previous topic Common Emitter Amplifier. And, in the previous class, what we have discussed about the relevant theories and today, we are going to discuss more detail of some numerical problems.

This chunk serves as an introduction to the session on numerical examples related to the Common Emitter Amplifier (CE Amplifier). The instructor, Pradip Mandal, welcomes the students back and highlights that the focus will be on resolving practical numerical problems that will help clarify doubts regarding the previous theoretical discussions on Common Emitter Amplifiers.

Think of learning a new recipe in cooking. Before you can cook a dish perfectly, you need to understand the ingredients and techniques involved. This session is similar; it builds on earlier lessons (theory) and moves to practical examples (the recipe in action).

So, as I said that primarily we will be focusing on common emitter amplifier. And, it is having two basic biasing schemes namely the fixed bias and cell bias, and we have discussed about the disadvantage of each of them and then advantages and all.

In this section, the lecturer emphasizes that the Common Emitter Amplifier operates through two main biasing schemes: fixed bias and cell bias. Each method has its own advantages and disadvantages which have been discussed in earlier lectures. Understanding these biasing schemes is crucial as they play a significant role in the stability and behaviour of the amplifier.

Consider a car that can be powered by two different fuel types. Each fuel has benefits and drawbacks (e.g., efficiency, cost). Understanding these options allows a driver to make the best choice under various conditions—similarly, understanding biasing schemes helps in designing effective amplifiers.

So, the first one it is we will be talking about the bias point stability where we shall demonstrate that fixed bias CE amplifier it is having a major issue in case if the β of the transistor it is getting changed.

This segment introduces the topic of bias point stability in the CE amplifier. It is particularly relevant when the transistor's beta (β), which signifies its current gain, changes. The fixed bias configuration can become unstable and may require redesigning of the circuit to accommodate alterations in β, potentially affecting the performance of the amplifier.

Imagine trying to balance a seesaw. If one person (representing the current gain) changes their weight unexpectedly, the balance shifts, and adjustments must be made. In electronics, when β changes, it can shift the operating point, requiring adjustments for stability.

Now, here let us try to find the operating point for β = 100. So, how do we proceed? First of all, we can consider the input port and we can replace the base to emitter junction by its corresponding equivalent circuit namely the r and in series with V_BE(on).

The instructor explains how to calculate the operating point when β is set to 100. This involves replacing the actual components in the circuit with their equivalent circuit models for analysis. Specifically, the base-emitter junction is replaced with an equivalent resistance and voltage. The aim here is to determine values such as base current and collector current, which are fundamental to understanding amplifier performance.

Consider analyzing the pressure in a pipeline. You might replace different elements of the system with simpler models to make calculations easier without losing the core functionality you want to examine. This is similar to how complexities in a circuit can be simplified for analysis.

So, let me recalculate whole thing for β = 200. So, what we have here it is let me use a different color here, maybe I can use red color. So, again we can consider the input port circuit containing R connected to V and then we do have the V_BE(on) which is around 0.6V.

In this section, the operating point is recalculated with β set to 200 to observe the impact of this increased current gain on circuit performance. The methodology remains the same, but the resulting calculations show that if the circuit demands more collector current (e.g., 4 mA), the voltage drop calculation leads to an unrealistic scenario where more voltage is needed than what is available from the power source. This highlights the sensitivity of circuits to changes in transistor parameters.

Think of a power grid that supplies a certain amount of energy. If the demand suddenly increases (analogous to increasing β), without additional sources of power, some areas may experience outages (making the supply scenario unsustainable). Similarly, if the required current exceeds supply limits, the amplifier cannot function efficiently.

So, let us see that cell bias circuit and the same situation if we consider namely we consider two values of beta and then we will see the changes of operating point of the cell bias circuit.

Here, the lecturer shifts focus to the cell bias configuration, which is expected to demonstrate greater stability than the fixed bias circuit. The aim is to analyze how varying β (to both 100 and 200) affects the operating point in this setup, showing that cell bias circuits are less sensitive to changes in transistor parameters and thus more stable.

Imagine a well-balanced diet. Some nutrients can vary in quantity without causing significant health issues; similarly, cell biasing maintains stability even when transistor parameters fluctuate. This resilience makes it a preferable choice in circuit design.

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