75.2. Overview of Differential Amplifier

Differential Amplifier: Basic Structure and Principle of Operation Dear students, welcome back to our NPTEL online certification course on Analog Electronic Circuit; myself Pradip Mandal from E and EC department of IIT, Kharagpur. Today’s topic of discussion is Differential Amplifier and we will be talking primarily the Basic Structure and Operating Principle. (Refer Slide Time: 00:54) So, in our overall flow where we stand, we are in module-7 and we are talking about different circuit modules and this module particularly in our discussion today it is differential amplifier.

In this section, we introduce the concept of a differential amplifier. It's an important circuit module in analog electronic circuits that amplifies the difference between two input voltages while rejecting any signals that are common to both inputs. This type of amplifier is crucial in many applications where signals are small and need to be amplified without interference from noise or unwanted signals.

Think of a differential amplifier like a referee in a game. The referee focuses on the actions of the two teams (the two input signals) and makes decisions based on their differences, ignoring any repeated actions that both teams have in common (common signals). Just as the referee allows a fair play, the differential amplifier allows only the correct signal to pass through by amplifying the difference.

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.

The fundamental operation of a differential amplifier is based on its ability to process two input signals. It amplifies the voltage difference between the inputs, resulting in an output that reflects this difference. This process is critical in circuits where noise and interference are present, allowing the amplifier to deliver a clearer output signal that conveys the true information carried by the difference.

Imagine you are listening to your friend speaking in a crowded room (one input), while at the same time, there is a lot of background noise (the other input). A differential amplifier is like your brain, which focuses on your friend's voice, amplifying it and filtering out the irrelevant background noise so you can understand what is being said.

And then we will be talking about the basic parameter values particularly, differential mode gain and common mode gain and so and so, from a given circuit. And then we will be talking about realization of differential amplifier at transistor level.

Characterization of a differential amplifier involves defining its key parameters: differential mode gain and common mode gain. Differential mode gain refers to how well the amplifier can amplify the difference between the two input signals, while common mode gain indicates how much of the common signals are amplified. Ideally, we want a high differential gain and a low common gain, emphasizing the importance of how well the amplifier rejects noise.

If differential mode gain is like a magnifying glass focusing on a specific small detail, common mode gain is like a foggy window that shows everything outside but doesn’t give clarity to the main picture. The clearer we want to see the details that matter (differential gain), the more the fog (common mode) needs to be minimized.

So, we may consider two structures namely, one is using BJT and other one it is using MOSFET. So, let us discuss about the basic operation and characterization of differential amplifier.

Differential amplifiers can be realized using different transistor technologies, primarily Bipolar Junction Transistors (BJTs) or Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs). Each type has its advantages and is suited for specific applications. BJTs are known for their high gain, while MOSFETs offer high input impedance and are often used in integrated circuits, allowing better performance in specific scenarios.

Choosing between a BJT and a MOSFET for implementing a differential amplifier is like choosing between a traditional film camera (BJT) and a digital camera (MOSFET). Both can take excellent pictures (amplify signals), but each has features that cater to different needs in terms of convenience, processing speed, and the types of environments where they work best.

So, in this connection supply connection we do have only one supply namely, V and the common terminal is ground here invariably we need a +ve DC voltage at both terminal-1 and terminal-2.

In the context of a differential amplifier, it is important to provide a proper DC voltage to the input terminals along with the AC signals. The DC voltage ensures that the transistors operate in the correct region (active region) for the amplifier to function effectively. This means the signals can swing above and below this preset DC level without distortion, ensuring accurate amplification of the input signals.

Think of the proper DC voltage like the base level in a swimming pool. The water level represents the usable range of the signal. If the pool is too shallow (insufficient DC voltage), you might hit the bottom (distort your signal), whereas if the pool is appropriately filled (right DC voltage), you can dive in (amplify the signal) without issues.

And then we do have the corresponding output we are observing at either terminal output-terminal-1 and output-terminal-2 or maybe we can compare this to voltage and then we call this is the differential output.

The output of a differential amplifier consists of two signals that are complementary to each other, resulting from the input signals processed by the amplifier. The output signal reflects the amplified difference between the two input signals. This complementary nature is crucial for the effective performance of the amplifier, specifically in how it deals with noise and distortion.

Consider the output of the differential amplifier as the final pizza delivery that has to balance the toppings (signals) between two plates (outputs). If one plate has more pepperoni (one input), the other must have less to keep the delivery consistent (amplifying the differential input). This ensures both plates (outputs) represent the correct and balanced order (the amplified output).

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