21.5. Lecture - 21

Welcome back after the short break on the topic of Linearization of input or output transfer characteristic.

So, we are discussing about this a small signal equivalent circuit we are about to start that, and the intension there it is of course, to get the simplified circuit. So, you may recall the equivalent circuit if I consider the large signal behavior, what we said it is and the transistor may be replaced by a current source dependent current source.

This chunk introduces the concept of a small signal equivalent circuit, which is a simplified version of the larger, more complex circuit that involves a MOSFET. The goal is to create a model that can help analyze circuits more easily. It begins by describing how a transistor can be modeled as a dependent current source, where the current depends on the voltages across its terminals.

Think of a small signal equivalent circuit like a simplified version of a complicated recipe. Instead of following every single detail (like the larger signal model), you focus on just the important ingredients and steps that affect the outcome of the dish.

In contrast to that whenever we are doing the linearization what you are doing is we are simplifying this current equation, namely the drain to source current we are considering only the small signal part, which means that i, which is having an expression like one of the 3 expressions or ok.

This chunk discusses the key difference between large signal and small signal models. In the small signal model, we focus solely on the small variations in current and voltage, allowing us to analyze the circuit under small-signal conditions, which leads to simplified calculations.

Imagine you are trying to measure how a car accelerates from a stoplight. If you consider the entire journey (large signal), you have to account for every traffic light and stop sign. But if you only focus on small adjustments in speed between red and green lights (small signal), the calculations become much simpler and more manageable.

Now if you see in this model and this model, here we do have one factor. So, this factor it is it depends on the size of the transistor, it depends on the device parameter, it depends on the operating point also. And, so, if I say that this is one parameter called g; why g , it correlates the v to ids. So, it is conductance so, that is why g and m stands for trans mutual from input port to output port.

This chunk introduces the transconductance parameter (gm), which is crucial in small signal analysis. This parameter relates the small changes in gate-to-source voltage (vgs) to the small changes in drain-to-source current (ids), allowing us to characterize how effectively the MOSFET can control current flow based on small input changes.

Think of transconductance like the responsiveness of a thermostat. Small changes in room temperature (input) result in small adjustments in heating or cooling (output). A sensitive thermostat responds quickly to tiny temperature changes, just as a MOSFET responds to small changes in voltage.

So, whenever you are talking about small signal model of the device, it involves one new parameter or new set of parameters rather and one of them is the g. So, in the while you are going from large signal equivalent circuit to small signal equivalent circuit. First thing is that we need to drop the dc part and also we have to get this model involving new set of parameters.

In this chunk, we learn that when transitioning from a large signal to a small signal model, it is important to identify the operating point (also known as the quiescent point or Q-point). At this point, you typically ignore the DC components of the voltages and currents to focus on the AC small signal response. The new parameters derived from the small signal model reflect the transistor's behavior around this operating point.

Consider how a car's speedometer works. When you are parked (DC condition), the speed is zero. If you start driving slowly (small signal operation), the speedometer starts responding to small changes in speed. The parked condition is like our DC operating point; we ignore it to focus on responsive speed changes.

So, if you consider that, what we will be getting is that of course, this I is dependent through this part, we may get a drain to source one conducting element. And, that conducting element is nothing but the output conductance or drain conductance.

This section discusses output conductance (gd) as another important parameter of the small signal model. The output conductance measures how much the drain current (ids) changes in response to a change in the drain-source voltage (Vds), providing insights into the circuit's performance under different conditions.

Imagine a water faucet. The output conductance would be like how much water flows out for a given turning of the faucet handle. If you're not careful, just like with a MOSFET, small changes in your handle position can have large effects on how much water (current) flows out!

So, just to simplify it is normally referred as high frequency small signal model, on the other hand if we drop this to capacity of element it is simply said it is small signal model of the MOSFET.

This chunk explains practical applications of the small signal model in engineering. Engineers use the small signal model for simplifying the analysis of circuits, especially as they design or analyze circuits that involve MOSFETs at varying frequencies. It's important to understand that at higher frequencies, we must also consider additional components like capacitors.

Think of this like tuning a musical instrument. At different frequencies (like tuning different notes), the string tension (circuit components) needs to change to maintain good sound (circuit performance). Ignoring these adjustments at high frequencies can lead to poor sound quality or circuit operation.

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