14.1.1. Lecture – 14

Welcome back to this course on Analog Electronic Circuits, myself Pradip Mandal associated with E and ECE Department of IIT, Kharagpur. So, after our previous modules in week-1, now we are in week-2 and we are going to discuss about the BJT and MOS related circuits. So, we will start with Analysis of simple non-linear circuit containing one BJT and later we will be discussing about one MOS and so and so.

In this introduction, Professor Pradip Mandal sets the stage for the next part of the course. He informs students that they have completed the first week's module and are now transitioning into week two, where the focus will shift towards analyzing circuits that include Bipolar Junction Transistors (BJT) and Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs). The analysis will commence with simple non-linear circuits using a BJT, which is a fundamental component in analog electronics.

Think of this transition like moving from a basic math course to algebra. In the basic math course, you learn how to add and subtract. Then, as you advance, you start learning about variables and equations, just like progressing from understanding simple components to analyzing complex circuits.

So, let us look back what is the topic we do have and what are the overall flow we do have in our program. So, we have completed the component things... So, we are going to start with analysis of simple non-linear circuit containing transistor and today’s focus is BJT.

Professor Mandal emphasizes that the objective for today's lecture is to analyze a non-linear circuit featuring a BJT, specifically looking at its transfer characteristics from input to output. This part of the lecture serves as a bridge from previously covering basic electronic components to more nuanced circuit configurations, illustrating how these components work in practical scenarios.

Imagine you're cooking a recipe where you've already learned about the ingredients (the basic components) and now you're faced with cooking techniques (circuit configurations). Understanding how to analyze these circuits is akin to mastering cooking techniques that enhance the flavors of your dish.

So, as I said that we will be analyzing non-linear circuit containing one BJT and the configuration will be discussing primarily it is common emitter configuration.

The common emitter configuration is a popular circuit setup for BJTs. It allows the transistor to function effectively as an amplifier and switch. The professor notes that his focus will be on how input voltage translates into output characteristics, which includes understanding how to manipulate the BJT's configuration for desired output responses.

Think of the common emitter configuration like a public speaker system. The speaker (transistor in this case) takes a small voice input (input signal) and amplifies it to be heard clearly (output signal) by a larger audience.

What we will be doing is that we will be focusing on input to output transfer characteristic of non-linear circuit.

In this part, the focus will be on the relationship between the input signal applied to the base of the BJT and the output signal that is produced at the collector. The professor highlights the significance of these characteristics in understanding how changes in the input signal will affect the output, laying the groundwork for discussions on signal amplification in subsequent sections.

This concept can be likened to a garden hose system. When you turn the faucet (input signal), water flows out of the hose (output signal). Depending on how much you open the tap (the input voltage level), the flow of water changes, demonstrating a direct relationship between input and output.

In this problem what you have to do, we need to find the operating point of the transistor or operating condition of the transistor; namely the base voltage intuitive is given.

Professor Mandal outlines the process for finding the operating point of the BJT. There are specific values that need to be derived based on given parameters, including base voltage, base current, collector current, and collector-emitter voltage. By systematically analyzing the circuit as described, students will learn to extract these key values that are critical for understanding BJT operation.

Consider the process akin to planning a trip. You need to know your starting point (base voltage), your accommodations along the way (base current), the final destination (collector current), and the route you’ll take (collector-emitter voltage). Each factor affects how effectively you can make it to your endpoint.

For simplicity that is what normally it is done, we assume that this early voltage it is very high compared to the V and hence we drop this part.

The early voltage relates to how the collector current varies with changes in the collector-emitter voltage. In practical applications, assuming a high early voltage simplifies calculations without significantly sacrificing accuracy. This is an important concept in managing the complexities encountered in transistor circuit analysis.

It’s similar to assuming that a car can drive straight without much resistance from the road. If you simplify the road conditions, it makes it easier for you to chart your route without getting bogged down in every small bump.

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