45.1.2. Common Collector and Common Drain Amplifiers (Contd): Analysis (Part A)

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, the instructor welcomes students back to the course and sets the agenda for the day. The focus is on analyzing two types of amplifiers: the common collector amplifier and the common drain amplifier. This signifies a continuation of previous discussions, indicating that students should be familiar with basic concepts surrounding these amplifiers. It establishes the learning goals for the day, which include a deeper understanding of amplifier performance with practical considerations.

Think of this as a workshop where previously you learned how to build a basic chair, and now the instructor is diving deeper, discussing how to make that chair more comfortable and stable by choosing the right materials and designs.

We are going to go a little detail of analysis of voltage gain and impedance, input capacitance, considering realistic biasing and their associated components.

This chunk introduces the key topics of the day: voltage gain and impedance. The instructor emphasizes the importance of real-world conditions in which amplifiers operate. Discussions will include how realistic components affect these parameters, reflecting an objective to understand how theoretical knowledge applies in practical situations. This is crucial for designing effective electronic circuits as real components often differ from ideal ones in characteristics and behaviors.

Imagine a car engine that is supposed to operate efficiently. If you only studied it in ideal conditions, like in a lab, you wouldn't account for factors like fuel quality or altitude. Here, realistic biasing is similar to these real-world factors that significantly affect engine performance.

Let us start with the common collector amplifier, considering the R_L... our analysis and here we do have small signal equivalent circuit of the common collector amplifier having this R_L included.

The section now focuses on practical analysis, specifically including the load resistor (R_L) in the common collector amplifier circuit. This addition reflects a more realistic approach to amplifier design, as it allows for the consideration of circuit behavior under various loads and influences on signal transfer. It points towards developing an accurate small signal model capturing real operational details vital for performance analysis.

Consider planning a musical concert. If you only think about the sound system without considering how the size of the venue (represented by R_L) affects sound quality, you won’t achieve the best performance. Similarly, in circuits, accounting for load resistances can fundamentally change the amplifier's output.

In our previous analysis where we have excluded this R_L... the expression of the input capacitance.

This part discusses how the input capacitance of the common collector amplifier is affected when we include R_L in our analysis. The capacitance between different circuit terminals can significantly influence how quickly the amplifier responds to changing signals. By calculating the input capacitance, it helps designers determine how this affects amplifier stability and performance under normal operation conditions.

Think of this as a sponge soaking up water. The input capacitance determines how quickly the amplifier can 'soak up' the input signal and respond. If you don’t account for the size and properties of the sponge (input capacitance), you can underestimate or overestimate how fast it can react in real situations.

Now, if we have this R_L which is essentially coming in parallel with r_o... that can be as well utilized just by replacing this r by r_O and R_L in parallel.

Here the lecturer explains how the expressions for voltage gain are adjusted when R_L is included in the analysis. Since R_L is in parallel with r_o (the output resistance), this impacts the computation of the overall voltage gain. By substituting r with the parallel combination of these resistances, you refine the model to reflect real circuit conditions accurately.

This is akin to calculating the amount of paint needed for a wall. If one part of the wall is textured (rough, like the parallel components), it will require more paint to cover than if the whole wall was smooth. Accounting for R_L modifies your calculations for voltage 'coverage,' just like variation in resistances adjusts voltage gain.

Now, if you want to know what will be the output resistance... total conductance coming from all these four elements.

This section focuses on the output resistance of the common collector amplifier derived from various current components flowing through the circuit. Understanding output resistance is vital as it affects the damping and stability of the amplifier when subjected to changing signal conditions. The ratio of voltage to current helps derive how much resistance offers to the output signal.

Imagine you have a garden hose connected to a fountain—the output resistance reflects how easily the water flows out. If the hose is narrow (high resistance), water flows less freely than if it’s wide (low resistance). Similar principles apply to output resistance in amplifiers: it determines how effectively they can drive loads.

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