35.1. Analog Electronic Circuits

Yeah, dear students welcome back to this NPTEL online program on Analog Electronic Circuits, myself Pradeep Mandal from the E and ECE Department of IIT Kharagpur. We are going through different modules and presently we are in the 4th module of this course. And, today’s topic of discussion it is Frequency Response of CE and CS Amplifier; Common Emitter and Common Source Amplifier.

In this introductory section, Professor Pradeep Mandal welcomes students back to the course on Analog Electronic Circuits. He highlights that they are currently in the fourth module, focusing on the frequency response of two types of amplifiers: the Common Emitter (CE) and Common Source (CS) amplifiers. Frequency response refers to how the gain of these amplifiers changes with varying input signal frequency.

Think of a speaker system: just like how speakers handle different frequencies differently (some may sound better with bass, while others handle treble well), amplifiers also respond differently to various signal frequencies. This section sets the stage for understanding how each type of amplifier manages frequency changes.

In the previous module we have seen how to find the gain of common emitter and common source amplifier. And, today what we will see that if we change the frequency of the input signal how the gain of the circuit whether it is common emitter or common source amplifier, it is it changes with frequency.

Here, the professor remarks on the learning from the previous module, where students learned to calculate the gain of CE and CS amplifiers. Now, they are going to explore how this gain varies when the frequency of the input signal changes. Understanding this relationship is crucial for designing circuits that perform reliably across different frequency ranges.

Imagine tuning a guitar: as you press different frets, the pitch of the string changes. Similarly, as the frequency of input changes in an amplifier, the gain adjusts accordingly, much like how a musician must adjust their playing to capture different notes.

Now, what do we have in this plan, in this module; it is the or rather today and the next classes are the following. So, we are planning to cover as I said that we are going to cover frequency response of common source and common emitter amplifier.

In this chunk, the professor outlines the structure for today's lesson and the upcoming classes. He emphasizes that the main focus will be on the frequency response of both the common source and common emitter amplifiers. This gives students a clear understanding of what to expect in their learning.

Think of this section like a syllabus for a class trip: it tells you where you will go, what you will learn, and the activities to expect. Just as you prepare for an exciting day out, students can prepare mentally for the topics ahead.

To understand that first what we will do that we will revisit the frequency response of R-C circuit and C-R circuit. And, then based on the R-C and C-R circuit we will be talking about the transfer function of a typical system R-C or C-R combination.

This part of the lecture explains that to grasp the frequency response of the amplifiers, the class will first revisit R-C (resistor-capacitor) and C-R (capacitor-resistor) circuits. The professor indicates that understanding these circuits is foundational for discussing the transfer functions, which describe how input signals are transformed into output signals in the circuit.

Consider a water pipe system where water flow is regulated by a gate (resistor) and a storage tank (capacitor). Understanding how this setup works will help you appreciate how electrical signals are processed in amplifiers, similar to how water flows and fills the tank.

And, then we will also discuss about what is the relationship between transfer function and then frequency response and then location of the pole zeroes in Bode in Laplace domain transfer function.

In this segment, the professor mentions the discussion of the relationship between transfer functions and frequency responses. Transfer functions help determine how a circuit responds to different inputs. The locations of poles and zeros (specific points that affect the circuit’s response) in the transfer function, especially in Bode plots, will be examined.

Think of taking a delivery route: the poles are like traffic lights that stop the flow of delivery at certain points, while zeros might represent shortcuts or points with no delay. Knowing these points helps optimize the delivery route, just as understanding poles and zeros helps engineers design better circuits.

So, and then what is the relationship between the pole and zeroes and the cut off frequency in the frequency response.

The professor continues by linking the concepts of poles and zeros to the cutoff frequency in the frequency response. The cutoff frequency marks the point where the output begins to significantly drop compared to the input, which is critical for understanding amplifier behavior.

Consider a filter in a pool that only allows certain sizes of debris to pass through while blocking larger items. The cutoff frequency is like the size limit—particles smaller than this can pass, while larger ones are trapped. Similarly, amplifiers allow certain signal frequencies to pass while attenuating others.

Then also we do have planned to cover design guidelines, once we once we recover the frequency response.

Here, the professor highlights that they will also cover design guidelines after discussing the frequency response. These guidelines will help students understand how to design amplifiers that meet certain performance criteria related to frequency behavior.

Designing an amplifier is akin to planning a nutrition plan—understanding which ingredients (resistors, capacitors) to include and in what amounts will help achieve the desired outcome (sound quality) just like the right balance of nutrients ensures good health.

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