45.2. Common Collector Amplifier Analysis

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. Previous day we have discussed about relatively idealistic bias situation and today we are going to a little detail considering some practical circuit components also.

In this introductory section, the instructor sets the stage for the analysis of common collector amplifiers by referencing previous lectures. He emphasizes that today's discussion will build upon earlier concepts and delve into more practical aspects of circuit components.

Think of this as a classroom where students learned the basics of cooking before. Now, the teacher is taking them to a kitchen to apply those basic cooking skills using real ingredients and tools, enhancing their knowledge with practical experience.

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.

The instructor outlines the key areas of focus for the analysis of the common collector amplifier. These include its voltage gain, impedance, and input capacitance, particularly when realistic biasing conditions are applied, contrasting this with the earlier ideal scenarios.

Imagine studying the theory of driving a car in a classroom and now preparing to actually drive on the road. You need to understand the mechanics of how the car operates under realistic conditions, including adjustments based on varying traffic situations.

In fact, in the previous class we have discussed about the analysis of the circuit for voltage gain, impedance and input capacitance ignoring these components, and today we are going to see what will be there you know consequences if we consider on practical components. So, let us start with the common collector amplifier, considering the R_L, and then we will be moving to the next one is considering the source resistance R_S.

This chunk highlights the transition from theoretical analysis to practical circuit considerations. The analysis looks at how introducing real-world components (like resistors) affects the voltage gain and input capacitance of the common collector amplifier. The instructor introduces two specific resistors that will be analyzed: R_L (load resistor) and R_S (source resistance).

Just like adjusting the settings on a sound mixer to manage real audio recordings, understanding how each resistor impacts the signal ensures optimal performance of the amplifier in real-world scenarios.

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_L. So, this R_L may be coming from the bias circuit, representing the finite conductance of the bias circuit or maybe and or maybe additional load resistance we are connecting at the output node with respect to ground.

In this section, the focus shifts to R_L, which is a crucial part of the common collector amplifier circuit. The instructor explains that R_L can originate from various sources, including biasing circuits or load resistors connected to ground, affecting the overall circuit behavior.

Thinking about this in terms of a water system, R_L can be compared to pipes of different diameters added to a water flow system; they influence how water flows through the pipes depending on their size and configuration.

Now, in our previous analysis where we have excluded this R_L there we have seen the expression of the input capacitance. Basically, the input capacitance at the base with respect to the AC ground.

Here, the instructor discusses the input capacitance of the amplifier, which is the capacitance observed at the base when considering the AC ground. The previous analyses did not factor in R_L, and now this component will be included to see how it influences the input capacitance.

Consider this as a sponge being filled with water—without the sponge (R_L), the water (signal) can flow easily, but as you introduce sponges of different sizes (resistors), the water’s flow changes, simulating how input capacitance affects signal transmission.

So, if we put this expression of this voltage gain here, so this is A_v. So, if you put that voltage gain expression here, what we will be getting here it is C_in having C and then C part it is having a factor which is a having in the numerator we do have r and in the denominator we do have (R_L // r_o) + this r_pi.

The focus now shifts to how the inclusion of R_L influences the voltage gain of the common collector amplifier. The expression reveals that the voltage gain A_v is dependent on the resistive elements in the circuit, showing how the presence of R_L alters the previous gain approximation.

Think of this like adjusting the volume control on a speaker. As you adjust it, the sound level (voltage gain) changes based on the input source impacted by the environment (R_L), illustrating the dynamic nature of electronic signals.

In fact, you can further simplify this one wherever we do have the you may replace this by β of the transistor. So, here also we can write this is (r_pi + R_L) = ... .

This section discusses the impact of source resistance R_S on the input and output characteristics of the amplifier. By estimating the input resistance in the presence of R_S, the instructor highlights how simple substitutions (like using β) can streamline calculations.

Imagine tuning a musical instrument where subtle adjustments make a significant effect on sound quality. Here, just a small resistance can change the output and input characteristics, much like the strings of an instrument impact its sound.

Now, that is about the common collector amplifier if you consider its counterpart mass counterpart namely the common drain stage and then if you consider this R_L for that, what you will be getting it is similar kind of things we can get only difference is that this r_pi, it will not be there.

In this part, the instructor compares common collector amplifiers to common drain stages, examining output resistance characteristics. It is concluded that while there are similarities, there are also distinct differences regarding specific parameters.

It's similar to comparing two types of musical instruments. Both may play music, but the sound output (resistance) and the way they resonate (circuit behavior) can be quite different, showcasing their unique characteristics.

In conclusion, we have discussed various aspects of the common collector amplifier, including the effects of R_L and R_S on voltage gain, input capacitance, and output resistance.

The instructor summarizes the detailed analysis of the common collector amplifier, reflecting on how the inclusion of resistors like R_L and R_S influences key parameters. This wrap-up reinforces the practical implications of these components in real circuit designs.

Just as a writer revises their drafts considering feedback to improve the final story, engineers must also refine their circuit designs based on practical components, ensuring the final performance meets desired specifications.

--