57.2. Lecture – 57

So, dear students welcome back after the break. So we are talking about composite transistor. And what we said is that if we have amplifier particularly multi configuration amplifiers where two transistors are having different configuration or maybe the same configuration. Then the analysis can be done slightly you know smarter way.

In this chunk, we are introduced to the idea of composite transistors. Composite transistors involve connecting two transistors together, either in the same or different configurations. This approach allows for smarter analysis of multi-configuration amplifiers, which can make the circuit's behavior easier to understand and analyze.

Consider a team of two athletes working together in a relay race. Each athlete (transistor) brings their own strengths to the race. When they work together, they can complete the race (amplifier) more efficiently. Just like analyzing their combined performance might involve looking at each athlete's contribution, analyzing composite transistors requires understanding how each one works within its configuration.

And to do that what you can do two transistors together we can consider a single one say for example, you do have Q and Q together. Where Q it is its collector is connected to V_dd supply. And its emitter it is directly connected to transistor-2 and then at the emitter will may or may not be having this bias current. And then the second transistor collector we can consider it is collector of the composite structure. And then emitter of Q it can be considered as emitter of the composite structure on the other hand base of the Q transistor it can be considered base of the composite structure.

This chunk explains how to visualize composite transistors as a single unit. The first transistor's collector connects to the supply, while the emitter connects to the second transistor. Here, the second transistor's collector and the first transistor's emitter serve as the composite transistor's collector and emitter. This simplification makes it easier to analyze the overall behavior of the composite transistor.

Think of building a bridge. Each part of the bridge is like a transistor. When constructing the entire bridge (the composite transistor), you combine all parts (transistors) into one structure. Just like understanding the overall strength of the bridge requires knowing how each part fits and works together, understanding a composite transistor requires knowing how both transistors connect and interact.

So, with that here the whole things, the shaded portion if I consider one transistor and then if you find its small signal parameter. And then we can use this transistor for two configuration say common collector configuration and then common emitter configuration.

In this section, the lecturer discusses how to use small signal parameters to analyze the composite transistor in two configurations: the common collector (CC) and common emitter (CE). By understanding these configurations, we can predict how the composite transistor will behave in various situations, enabling more effective design and troubleshooting of amplifier circuits.

Imagine you are planning a school event with different roles. For instance, you might assign one student to manage decorations (common collector) and another to handle music (common emitter). Understanding what each student brings to their role helps you organize the event efficiently. Similarly, analyzing how a composite transistor operates in various configurations allows engineers to optimize circuit functionality.

So, let us try to see that how we are getting it say. We do have the basic composite structure here whether we do have this bias circuit or not if we connect the circuit in say CE configuration. So, which means that the emitter we can connect to ground and at the collector we can put a bias resistor R connected to V_dd. And so, we can call this is the output port and at the base we can connect maybe R providing the bias current for Q_base bias current for Q and then we can feed the signal there with a meaningful DC.

This chunk describes how to set up a composite transistor in a CE configuration. It explains the connections involved: the emitter to ground, a bias resistor at the collector, and a base resistor for providing bias current. This configuration directs the output at the collector, enabling the analysis of the composite transistor's performance and behavior.

Think of setting up a speaker system for a concert. You need to position the speaker (collector) correctly, connect it to a power source (bias resistor), and make sure it's tuned (bias current) to deliver sound effectively. Setting up a circuit with a composite transistor is similar: proper connections ensure the circuit functions as intended.

So, if I say that entire circuit equivalently it is working as one transistor where the collector we call C(C), emitter it is E(C) and then base is B(C). Then whenever we like to get see performance of the corresponding amplifier coming out of this composite transistor first thing we need small signal parameter.

In this part of the lecture, the concept of simplifying the entire composite structure into an equivalent single transistor model is introduced. Naming the equivalent collector, emitter, and base helps in analyzing the small signal parameters necessary for evaluating the amplifier's performance.

Imagine looking at a large production line and focusing on just one machine that represents it. By treating the entire line as a single machine, you can analyze its productivity and efficiency more easily. Similarly, simplifying a composite transistor to a single equivalent model allows engineers to effectively assess circuit performance.

So, what are the small signal parameters? So, β of the transistor then r collector to emitter resistance. Then trans conductance and then base to emitter resistor resistance r_π. And just to consider this is this set is parameters of the composite transistor let you put the superscript C here for each of the parameter.

This chunk gives specific definitions for the small signal parameters critical for analyzing the composite transistor. Parameters such as beta (β), the resistance from collector to emitter (r), transconductance (g_m), and base-emitter resistance (r_π) form the foundation of evaluating the composite transistor's performance.

Think of a car as the composite transistor. Each parameter (speed, fuel efficiency, weight) is important for assessing its overall performance. Just like you would consider each factor to understand how well the car performs, engineers analyze these parameters of composite transistors to predict how well they will work in circuits.

So, we can say that this is R ⫽ r. So, that is the first thing and then, so this is very straightforward to get. Then next one it is if you see the β of the transistor which means that the current gain current gain if I consider we do have some base current is flowing and then whatever the current is going flowing through the collector to emitter which is β(C)i_b.

In this section, the lecturer discusses how to derive and express various small signal parameters in terms of the internal characteristics of the transistors making up the composite transistor. This includes expressing beta in terms of input and output current relationships, emphasizing the interdependence of these parameters.

Think of making a smoothie. Each ingredient (parameter) adds flavor (performance) to the final smoothie (composite transistor). Just as you can express the final flavor in relation to the amounts of each ingredient you add, you can express the composite transistor's performance in terms of its component parameters.

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