56.4. Indian Institute of Technology, Kharagpur

So, in the next slide we will be mixing CE and CC. So, for our reference so, we do have the main table we are keeping it here and we will see that how the CE and CC will be helping us to improve the performance.

In this segment, the discussion revolves around two essential transistor configurations: Common Emitter (CE) and Common Collector (CC). The focus is on how these configurations can be mixed to enhance amplifier performance. The teacher suggests that they will be showing this through a table that summarizes important characteristics of both configurations, noticing how each plays a role in improving the overall circuit performance.

Consider CE and CC configurations like spices in cooking. Just as combining the right spices can enhance the flavor of a dish, mixing these transistor configurations can improve the performance of an electronic circuit.

So, we do have the basic CC configuration and its main characteristic or main rather from requirement is that input is at the base and output is at the collector.

The Common Collector (CC) configuration is defined primarily by its input and output terminals. In this configuration, the input signal is applied at the base of the transistor, while the output is taken from the collector. This design is particularly useful for converting a higher voltage to a lower one while maintaining the current level, making it ideal for certain amplification tasks.

Consider the CC configuration like a water tap. The input water (base signal) comes in, and the output stream (collector signal) can be directed elsewhere. The tap allows us to control the flow without changing the nature of the water (current).

So, if I say that if we do not have any bias current here this means that the base current of the second transistor it is same as the emitter current of the first transistor.

Biasing in a transistor is crucial for controlling its performance. In the context of these configurations, if there's no external bias current flowing into the system, the current flowing into the base terminal of the second transistor will equal the emitter current from the first transistor. This means that the first transistor effectively helps establish the working conditions for the second one based on its current flow.

Imagine a relay race where the runner passing the baton represents the current flow. If the first runner (transistor one) doesn't need to alter their speed (no bias current), they simply pass on their exact momentum (current) to the next runner (transistor two) without any adjustments.

So, if I draw this small signal equivalent circuit and if we are feeding the signal directly to the base to base of the second transistor, then we can draw the small signal model of the first transistor and then followed by the second transistor.

Creating a small signal model is a method used for analyzing circuits when dealing with variations in small input signals. In this case, the small signal model allows us to depict the behavior of both transistors by connecting the output of the first transistor directly to the input of the second, thus simplifying the analysis of how signals will propagate through these configurations.

Think of a game of telephone. The small signal model represents how whispers (signals) can travel from one player (transistor) to the next, allowing us to understand what the final message (output) will be and how clearly it will come across.

So, let us see how this configuration on this kind of mixing it is helping to get higher input resistance.

One of the primary goals of combining CE and CC configurations is to enhance input resistance. The teacher explains that the mixed configuration creates a system where the input resistance seen by the input signal is significantly higher, which is often desirable in amplifier design as it minimizes the loading effect on the preceding stage.

Imagine a sponge soaking up water. A high input resistance means the sponge can absorb much more without overflowing, making it less likely for the prior stage (source) to be affected by the process. This is particularly beneficial for ensuring signal integrity.

So, in summary what we can say of course, this β it is a second transistor β so, we should write β .

In summarizing the explanatory section, the focus is placed on the advantages achieved by using mixed configurations of transistors. Notably, it leads to increased input resistance and decreased output resistance, which are vital characteristics for effective amplification in circuits. The text emphasizes the value of the feedback factor represented as β, which influences performance across both transistors when cascaded.

Think about a teamwork scenario where each member's contribution (β for each transistor) amplifies the overall impact of the group. Just like a strong team can achieve more than the sum of individual efforts, the mixed transistor configurations leverage their properties for improved circuit performance.

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