75.5. Realization of Differential Amplifier Circuits

So, how do we realize this differential amplifier? There may be different possible realizations 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 realized in various ways, but two commonly used implementations are based on BJT (Bipolar Junction Transistor) and MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor). These circuits are fundamental because they can help in grasping basic concepts of differential amplification. The configurations of both types are practically utilized in circuits, showcasing their importance in the electrical engineering field.

Consider a chef who has two different methods to prepare a dish (BJT and MOSFET). While both methods achieve the same end result (a delicious meal), each might use different cooking techniques and ingredients specific to its method. Just like how the chef uses different kitchen tools to enhance flavor, engineers use BJTs or MOSFETs based on their advantages, like operational speed or power efficiency.

Now, here I like to tell you one important point here 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 a differential amplifier, the two input signals lead to corresponding outputs that reflect changes in the original signals. If one input increases positively, the output will shift positively, while the other output will simultaneously shift negatively if designed to do so. This characteristic allows for amplification of the difference between the two input signals while rejecting any common signals, which is essential for noise reduction in many applications.

Imagine two siblings arguing. If one sibling raises their voice (positive input), the other sibling may respond with a counter-argument that lowers their tone (negative output). The 'difference' in their combined responses reflects the overall situation at hand (the way a differential amplifier processes inputs), representing the real issue while filtering out the noise of their emotional responses.

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 halves 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.

For the differential amplifier to function optimally, it is crucial that the components on both halves of the circuit are identical. This includes resistors (R1, R2) and transistors (Q1, Q2). If these elements are not identical in both value and construction, the performance of the amplifier could become unbalanced, leading to unwanted variations in gain and distortion. Matching these components ensures that ideal conditions are met for analyzing the circuit's performance.

Think of a duet performance between two singers. If one singer has a much stronger voice than the other (i.e., unbalanced components), it can overshadow the other, resulting in a poor performance overall. However, when both singers are equally skilled (i.e., identical components), they can harmonize perfectly, delivering a smooth and pleasing experience (i.e., optimal amplifier performance).

Now of course, again we like to say that while we will do this operation in actual circuit we have to retain this DC voltage and then, we can see what kind of signal we can apply here to get this differential mode of stimulus and then common mode of stimulus. So, now let us see the realization of this circuit. There are different possible realizations and as I said that these two realizations are very basic and very fundamental.

Differential amplifiers can have several variants based on different configurations and component selections. These include using different transistor types or active components to enhance performance characteristics. The choice of component affects the performance such as gain and bandwidth. In practical applications, engineers have the flexibility to choose the best configuration suited for their specific needs.

Imagine a car manufacturer that produces different models of cars using the same engine design (basic realization). However, they may offer a racing model with a tuned engine (variant) or an electric version (another variant). Just as the car model serves different purposes based on its design, differential amplifiers are tailored to meet specific performance requirements relative to the application.

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