47.1.1. Lecture – 47

Yeah, dear students, welcome back to NPTEL online certification course on analog electronic circuits. Myself, Pradip Mandal from E and EC department of IIT, Kharagpur. So today, we are going to continue the discussion on Common Collector and Common Drain Amplifiers.

In this introduction, Professor Pradip Mandal welcomes students back to the online course. He highlights that the focus of the lesson will be on Common Collector and Common Drain Amplifiers, which are important concepts in analog electronics. These amplifiers are used in various applications such as buffering signals and level shifting.

Think of the Common Collector Amplifier as a 'buffer' in a relay race where one runner hands over the baton to another. The buffer ensures that the signal (or baton) is passed smoothly without losing speed. Similarly, these amplifiers help in passing on the signal from one stage of the circuit to another with high efficiency.

So, the outline of today’s presentation is given in the next slide. What we are going to do today it is primarily focusing on numerical examples and design guidelines of common collector and common drain amplifiers.

The professor outlines that today's lecture will concentrate on exploring numerical examples associated with the design and performance of Common Collector and Common Drain Amplifiers. Understanding these examples helps in grasping the practical implications and design considerations of these circuits.

Imagine you're learning to bake a cake. Before attempting to bake one, you would want to review recipes and examples of cakes that have been successful. The numerical examples in this lecture serve a similar purpose by guiding students through actual applications and design processes, akin to following a trusted recipe.

We will discuss the analysis of voltage gain, impedance, and capacitance of common collector and common drain circuits for ideal situations as well as considering the different parasitics such as source resistance, load resistance, and collector or drain terminal resistances.

The lecturer explains that the analysis will first consider ideal circuit conditions to understand the basic workings of Common Collector and Common Drain Amplifiers. Then, various parasitic components—like source resistance and load resistance—will be factored in to realistically evaluate circuit performance. This helps in comprehending how imperfections can affect amplifier behavior.

Consider a bike ride on a perfect road (ideal conditions) versus a bumpy path with potholes (real-world conditions). Understanding how speed and comfort are affected in both scenarios illustrates the importance of recognizing ideal versus real conditions when analyzing circuit performance.

Let us start with one numerical example where we do have idealistic bias. We have a common collector amplifier and then the bias circuits are given here..., Collector current, it is approximately equal to the emitter current.

In this numerical example, the professor begins with an idealized setup for a Common Collector Amplifier. He notes the biasing conditions and highlights that under these conditions, the collector current is approximately equal to the emitter current. Setting up this scenario allows students to grasp how these parameters interact within the amplifier's operation.

Imagine a well-planned event where everything runs smoothly. The collector current being equal to the emitter current is similar to ensuring that all participants arrive on time, allowing the event to proceed as intended. They rely on each other to maintain the event's flow.

If I analyze this circuit and if I consider bias current, it is 0.5 mA… The emitter voltage is the 6 V, the base voltage - V_BE(on), it is 5.4 V.

The professor calculates the operating point of the transistor based on the provided bias current and voltage levels, exploring how voltages at different terminals are derived. This step is crucial as it defines the operational state of the transistor, which directly affects the amplifier's performance.

Think of setting a thermostat in a room. The desired temperature (operating point) is established based on current room temperature and heating systems (bias conditions). Just like achieving comfort in a room, determining correct operating conditions for amplifiers ensures they function optimally.

Now, let us look into the small signal parameters values; namely, g_m and r_pi… values of small signal parameter.

The next step involves calculating small signal parameters like transconductance (g_m) and input resistance (r_pi) to understand how these affect the amplifier's response to signals. These parameters play significant roles in determining gain and stability of the amplifier's output.

It’s like knowing the horsepower and torque of a car engine—these figures help in understanding how the car will perform under different driving conditions. Knowing the small signal parameters equips engineers to predict how the amplifier will behave when processing real signals.

The voltage gain it is A = (g_m r_o)/(g_m + r_pi + r_o), which we expect to be close to 1…

Here, students learn how to derive the voltage gain (A) of the common collector amplifier mathematically, utilizing the small signal parameters previously calculated. The goal of achieving a gain close to 1 is discussed, which is typical for common collector configurations that function as buffers.

Imagine a translator at a conference—if the translator conveys messages exactly as intended without adding or altering the content, it's like achieving a voltage gain of 1. The goal of such amplifiers is to output a signal without distortion, just as the translator aims to faithfully reproduce the speaker's intent.

Let me again clear it. And, let you go for the output impedance… it is coming from r_pi and r_o.

The lecture moves on to discuss input and output impedances of the amplifier. The input impedance should be high, while the output impedance should be low for effective signal handling. These factors are essential for ensuring that the amplifier interfaces well with connected components.

Consider a hotel check-in process. A high input impedance is like a reception desk that can handle many guests (signals) without overloading, while a low output impedance is like a quick check-out area that efficiently leads guests away. Ensuring smooth guest flow mirrors the role of impedance in amplifiers.

Now, we can find what is the upper cutoff frequency… so, we have the bandwidth of the circuit coming from the output node it is quite big.

In this part, the professor discusses how to calculate the upper cutoff frequency for the amplifier circuit. The significance of this frequency is highlighted, as it dictates the bandwidth within which the amplifier can operate effectively without distortion.

Think of a concert sound engineer who must set an audio equipment's frequency limits. Setting the upper cutoff frequency ensures that only the required sound frequencies are amplified. If set incorrectly, unwanted sounds can become overwhelming, similar to how incorrect upper cutoff frequencies can impact circuit performance.

In the next circuit, we consider a load capacitance here… then the corresponding impact.

This segment prepares students for exploring how the presence of parasitic elements like load capacitance can affect overall circuit performance when analyzed under more realistic conditions. Understanding these aspects is crucial for designs used in practical applications where ideal conditions are often not met.

Consider a sports team dealing with injuries—these unexpected changes (like parasitic elements) can alter the team's performance and strategy. Recognizing how these factors affect the outcome is vital, just as engineers must account for parasitics in circuit design.

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