67.6. Small Signal Analysis of Common Source Amplifier

Yeah. So, welcome back after the short break. And we were discussing about the limitation of the voltage gain of the common emitter and common source amplifier particularly if it is having passive load. And intuitively we understand that, how it can be enhanced. Namely in case if we can get some characteristic load line characteristic like this, instead of having a linear characteristic. In fact, that is the center point of getting higher gain of any amplifier using active load.

In this chunk, we learn about the limitations of voltage gain in amplifiers like the common emitter and common source, especially with passive loads. The speaker hints at how these limitations can be overcome by utilizing active loads, which can modify the load line characteristics of the amplifier. By changing the slope of the load line, we can enhance the amplifier's gain. This is a fundamental concept in amplifier design.

Think of a common source amplifier as a water pipe system. If you have a fixed-size pipe (passive load), water flow (signal) through it will be limited. However, if you introduce a variable pipe (active load) that can expand when needed, you can increase the flow rate significantly, representing an increase in voltage gain.

Now naturally, then who defines this current? For proper operation, we require both the current should be equal and we need to satisfy some condition to ensure that I and I ; I rather I they should be equal. ... So, that the both the devices are in saturation region and of course, their current should be equal.

This chunk focuses on how to ensure that two transistors in an active load configuration are functioning correctly. For them to operate efficiently, they need to be in a saturation region where their currents are equal, which is essential for obtaining the desired amplification. The speaker emphasizes the importance of maintaining both transistors in saturation to prevent unwanted shifts to the triode region, which could adversely affect the amplifier's performance.

Imagine two trains running on the same track. For the system to function without collision or delay, both trains must maintain a set speed (saturation) and stay synchronized (equal current). If one train speeds up or slows down unexpectedly (entering triode region), it could derail the entire system (the amplifier's performance).

In other words the (Kd) into its corresponding K called K′ of transistor. ... And these two parts on the other hand particularly this part ( ) and this ( ) they are referred as the second order part; because they are influenced on the current as long as the devices are in saturation region is very small.

This section elaborates on the concept of load line characteristics, which graphically represent how the output current and voltage relate to the overall gain of the amplifier. The speaker denotes two different portions regarding load line: first-order parts that have strong influences and second-order parts that have minor influences on current in saturation. Understanding these nuances can help optimize amplifier designs.

Consider a seesaw. The first-order parts represent significant weight (influencing current) that can greatly affect the movement, whereas the second-order parts symbolize lighter weights that change the seesaw's position slightly but not significantly impact the larger balance.

So intuitively you may say that the voltage gain A equals to slope of this line it is gm, divided by slope of this line. ... So if I consider slope of this line, it is slope of this line it is change in IDS or I with respect to V .

Here, we analyze how to calculate the voltage gain of the amplifier when using an active load. The voltage gain is determined by the ratio of slopes between the output characteristic curve and the load line. By recognizing that both the load line and output characteristics influence gain calculations, engineers can assess performance and make necessary design changes.

Think of voltage gain like the efficiency of a ramp. The steeper the ramp (slope), the easier it is to lift a heavy object (increase voltage). If both the ramp and the object have a favorable slope, you can achieve more lift compared to using a flatter ramp (passive load), which would make lifting harder.

So, in the next slide we do have the we have drawn the small signal equivalent circuit for the common source amplifier having active load. ... So, we can say that this is AC ground and this is also any way this is DC voltage so, we can see that this is AC ground.

In this section, we delve into the small signal equivalent circuit model for the common source amplifier with an active load. This model simplifies complex circuits into manageable components that can be analyzed to understand how the amplifier responds to small signal variations. These equivalent circuits are crucial for predicting performance in real applications.

Imagine you're analyzing the structure of a building. Instead of looking at the entire building with all its complexities, you create a simplified model using basic shapes. This model allows you to predict how the building will behave under various forces. Similarly, the small signal equivalent circuit helps predict how small input signals will affect the amplifier's output.

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