20.1.2. Introduction to the Example Circuit

So, to simplify the analysis, we are considering example having only one MOSFET transistor in the circuit. We are basically linearization of input or output transfer characteristic of non-linear circuit containing BJT or MOS.

In this circuit analysis, the focus is on simplifying a non-linear circuit by using a single MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor). By concentrating on one transistor, we can better understand the effects of changing circuit parameters on the input and output transfer characteristics. The emphasis is placed on how non-linear circuits, which include BJTs (Bipolar Junction Transistors) as well as MOSFETs, behave and the need for linearization when performing circuit analysis.

Imagine trying to understand the flow of water through a complex network of pipes. If you simplify the system to only focus on one pipe at a time, it becomes much easier to analyze how changing the pressure at one end affects the flow. Similarly, in circuits, isolating one component (like a MOSFET) allows engineers to more clearly see how adjustments impact circuit behavior.

Today it will be similar kind of discussion, but then instead of BJT we are focusing on MOS transistor. From that we will see that there is the notion of small signal model of MOSFET transistor.

The module continues the discussion by highlighting the concept of a small signal equivalent circuit specifically for MOSFETs. A small signal model is used to analyze how small changes in the input signal (gate voltage) affect the output signals (drain current and output voltage). This concept is crucial for understanding the linearization of a non-linear circuit, as it helps in predicting how the circuit will respond to small perturbations around a certain operating point.

Think of a musician tuning a guitar. If the tuning is perfect (the operating point), slight adjustments to the strings will create small, predictable changes in pitch. The small signal model is like the musician focusing only on minor adjustments, allowing them to maintain a specific sound while playing without altering the overall tune drastically.

So, to come to the example; we do have the common source amplifier; at the gate we do have the voltage we are applying; and at the drain we are applying supply voltage through R.

The example circuit in focus is a common source amplifier configuration where the input voltage is applied to the gate of the MOSFET, and the output is taken from the drain across a load resistor. This section introduces the physical layout of the circuit, setting the stage for analyzing how changes in gate voltage affect the drain current and output voltage.

Consider a faucet controlling the flow of water. The gate voltage is analogous to how far you turn on the faucet – a small turn increases the flow minimally, but if you open it too wide, the flow becomes unpredictable. In a circuit, the 'water flow' is the current through the MOSFET and is influenced by how you manipulate the input voltage.

If I say that this is the input and if I say that this is the corresponding output; let me say this is also output, but let you consider this is the output; then input to output variation what is known as input to output transfer characteristic.

This section defines the concept of the input to output transfer characteristic. It explains how an input voltage variation (at the gate) leads to a corresponding change in the output voltage and current, creating a relationship that is typically non-linear in the case of MOSFETs. Understanding this characteristic is essential for analyzing circuit performance and behavior.

Similar to how adjusting the brightness of a dimmer switch gradually increases or decreases the light in a room, varying the input voltage at the MOSFET gate influences the output voltage and current. The relationship is not always straightforward, especially in non-linear circuits.

The input to output transfer characteristic is highly non-linear, because the device it is non-linear; and then we will be talking about how the non-linear characteristic curve it will be getting linearized.

The discussion emphasizes the need for linearization due to the highly non-linear nature of the MOSFET's input-output relationship. Linearization simplifies analysis by allowing engineers to work within a small, linear range around a chosen operating or quiescent point. This simplification is essential for making predictions about circuit behavior and for design purposes.

Think of driving a car uphill. If the incline is too steep (non-linear), the car's acceleration (output) varies unpredictably with gas pedal adjustments (input). However, on a gentle slope (linear range), minor pedal adjustments result in predictable acceleration. Linearization helps us work within the 'gentle slope' of the device's performance.

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