75.3. Basic Operation and Characterization

So, the concepts covered in this talk, in this lecture are the following. So, we are going to start with basic operation and then we will be going to discuss detail of characterization of differential amplifier. Namely, how we find the basic parameter values particularly, differential mode gain and common mode gain and so and so, from a given circuit.

This chunk introduces the key topics that will be discussed about the differential amplifier. The basic operation is about understanding how the differential amplifier functions, while characterization involves calculating important parameters such as differential mode gain and common mode gain. These parameters help in understanding the performance of the amplifier, especially in distinguishing the desired signals from unwanted noise.

Imagine a conversation at a loud party, where you are trying to listen to your friend (the desired signal) while ignoring the background noise (the unwanted signals). The differential amplifier works similarly to help focus on the 'desired signal' while minimizing the influence from 'background noise'.

So, to recapitulate whatever we have discussed in our previous lecture we do have say this is a main differential amplifier. Where we have the power supply DC supply and ground and then at the input we do have the differential input port having two terminals 1 and 2 and at this input we are given signal or I should say voltage signal and it is having a DC part meaningful DC part and on top of that we do have two signals.

This chunk explains the configuration of a differential amplifier, which includes a power supply and an input port with two terminals. The essential part of the input signal comprises both a DC component and two AC signals (v_in1 and v_in2) that are applied to the two inputs of the amplifier. It’s important that these signals are properly biased with a DC voltage so that the amplifier can function correctly.

Think of the differential amplifier as a balanced scale. The DC part is like the base weight you place on the scale to ensure it is level. The two signals (v_in1 and v_in2) are like the weights you add to each side of the scale. To keep it balanced (or to get a good output), you must ensure the weights are correctly applied.

Now once we are in small signal equivalent circuit, then it is basic parameters are differential mode gain and common mode gain as you can see here and we say that ideally we want this differential mode gain it should be as high as possible and this common mode gain on the other hand it should be as small as possible.

In this chunk, the lecture introduces gain parameters specific to the operation of the differential amplifier in its small signal equivalent circuit. The differential mode gain (A_d) should be maximized to ensure that the desired signal is amplified effectively, while the common mode gain (A_c) should be minimized to reduce the amplification of noise that is common to both input signals.

Think of a radio tuning to a specific station. You want the station (the desired signal) to be loud and clear (high gain), while noise from other stations (common mode gain) should be low so it doesn’t interfere with your listening experience.

Now let's see the output in terms of whatever the input we are applying here and in case if we have say these two parameters namely the common mode gain and differential mode gain. We need to translate this pair of signals in the form of common mode component and differential component.

This chunk describes the need to analyze the output signal concerning the differential and common components of the input signal. By separating the input into differential and common mode components, we can understand how each part contributes to the output, enabling us to calculate the respective gain parameters accurately and ensuring that the circuit is functioning optimally.

Consider a teacher trying to understand how much each student contributes to a group project. The differential contribution of each student represents their unique input, while the common input represents how well the group as a whole is working together. Analyzing both helps the teacher gauge the overall effectiveness of the project.

Now, to find A differential mode gain of this circuit, what do we do we take this circuit and then we consider that this part = 0. So, then the corresponding stimulus what you can see here it is common mode signal it is 0, we do have only the differential signal coming to the circuit.

This segment explains the method to calculate the differential mode gain by nullifying the common mode signal, effectively isolating the differential signal alone for analysis. Here, we assume the common mode part is zero to focus on how well the amplifier responds to the differential signal, allowing us to derive the gain from the output response.

This is like checking how well a singer can project their voice without any background music. By removing everything else, you can better understand the strength (or gain) of the singer's voice alone.

So how do we realize this differential amplifier? There may be different possible realization of differential amplifier, but here we do have a very basic realization of differential amplifier I should say it is basic, but still it is I should say practical circuit.

In this chunk, the importance of practical implementations of the differential amplifier is discussed. It introduces two primary realizations of the differential amplifier—one using BJT and another using MOSFET, outlining that while they might seem simple, they are fundamental in practice and serve as the basis for further advanced designs.

Consider building a bridge. There are several types of bridges (BJT and MOSFET variations) that can be used to cross a river, but each serves the same basic purpose of connecting two banks. Understanding how to construct a simple bridge helps you grasp more complex structures in engineering.

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