63.2. Lecture – 63 Multi-Transistor Amplifiers: Cascode Amplifier (Contd.) – Numerical Example (Part A)

Dear students, welcome back to our online NPTEL certification course on Analog Electronic Circuit. Myself, Pradip Mandal from E and EC department of IIT Kharagpur. Today we are going to talk about Multi Transistor Amplifiers; namely, in fact this is continuation of our previous lectures. So, today we will be talking about Numerical Examples of Cascode Amplifiers.

In this chunk, the lecturer welcomes the students and expresses the focus of the session, which is on cascode amplifiers. Moving forward from previous lectures, this session will delve into numerical examples, thus building on the conceptual understanding of the students.

Think of a car engine that has multiple components functioning together to increase its efficiency and performance. Each component represents different aspects of the cascode amplifier working in conjunction for better output.

So, the coverage of today’s lecture it is primarily cascode amplifier using BJT and cascode amplifiers using MOSFET. We do have two very in depth numerical problems... But based on the time availability, I may be giving you some hint, but you have to work it out, ok.

The lecture will focus primarily on two types of cascode amplifiers: those built with BJT (Bipolar Junction Transistor) and MOSFET (Metal Oxide Semiconductor Field Effect Transistor). The instructor mentions that there are two detailed numerical problems to work through, highlighting an interactive approach to learning where students are encouraged to engage with the material actively.

Imagine you're learning to cook a new dish. The instructor provides two recipes (numerical problems) to practice with—one recipe focuses on using a stovetop (BJT) while the other uses an oven (MOSFET). Both methods aim to achieve a delicious meal, just like how the different amplifiers serve similar functions.

Coming to the cascode amplifier using BJT, so here we do have the numerical problem. This circuit, the cascode amplifier we already have seen before... And for both the transistors C and C from base to collector are given here...

In this section, the lecturer outlines specific parameters relevant to the BJT-based cascode amplifier. These parameters include Early voltage, beta values of the transistors, and coupling capacitors. Understanding these parameters is crucial to analyzing the amplifier's function and performance in practical situations.

Consider building a house where each room (parameter) has a specific function. Knowing the dimensions and layout (early voltage, beta values, etc.) helps ensure everything fits together properly and serves its purpose effectively, just as parameters do in a cascode amplifier.

In the bias circuit on the other hand we assume that, the value of this resistance it is 2.8 k and the bias current here I bias for the first part of the example...

The operating point of the transistors is determined through calculations involving supply voltage and resistance values. The lecture elaborates on finding the base and collector voltages, ensuring the transistors remain in the active region. This foundational knowledge lays the groundwork for analyzing the amplifier's small-signal performance.

Think of tuning a musical instrument. The operating point helps achieve the right pitch (voltage and current levels) to ensure that the instrument sounds pleasant (optimal amplifier performance). If the tuning is off, the instrument won't play in harmony.

Now, so we obtain the operating point of the both the transistors. And let us see what will be the value of small signal parameters namely g, then r and r of the two transistors...

Upon determining the operating point, the next step is to calculate small signal parameters, which provide insights into the performance of the amplifier. Parameters like transconductance (g) and output resistance (r) are vital for understanding how the amplifier responds to input signals.

Calculating these parameters is similar to a race car team analyzing speed and handling characteristics. Just as teams quantify performance metrics to enhance outcomes during a race, engineers quantify small signal parameters to optimize amplifier designs.

So, what is the voltage gain? So, try to remember these values of the small signal parameters to get the voltage gain...

Here, the lecturer leads through calculating the voltage gain, a critical performance metric of the amplifier. The signal flow from the input to output exhibits how variations in small signal parameters affect the overall gain, emphasizing the importance of design choices.

Consider the voltage gain as the speed of a delivery truck. A truck's design (amplifier components) affects how quickly it can deliver goods (signals). Higher gain means the amplifier can amplify signals more effectively, much like a faster truck transports goods.

So, C in, input capacitance of this entire circuit looking at the base or transistor-1 which, is equal to we do have the C...

The calculation of input capacitance is essential for understanding how input signals interact with the amplifier. The relationship between capacitance and bandwidth determines how quickly the amplifier can respond to changes in input signal, essential in high-frequency applications.

Think of a water hose: the larger the diameter (higher capacitance), the more water (signal) can flow through without restriction. In amplifiers, a higher input capacitance can slow down response time, analogous to how a narrow hose limits water flow.

To summarize that, if I compare the common emitter amplifier and then cascode amplifier; we can say that, for both the circuits the gain it is very close to each other...

The lecturer compares the performance metrics of the cascode amplifier with the more traditional common emitter amplifier. While gains may be similar, the advantages of the cascode amplifier in terms of bandwidth and input capacitance are emphasized, demonstrating its enhanced performance for specific applications.

Imagine evaluating two cars: both get a similar mileage (gain), but one car has superior handling (bandwidth) and is more fuel-efficient (lower input capacitance). This highlights the nuanced performance differences that may favor one option over another.

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