22.1.4. Modelling of Voltage Amplifiers

Now, what are the concepts we are going to cover today? It is, basically as I said that the model of voltage amplifier, then model of current amplifier, model of trans-conductance amplifier and trans-impedance amplifier.

This chunk introduces the key concepts of amplifier models, particularly focusing on the voltage amplifier model. It highlights that the lecture will cover various types of amplifier models, inviting students to comprehend the broader context of amplifiers in electronics.

Think of a voltage amplifier model as a map. Just like a map helps you understand how to travel from one place to another without needing to know all the roads and pathways, a voltage amplifier model helps us understand how input voltage translates to output voltage without needing to dive deep into the complexities of the actual circuit.

Say for example, the first one it is; it consist of BJT, second one it is having a MOS transistor. And for each of the cases what we have here it is, the DC voltage source and then of course the ground and along with the biasing element resister.

In this chunk, two types of transistors are introduced: BJT (Bipolar Junction Transistor) and MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor). It explains that these transistors are powered by a DC voltage source and involve biasing resistors to keep the transistors operating in their active regions, which is crucial for them to function as amplifiers.

Consider a pump that needs a steady power source (like a water pump connected to an electric power supply) to function properly. Similarly, transistors like BJTs and MOSFETs need a stable DC voltage (the power source) to amplify signals effectively.

So, whenever we are going for say model or linearization basically what you do; as I said that the transistor we are keeping in the appropriate region of operation, and then after that we try to find what is the relationship between the applied voltage here to whatever the corresponding output we are getting.

This chunk discusses how to create a linear model of the voltage amplifier by maintaining the transistors in their designated operational regions. The focus is on mapping the input voltage to the output voltage, leading to the development of a mathematical relationship that characterizes the amplifier’s behavior.

Think of this process as adjusting the settings of a carburetor in a car to ensure the engine runs efficiently. Just as you need to make specific adjustments to maintain optimal performance, you must ensure that the transistor operates well in its active region to properly model its behavior.

So, what we mean by the voltage amplifier? It is an equivalent linear circuit and it is main purpose is to provide the dependency of output signal on the input signal.

The chunk clarifies that a voltage amplifier is conceptually represented as a linear circuit. Its primary function is to demonstrate how the output voltage changes in response to variations in the input voltage. Understanding this relationship is crucial for analyzing amplifier performance.

Consider a good advertisement: A well-crafted ad (your input) should lead to more customers (the output) who respond positively to what you offer. Similarly, the voltage amplifier takes an input signal and amplifies it to produce a larger output signal.

The input resistance and whatever the external source resistance we do have, they are forming one potential divider.

This chunk highlights the importance of resistance within the voltage amplifier. The input resistance and the source resistance function together as a voltage divider, which affects the voltage received by the amplifier’s input. This interaction helps in understanding how external factors influence amplifier performance.

Consider two people sharing a cake, where one has a larger piece than the other because of the way the cake was divided. In an amplifier context, the input and source resistances determine how much of the signal (the cake) the amplifier gets to work with, influencing the final output.

So, naturally just by having this voltage dependent voltage source is not representing the main amplifier. So, I do have the input port here and v is the voltage at the input port, that is how it is defined.

The discussion in this chunk emphasizes that the presence of a voltage-dependent source alone does not fully describe the amplifier. It highlights the significance of loading effects, which occur when a load is connected to the amplifier’s output, potentially altering the expected output voltage.

Imagine a restaurant where chefs depend on a particular vegetable supplier. If the supplier doesn't deliver sufficient vegetables (like a load affecting the output), the chefs cannot prepare their signature dishes as expected (the output voltage). Thus, loading effects can impact the performance of the amplifier.

And then at the output we can put the corresponding load resistance. And so, the output resistance of this amplifier one, so output resistance looking into this output port.

This chunk explains the concept of cascading multiple amplifier stages. It describes how the output stage of one amplifier can be connected to the input of another, and the resulting relationships must be carefully understood to maintain the overall system’s performance.

Think of it like a relay race, where one runner passes the baton (output) to the next runner (input). Each runner must perform well for the team to win; similarly, each amplifier stage needs to maintain its performance for the entire circuit to function effectively.

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