79.1. Analog Electronic Circuits

So dear students, welcome back to our NPTEL online certification course on Analog Electronic Circuits. Myself Pradip Mandal from E and EC Department of IIT, Kharagpur. Today’s topic of discussion it is continuation of Differential Amplifier.

In this introduction, the speaker is welcoming students back to a course on Analog Electronic Circuits. The focus is on discussing the differential amplifier, which is vital in analog electronics because it amplifies the difference between two input signals while rejecting any signals that are common to both inputs.

Think of the differential amplifier as a judge in a debate who listens to two speakers and only considers the differences in their arguments while ignoring any common points they make. Just like the judge, the differential amplifier focuses solely on the differentiating signals.

As I said that, the analysis part it is done in the previous 2 lectures, and we are going to talk about numerical examples, and we do have primarily differential amplifier using BJT then we do have differential amplifier using MOSFET and then also we do have another example where we do have the differential amplifier, we do have both types of transistor MOSFET as well as BJT.

The speaker outlines the agenda for the session, indicating that they will cover numerical examples of differential amplifiers built with both Bipolar Junction Transistors (BJT) and Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFET). This is essential for students as it provides practical applications of the theoretical concepts discussed in previous lectures.

Imagine learning how to cook. After understanding the theory behind cooking techniques, the next step is to practice with different ingredients. Here, the different types of transistors (BJT and MOSFET) serve as the various ingredients that will be used to create successful meals (i.e., amplifiers).

So, this differential amplifier having BJT’s it will be having different perspective; namely, the DC operating point and then small signal parameters, then differential mode gain, common mode gain and then going to the input range and output swing.

The speaker highlights the critical aspects of analyzing a BJT-based differential amplifier. This includes determining the DC operating point, which defines the amplifier's idle state, analyzing small signal parameters that affect its performance, and calculating gains for both differential and common modes. Understanding the input range and output swing is also crucial for ensuring the amplifier operates effectively without distortion.

Consider analyzing the performance of a car. You need to know how the car operates under normal conditions (DC operating point), understand its responsiveness to slight pressure on the gas pedal (small signal parameters), know the speed limits (input range), and comprehend how fast it can go without breaking down (output swing). Each of these factors is essential to determine the overall capability of the vehicle.

So we do have differential amplifier realized by BJT. So, this is the circuit we have discussed before and you may recall that in our most of our analysis we used to split this resistor R into two identical elements in parallel.

The speaker refers to a specific BJT differential amplifier circuit, emphasizing that in previous analyses, resistors were split into two identical parallel parts. This splitting helps in understanding how common mode and differential signals can interact within the circuit, providing insights into the circuit's behavior under different input conditions.

Think about how a balanced seesaw functions. If you have identical weights on either side, it stays level. Similarly, splitting the resistor helps analyze how the amplifier responds symmetrically to signals, akin to maintaining balance in weight distribution.

So, to start with, we do have this DC voltage given to us which is 2.6. In fact, this DC voltage should be sufficiently high, so that Q and Q should be in active region.

The speaker discusses the importance of setting the correct DC voltage (2.6V) for the transistors (Q1 and Q2) to ensure they operate in their active region. This ensures that the transistors can effectively amplify signals without entering saturation, which could lead to distortion.

Setting a comfortable temperature in a room thermostat ensures that the heating or cooling system works effectively. If the temperature (DC voltage) is too low or too high, the system might not function properly, similar to how transistors need proper voltage to amplify signals correctly.

Now, if I consider V drop of 0.6, then we do have the emitter voltage DC wise it is BE(on) 2 V.

After determining the DC voltage, the next step is to calculate the emitter voltage, which is affected by the base-emitter voltage drop (typically 0.6V for silicon transistors). This helps in assessing the overall voltage levels within the circuit, crucial for predicting how the circuit will perform.

Imagine pouring water through a pipe with a drop in height; the water pressure (emitter voltage) at the bottom will depend on the height difference caused by an obstruction. Similarly, the voltage drop across the transistor influences the voltage levels at various nodes of the amplifier circuit.

So, to summarize the DC operating point, we do have 2.6 V is the base voltage and then at the emitter. So here also, it is 2.6 V and at the emitter we do have 2 V. Then, voltage here it is 6.8 V and here also it is 6.8 V.

The speaker summarizes key DC voltage values obtained during the analysis. These include the base voltage, emitter voltage, and collector voltage, which are critical for establishing the operating point of the transistors and ensuring they function correctly.

When creating a budget for a project, you summarize all income and expenses to determine if you can afford it. Similarly, summarizing the operating points helps verify that the circuit will operate efficiently without issues.

So now we obtain the output swing. So, we obtain operating point, we obtain the DC voltage; now, next thing is the small signal parameter of transistors.

After establishing the operating points, the next focus is on evaluating the output swing and small signal parameters. The speaker indicates that understanding how the circuit will react to small variations in input (small signals) is vital for predicting its overall performance.

It’s like testing a car's suspension by driving over bumps. You want to see how the car (circuit) responds to these small changes in terrain (input signals), ensuring that it maintains stability and comfort during the drive.

So, the differential mode gain A = g R and this is equal to R it is 5.2 and g = and this is of course, it is kΩ.

The conversation turns to calculating gains, specifically the differential mode gain (A) from the small signal parameters and load resistance values. These calculations are critical for determining the efficiency and effectiveness of the amplifier in real-world applications.

Calculating gains in a circuit is similar to determining an athlete's performance. Just as you measure a runner's speed and distance covered to evaluate their performance, similar calculations in an amplifier determine how effectively it can amplify signals.

So, we do have the common mode component and we do have the differential mode component here and we do have the differential mode gain and common mode gain.

Differential and common mode signals are analyzed to understand their behavior within the amplifier. The speaker highlights the significance of calculating both types of gains to optimize the amplifier's performance while minimizing the effects of unwanted signals (common mode).

Think of a good speaker at a concert. They focus on their unique voice (differential mode) while ignoring background noise (common mode). By enhancing their unique sound while dampening the noise, they can ensure their message is clear.

To get the individual signal first thing is that, we need to see what is the differential input, and then what is the common mode input...

The process of estimating the output signals begins by differentiating between the differential and common mode inputs. The speaker explains how these components are handled to predict the overall output signals of the amplifier, emphasizing the methodology used in numerical examples.

This is like assessing a class of students. You evaluate each student's unique strengths (differential inputs) while also considering the class's overall performance (common mode inputs) to effectively measure educational outcomes.

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