47.1.2. Common Collector And Common Drain Amplifiers (Contd.): Numerical Examples

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

In this introductory chunk, Prof. Pradip Mandal welcomes students back to the course and outlines what will be covered. The focus will be on practical numerical examples and guidelines for designing common collector and drain amplifiers. This sets the context for understanding the theoretical concepts through application in real situations.

Consider this as a cooking class where the instructor first introduces a recipe (numerical examples) before moving to the actual cooking (real-time calculations). Just like a good chef will explain the steps before getting started, in electronics, understanding theory through examples helps solidify the knowledge.

Whatever the knowledge we have gathered in our previous discussion namely, the analysis of voltage gain impedance and capacitance of common collector and common drain circuit for ideal situation as well as considering the different parasitics namely, source resistance, load resistance and collector or drain terminal resistances that will also be getting utilized in the numerical examples.

This chunk emphasizes the importance of parameters to analyze in amplifier circuits, including voltage gain, impedance, equivalence capacitance, and other parasitic elements. Understanding these factors is significant for numerical examples because they affect the amplifier's performance under various conditions.

Think of it like tuning a musical instrument. Just as musicians must consider various factors (like string tension and humidity) for the best sound, engineers must analyze multiple parameters in amplifiers to ensure they function optimally.

Let us start with one numerical example where we do have idealistic bias. So, we do have the common collector amplifier given here; and then we do have the bias circuits, it is given here. In fact, I should say that V it is BB BB making a bias at the base terminal and then we do have the DC supply of 10 V. So, V BB it is given here, it is 6 V. And then we assume that the thermal equivalent voltage it is 26 mV; then, we are also considering that load capacitance connected at the output node; C and its value it is say L 100 pF.

In this chunk, the stage is set for the numerical example, detailing the components and parameters. Voltage sources and thermal characteristics of the amplifier are introduced, highlighting their significance for the calculations to follow. It helps understand the system's limitations and design considerations.

Imagine building a model airplane. Just as you need to carefully select the materials (like wings, engine, and body) based on precise measurements, in amplifiers, we need to define clear specifications and parameters before we calculate performance.

So, if I analyze this circuit and if I consider bias current it is, 0.5 mA it is given to us. So, we can say that the collector current, it is also approximately equal to the emitter current. So, that is 0.5 mA, then the base current quotient current it is . That is µA. So, 5 µA, then the V it is given to us; so, V it is approximately 0.6 BE BE.

This chunk explains how to determine the operating point of the transistor by analyzing the bias current and corresponding voltages. It reinforces the relationship between base, collector, and emitter currents, which is fundamental to transistor operation. The calculations help verify whether the transistor is in its active region.

Think of tuning a guitar. You need to know exactly how tight each string is (the operating point) to ensure it produces the right sound. In amplifiers, having the correct operating point is essential for optimal performance.

Now, let us look into the small signal parameters values; namely, g and then r and then the r or rather in this case it is not m π r rather, r collected to emitter terminal resistance. So, let it go one by one small signal parameter; values of small signal parameter. Let we start with g and you may recall its expression in terms of the quotient current, it is collector current divided by thermal equivalent voltage and collector current it is 0.5.

In this section, the process of calculating small-signal parameters like transconductance (gm) and resistances is introduced. These calculations are crucial as they directly impact the performance of the amplifier, particularly gain and frequency response.

This is similar to optimizing a recipe where you need to measure the perfect amount of each ingredient to achieve the desired flavor. Just as adjusting one ingredient affects the overall taste, changing a parameter in the amplifier influences its performance.

The voltage gain it is A = . The other form it was having some any v v anyway, this this is the correct one. So, now ( = β; + 1) r ; and in the denominator we do have ( ) .

Here, the formula for calculating the voltage gain (A) is introduced. This is a key performance metric of the amplifier, as it determines how much the input signal is amplified. Students are urged to recall previous discussions and align calculations with expectations of the gain being close to one.

Think of a megaphone—its job is to amplify your voice. The voltage gain tells us how effectively the amplifier translates the input signal into a larger output, similar to how a megaphone projects your voice.

So coming to the output impedance, what we have it is at this point. So, we do have conductance here, conductance of the r , then r and then also the g part. The output resistance it is namely, g then due to r , and then due to r o .

This section discusses calculating input and output impedance critical for understanding how the amplifier interacts with other circuits. It explains how different resistances in the circuit affect the amplifier’s loading and input conditions, which in turn influence performance.

Imagine a racetrack. The impedance is like the track’s layout—smooth curves allow for high speed (low impedance), while sharp turns slow down the cars (high impedance). Similarly, the impedance in circuits affects the speed and efficiency of signal processing.

Now the input capacitance; so, we do have input capacitance C looking into this circuit, it is what we said it is C multiplied by a very small factor. So, what was that factor? It was r divided by . And then C as is; so again if you consider this is β and this is very high.

In this part, the role of capacitance in determining the frequency response of the circuit is discussed. It is essential to understand how capacitances affect the circuit's behavior at different frequencies, which helps in bandwidth calculations.

Think of traffic lights at an intersection. Just as high traffic at peak times can slow down vehicles (like high capacitance affects signal), understanding how to time the lights can help keep traffic flowing smoothly (stable frequency response in a circuit).

So, we can say that the upper cutoff frequency now, so this is done. Now the upper cutoff frequency if I say that f upper cutoff frequency, it is . So, that is, let me use different color should see ( ) R C .

This chunk provides insights on how to calculate the upper cutoff frequency, which is critical for determining the bandwidth of the amplifier. Understanding upper cutoff frequency helps to set limits on the frequency range where the amplifier works effectively.

Consider a radio antenna. The upper cutoff frequency is like the frequency beyond which the signal can't be received clearly. Just as the antenna's effectiveness is limited to certain frequencies, amplifiers have a defined operational frequency range.

So let me take a small break and then we will come back with some more numerical examples.

In concluding this section, the speaker breaks to prepare for further numerical examples. This highlights that learning is a continuous process, and the next steps will build on what has been covered, reinforcing the importance of practical application.

Like in a school session, after a lesson, a teacher may take a break to indicate that students should reflect on what they've learned before diving into the next topic. This pause allows time for comprehension and readiness for further complicated concepts.

--