22.1. Linear Models of Amplifiers (Part A)

Dear students, welcome back to this NPTEL course on Analog Electronic Circuits and today we are going to cover the topic of Linear Models of Amplifiers. So, in fact if you see, we already have covered few topics and today actually we are in the first day of 3rd week. So, let see what where do we stand today, compared to our overall plan. This 3rd week we are planning to cover amplifier models and followed by cascading multiple amplifier stages; and then followed by CE amplifier, common emitter amplifier and common source amplifier. And under this topic of amplifier models what you are going to cover it is; the voltage amplifier, current amplifier and trans-conductance amplifier and trans-resistance or trans-impedance amplifier. So, these are essentially simplified model of the different types of voltage amplifier. Today we are going to discuss with simple example and we will cover the purpose and the little detail of the model different, these four models.

In this overview, the instructor introduces the course and the topic of linear models of amplifiers, guiding students on the path of the week's learning objectives. The focus is on amplifier models, including voltage, current, trans-conductance, and trans-resistance amplifiers, which are simplifications of various amplifier types. The aim is to provide a foundational understanding that will be applied through examples and discussions in the upcoming lectures.

Think of amplifiers as different types of vehicles. Just as cars and trucks serve different purposes, amplifiers come in different forms based on their intended function, whether amplifying voltage, current, or another parameter.

Now, what are the concepts we are going to cover today? It is, basically as I said that the model of voltage amplifier, then model of current amplifier, model of trans-conductance amplifier and trans-impedance amplifier. So, primarily we will be discussing more detail of these two topics and whatever the idea we will be gaining from that, we will be extending to the other two types of amplifiers quickly.

This chunk focuses on the specific concepts the lecture will cover, highlighting the models of different types of amplifiers, particularly emphasizing voltage and current amplifiers. The intent is to delve deeper into understanding voltage and current amplifiers, and then broaden the knowledge to include trans-conductance and trans-resistance amplifiers, promoting an interconnected understanding of amplifier models.

Imagine you are learning to cook. You start with a specific dish (voltage amplifier), and once you master that, you can use those skills to create other dishes (current, trans-conductance, and trans-impedance amplifiers).

Say for example, the first one it is; it consist of BJT, second one it is having a MOS transistor. And for each of the cases what we have here it is, the DC voltage source and then of course the ground and along with the biasing element resister. The transistor it is in this case the BJT is kept in the active region of operation with the help of V_BE, the DC voltage here. And then on top of the DC voltage we do have the small signal called v_be.

In this chunk, the instructor presents practical examples of voltage amplifiers using a Bipolar Junction Transistor (BJT) and a Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET). The BJT is described as being kept in the active region, which is crucial for its function. The concept of DC voltage and its role alongside small-signal variations is introduced, aiming to set the foundation for understanding how these amplifiers operate.

Think of the BJT amplifier as a water faucet. The DC voltage is like the water pressure that keeps the faucet at a constant level (the active region), while the small signal is like a small adjustment of the faucet handle, potentially allowing for small variations in water flow (signal output).

Whenever we are going for say model or linearization basically what you do; as I said that the transistor we are keeping in the appropriate region of operation, and then after that we try to find what is the relationship between the applied voltage here to whatever the corresponding output we are getting. So, in our main operation the DC conditions are kept intact, only the small signal part changes with time and then the corresponding effect we observe here.

The focus shifts to the modeling of voltage amplifiers, underscoring the importance of keeping transistors in their appropriate operational regions to achieve accurate linear models. In this modeling process, the DC conditions provide a stable reference point, while small-signal variations reflect the effective changes in output based on input changes.

Consider a performer on stage. The background light (DC conditions) remains constant, allowing the small changes in spotlight intensity (small signals) to be prominent, much like how the amplifier responds to small variations in input while keeping a stable DC condition.

What we mean by the voltage amplifier? It is an equivalent linear circuit and it is main purpose is to provide the dependency of output signal on the input signal. The DC part we are not changing. So, we can probably eliminate the DC part in the simplified model to make the circuit simple enough or probably the analysis we can make it simpler.

This chunk defines the voltage amplifier as an equivalent linear circuit designed to illustrate the relationship between input and output signals, essentially simplifying the analysis by disregarding the stable DC component. This approach allows for better focus on the AC characteristics that are often of greater interest in amplifier performance.

Imagine a chef perfecting a recipe. They focus on the flavors (AC signals) rather than the ingredients' basic qualities (DC conditions). By concentrating on the flavors, they can better understand how to adjust and improve the dish.

So, we may say that to again to summarize what we said is; that we like to get the input to output signal relationship primarily. So, this is the say, you may say that input signal and this is the corresponding output. So, the input signal of course, it may be coming from the signal source it need not be same. And since we are expressing this output in terms of the input signal, we the simplified model must capture one basic element called voltage dependent voltage source.

This section emphasizes key elements required in modeling voltage amplifiers, including the need for a voltage-dependent voltage source that illustrates how variations in input correspond to changes in output. This is a crucial aspect of understanding amplifier behavior since specific parameters, such as gain and loading effects, must be taken into account.

Consider a barista preparing coffee. The relationship between the amount of coffee grounds (input) and the strength of the coffee (output) depends on how much water is used (voltage-dependent source). The barista adjusts the amount of water to achieve the desired coffee strength.

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