75.2.3. Realization of Differential Amplifier

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. Many of times it is also used in actual circuit and whatever the concepts we will be discussing related to differential amplifier for that this circuits are good enough to discuss that. So, here we are showing that the realization of this differential amplifier. So, like so, this is BJT version and here it is MOSFET version.

The differential amplifier can be constructed using various components, including transistors such as BJTs (Bipolar Junction Transistors) or MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors). These components can be configured in different ways to optimize performance. In this basic realization, two identical transistors are utilized—one for each input signal. This allows the differential amplifier to effectively amplify the difference between two signals while suppressing common noise.

Think of a differential amplifier like a pair of trained judges in a competition. Each judge hears the same performance (the input signals) but evaluates them differently based on subtle differences (the differential input). This structured approach allows them to provide a more objective and fair assessment, similar to how a differential amplifier enhances the difference between input signals.

Now coming to the other information which is also very important, that you might have seen that we do have this circuit is having two parts structurally they are identical. In fact, this R and R we want they should be equal, R we want R should also be identical. Likewise Q and Q are identical right. So, once we have these two halfs they are identical not only in schematic, but their actual value then only we can see that whatever the parameters other parameters we have ignored namely, A = 0 and A = 0. c_d d_c Which means that whenever we are applying say differential signal at the input the corresponding output it is perfectly differential and whenever we are applying say input signal in the form of common mode, perfectly common mode which means differential component is 0 then you can say that at the output whatever the signal we are getting signal at terminal-1 and terminal-2 they are identical and hence differential output is 0.

In a differential amplifier, achieving identical structures and components is crucial for its performance. Components such as resistors (R1 and R2) and transistors (Q1 and Q2) should be matched to ensure that when a differential signal is applied, the outcoming signals are properly amplified without introducing unwanted variations. If the components are not identical, it can lead to imbalances that affect the amplifier's ability to nullify common mode signals, which is essential for maintaining signal integrity.

Imagine you are baking cookies and you have two identical measuring cups to measure out sugar and flour. If one cup is slightly larger, your cookie recipe will turn out differently than expected, affecting the taste and texture. Similarly, in a differential amplifier, using identical components ensures that both halves of the circuit respond equally to their inputs, producing a consistent and reliable output.

Now, the natural question is that in practical realization if they are they are not possible to achieve whatever the little variation is there. So, same thing it may happen for Q and Q, then their consequences it will be if they are not and if are not identical then these two parameter we cannot consider they are 0 and we have seen their consequences. Particularly A it is very dangerous. So, we prefer to avoid such kind of situation.

Now next thing is that here of course, there are different possible realizations and as I said that these two realizations are very basic and very fundamental.

In practice, it may not always be feasible to create perfectly identical components for a differential amplifier due to limitations in manufacturing or component tolerance. Even minor discrepancies can significantly impact performance. Consequently, various design approaches and modifications can be employed to maintain acceptable performance levels. Using different configurations that help mitigate the effects of non-ideal components represents a crucial step in practical applications of differential amplifiers.

Consider customizing a car. While two identical models leave the factory the same, minor tweaks such as tire selection or engine tuning can introduce differences in performance. Differential amplifier designs must cater to these tweaks by introducing alternates to balance performance, much like tuners who adjust their vehicles for optimum operation despite variations in components.

So, if we consider say this is one terminal input terminal, this is the other input terminal in-1 and in-2 and then if I consider the corresponding output, output at this point; it is in phase with this input and hence instead of calling this is v, we are calling this is v ok. Just to say that if this is +ve side of the signal and this is ‒ve side of the signal, the corresponding output will be having this side is this terminal showing the +ve side of the output and on the other hand this terminal will be showing the ‒ve side of the output.

In more advanced designs of differential amplifiers, active devices—such as additional transistors—can replace passive resistors to enhance performance. This can lead to higher differential gain while concurrently reducing common mode gain, optimizing the amplifier for real-world applications where signal integrity and amplification are paramount.

Think of upgrading your home sound system. By replacing standard speakers with powered ones, you achieve higher sound quality without distortion. Similarly, using active devices in a differential amplifier enhances signal quality and boosts performance, resulting in more accurate signal amplification.

--