77.2.4. Differential Mode Analysis

So, in our previous lecture we have talked about basic structure and working principle of differential amplifier...operation and their corresponding gain.

In this section, we delve into the differential mode analysis of amplifiers. The differential amplifier is a fundamental component that amplifies the difference between two input signals while rejecting any signals that are common to both inputs. The basic principle behind this analysis is understanding how these amplifiers work when they are given different types of stimuli, specifically, differential versus common mode signals. The primary focus here is on the gains associated with these different operational modes.

Imagine a team of reporters trying to figure out the truth behind a rumor. If two reporters from different outlets (the inputs) report on the same issue (common mode), they might end up with biased information. However, if they each investigate different angles of the rumor (differential mode), they can compare findings to reveal the actual story. This illustrates how a differential amplifier works: it focuses on the 'true difference' between signals.

So, for small signal analysis we require small signal equivalent circuit...we need to have equivalent circuit where, we can drop the DC part and components of this current as well as the voltage.

The small signal equivalent circuit is used to simplify the analysis of differential amplifiers under small signal conditions. This involves linearizing the circuit by removing DC components and focusing on AC variations. By applying small signal analysis, we can consider how the amplifier responds to small changes in input signals, which is critical for understanding its gain and other dynamic behaviors. Both BJT and MOSFET implementations are similarly analyzed to draw meaningful insights.

Think of small signal analysis like tuning a musical instrument. When a musician plays a note, they listen for the pitch (the small deviations) after adjusting the strings (the DC part). By focusing only on how minor adjustments affect the sound (the small signal), the musician ensures the instrument is precisely tuned for performance.

So, we may say that the output one of the output we are observing here, which is named as v1....difference of these two voltages we call say v_d.

The differential mode gain (Ad) quantifies the amplification of the output signal relative to the difference in input signals. It is defined mathematically and depends on how the circuits respond to differential stimuli. For practical applications, Ad allows engineers to design systems that maximize signal transmission while minimizing noise, which is vital in communication and sensor technologies.

Imagine a referee in a soccer game who focuses on the differences in performance between two players rather than their entire gameplay. The referee's score represents the differential mode gain, highlighting the best player based on specific feedback. This approach allows for more precise judgments, paralleling how differential amplifiers work by amplifying just the necessary differences.

So, in summary we have drawn the clean small signal equivalent circuit here...for differential mode of operation namely if the input-2 and input-1, if they are complementary to each other.

The application of differential mode analysis can be realized using different technologies, such as Bipolar Junction Transistors (BJT) and Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFET). While the underlying principles remain the same – amplifying the voltage difference between two inputs – the characteristics of BJT and MOSFET differ in terms of performance metrics like gain, bandwidth, and noise figure. Understanding these differences is crucial for selecting the right type of amplifier based on the application context.

Choosing between BJT and MOSFET is like selecting the right type of vehicle for a delivery. A truck (BJT) might be robust for heavy loads, while a sports car (MOSFET) might be faster in simpler tasks. Depending on the delivery specifics, one will outperform the other, just as different applications benefit from different transistor types in amplification.

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