45.1. Analog Electronic Circuits

Dear students, welcome back to our online NPTEL certification course. The topic of this course is Analog Electronic Circuit. Myself, Pradeep Mandal from E and ECE department of IIT Kharagpur. Today’s topic of discussion it is Common Collector and Common Drain Amplifiers, rather I should say it is continuation of this topic.

In this introduction, Professor Pradip Mandal welcomes the students back to the course and recaps the focus on Analog Electronic Circuits, specifically common collector and common drain amplifiers. This sets the stage for a continuation of the previous discussions, emphasizing the practical aspects of these amplifiers and their components.

Think of this course like a cooking class where each session builds on the last. Just like how each recipe might use ingredients from the previous ones, today’s lesson will use concepts we already covered about amplifiers and add more practical details.

So, what we have to cover today, as I said we have discussed the motivation part of the common source and sorry common collector and common drain amplifier, basic operation and biasing also it is done. And we are going to go a little detail of analysis of voltage gain and impedance, input capacitance, considering realistic biasing and their associated components.

In this chunk, the professor outlines the topics for the day's lecture, which include a more in-depth analysis of common collector and common drain amplifiers. Topics include voltage gain, impedance, and input capacitance while taking realistic components into account. This provides a clear roadmap for what students can expect to learn.

Just like a construction project where the blueprint gives a detailed plan, this outline helps students understand what to expect in the session. If you were building an amplifier, you'd want to know about its strength (gain), foundations (impedance), and size (capacitance) before calling it complete.

So, to start with let we let you consider the common collector amplifier and also in the common collector amplifier we are including this R . So, this R may be coming from the bias circuit...

This section begins with an analysis of the common collector amplifier, emphasizing the inclusion of a load resistance (R). The professor explains the significance of considering this resistance in the amplifier's analysis to better understand its practical implications. It highlights the transition from theoretical concepts to real-world applications.

Imagine you've built a bridge. The load resistance is like the weight of vehicles using the bridge. While designing it, considering the weight ensures the bridge will be stable and safe under real usage conditions.

we do have small signal equivalent circuit of the common collector amplifier having this R included. So, we do have r here and then g v , r coming from the MOS at the BJT transistor...

Here, the professor discusses the small signal equivalent circuit of the common collector amplifier. This model allows for a simplified analysis of the amplifier's performance under small signal conditions by representing active components with their small signal parameters. The inclusion of specific resistances and transistor parameters is crucial for accurate calculations.

Think of a small signal equivalent circuit like a miniature model of a complex machine. Just like a model can help you understand how the actual machine works, this simplified version helps us analyze the bigger circuit's behavior in real conditions.

Now, if we have this R which is essentially coming in parallel with r then whatever the derivation we have done before that can be as well utilized just by replacing this r by r and R in parallel.

In examining the impact of resistances on voltage gain, the professor explains how the parallel arrangement of the load resistance (R) and the internal resistance (r) changes the voltage gain calculation. This adjustment reflects the real-world scenario where these resistances affect how the amplifier performs.

Consider trying to pour water through two size openings (resistances). If one is wider (lower resistance) than the other, the flow (gain) is increased. In practical circuits, all these openings must be accounted for to understand the overall flow of electrical signals.

Likewise, when you consider the input capacitance if you see the expression or if you recall the previous expression of the input resistance without considering this R , what we had it is...

The professor delves into the concept of input capacitance, emphasizing how its calculation changes when the load resistance is included. He reiterates how previous theoretical formulations must be updated to consider practical components, enhancing students' understanding of circuit behavior.

Imagine trying to fill a balloon (input capacitance) with different types of liquids (currents). The thickness of the liquid will impact how quickly you can fill it (the capacitance behavior). Understanding how changes in components affect this filling process helps predict the performance of the circuit.

So, if you see this circuit and at this point if you see what is the output resistance, it is basically the output resistance coming from the rest of the circuit coming in parallel with R .

This chunk focuses on output resistance in the common collector amplifier. The professor explains how output resistance is calculated based on the parallel configuration of various resistances in the circuit. Understanding output resistance is vital for assessing how the amplifier interacts with subsequent stages in a circuit.

Think of output resistance like a team of athletes running a relay. If some athletes (resistances) are faster (lower resistance), the team’s overall performance will improve. Similarly, in circuits, proper analysis of output resistance ensures that the heart of the circuit can pass signals effectively.

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