49.2. Discussion on Common Base and Common Gate Amplifiers

So, based on our overall plan let us see what our situation is now if we are in week-5 and we are in the building blocks of analog circuits. And in week-5 we have completed common collector and common drain. Today we are going to discuss about the common base and common gate amplifiers. Under that we will be discussing about basic operation biasing, analysis and design and numerical examples will be covered will be covering later in the next class.

In this section, the instructor outlines the agenda for the lecture on common base and common gate amplifiers, essential components of analog electronics. He shares that they will explore the basic operations, biasing techniques, and analysis related to these amplifiers, keeping in mind that previous topics about common collector and common drain configurations were already discussed in prior classes.

Think of it like learning different cooking techniques in a culinary class. Having learned how to makebasic dishes like pasta and rice (common collector and common drain), today you're moving on to more specialized methods like grilling (common base) and baking (common gate). Each technique has its unique flavor and method, just like each amplifier configuration.

So, the motivation of these two new configurations; as we have discussed already about the common emitter and common source amplifier followed by common collector and then common drain. Now, we are entering into the third configurations. So, let us see what the basic motivation is as I said.

The instructor highlights the significance of transitioning to common base (BJT) and common gate (MOSFET) configurations. These configurations are motivated by the need to address performance degradation observed in earlier stages, especially concerning loading effects and bandwidth limitations when cascading amplifiers. The common base and common gate setups act as buffers in current mode, alleviating issues in voltage-based configurations.

Imagine you're running a relay race, where each runner's speed affects the next. If the previous runner tires too quickly (loading effect), it affects the overall team performance. By introducing a ‘strong helper’ between each runner (common base or common gate), you ensure that each participant maintains their speed, thus optimizing the team’s overall performance.

The basic operation it is I am keeping both the common base as well as common gate simultaneously because they do have a lot of similarities. So, the analysis can be shared across these two configurations.

Here, the instructor describes that both common base and common gate amplifiers share operational similarities. He indicates that they will discuss the basic function of these configurations while ensuring they can also apply findings from one configuration to the other. He emphasizes maintaining an adaptable approach in understanding how these configurations can cater to both current and voltage amplification needs.

Consider the concept of a universal remote control for different devices in a home. While the TV, sound system, and gaming console (common base and common gate) may differ in function, the basic idea of controlling them (amplification) remains the same. Just as the universal remote manages various devices, these amplifier configurations share core principles of operation.

Now, coming to the biasing of the common base amplifier, here how do you practically make this bias. Suppose you do have the BJT main BJT is here…

This chunk addresses the practical aspects of biasing, which is crucial for the stable operation of common base amplifiers. The instructor discusses various biasing methods, including both passive and active schemes. By using other transistors as part of the biasing network, one can ensure appropriate DC conditions required for the amplifier to function correctly.

Think of biasing like setting the correct temperature on a thermostat. Just as you need the right temperature for your heater (biasing) to work optimally, ensuring that the transistors in the amplifier have proper bias allows them to function effectively in amplifying signals. If the temperature is too low or too high, the heater won't work right, similar to how incorrect biasing can disrupt amplifier performance.

So this is the current mode buffer requirements it is a listed in this part. So, this is the current mode buffer, this is for our reference we are keeping the voltage mode.

In this section, the instructor outlines the specific requirements for current mode buffers using common base and common gate configurations. He contrasts these requirements with those for voltage mode buffers, highlighting how the configurations enable high output impedance and low input impedance, thus optimizing performance for current amplification.

Imagine using different types of bridges based on the type of traffic they carry; a pedestrian bridge serves foot traffic (voltage mode), whereas a highway bridge supports heavy vehicles (current mode). Similarly, the current mode buffers are designed to handle higher currents efficiently, just like the highway bridge is built for heavier loads.

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