20.1. Linearization of Non – Linear Circuit Containing MOSFET

So, welcome back to this course on Analog Electronic Circuits, we are almost to the verge of second week of on this course. And today’s topic of discussion is Linearization of a Non-Linear Circuit which contains MOSFET. So, to simplify the analysis, we are considering example having only one MOSFET transistor in the circuit.

In this introduction, we set the stage for discussing linearization in circuits that incorporate MOSFETs. The focus will be on simplifying the analysis by using a circuit with just one MOSFET. The linearization process is essential to understand how non-linear behaviors can be approximated as linear under certain conditions.

Think of linearization like flattening a hilly terrain to build a road. You may need to create a smooth path even if the natural landscape is undulating. In electronics, linearization helps create a 'smooth path' through the complex behaviors of non-linear components like MOSFETs.

In our overall flow, the discussion is about linearization of input or output transfer characteristics of non-linear circuits containing BJT or MOS. In previous modules, we already have seen linearization of input to output transfer characteristics of circuit containing BJT. Today it will be similar kind of discussion, but then instead of BJT we are focusing on MOS transistor.

This chunk outlines the context of discussing linearization within circuits, specifically shifting the focus from BJT (Bipolar Junction Transistor) to MOSFETs. It emphasizes that the approach to linearization remains consistent, but the characteristics of MOSFETs will be the primary concern today.

Imagine transitioning from one type of vehicle to another on a familiar journey; you still follow the same road rules but might have to adapt slightly depending on the vehicle's features. Similarly, when switching from BJTs to MOSFETs, the fundamental principles remain intact, but their unique characteristics necessitate specific modifications in analysis.

Similar to the BJT circuit, we will also be having a notion called small signal equivalent circuit and from that we will see that there is the notion of small signal model of MOSFET transistor. So, and then we will see that how the small signal model helps us to speed up the analysis and helps us to find a gain of the circuit.

The discussion introduces the small signal equivalent circuit concept, which is crucial for analyzing how a MOSFET behaves under small voltage variations. This model simplifies calculations by allowing engineers to focus on small perturbations around a quiescent point, making it easier to determine the circuit's gain and response.

Consider a small change in your daily routine, like switching from coffee to tea. While most of your routine remains the same, you are tweaking one element slightly. In electronics, the small signal model allows us to analyze small changes in voltage/current without having to reconsider the entire circuit.

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

This chunk introduces the common source amplifier configuration, essential for understanding MOSFET functionality. The gate voltage determines how the transistor operates, while the output voltage observed at the drain indicates the amplification effect of the amplifier.

Imagine a faucet controlling the flow of water into a basin. Turning the faucet (gate voltage) dictates how much water (current) flows out into the basin (output). Just like the water flow amplifies depending on how wide you open the faucet, the MOSFET amplifies signals based on the gate voltage.

So, if we vary the gate voltage Vg, we like to observe the corresponding effect on the Id and on the Vds, which is the output of the circuit. This is the information it is known to us; if the transistor is in saturation region, if it is in saturation region the expression of this Id in terms of Vgs and Vds is given here.

This section addresses how changing the gate voltage affects the drain-source current (Id) and the output voltage (Vds). The saturation region is key in understanding the MOSFET's operation, which tells us how the output responds to input variations and establishes the relationships between the voltages and currents involved.

Think about a dimmer switch controlling the brightness of a light bulb. Adjusting the dimmer (gate voltage) changes the brightness (output voltage), demonstrating how input variations directly influence output behavior. Similarly, varying the gate voltage adjusts Id and Vds, allowing for control over the circuit's performance.

So, we are expecting this transfer characteristic it will be 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.

In this section, the expectation of highly non-linear input-output transfer characteristics due to the nature of the MOSFET is discussed. The goal is to show how we can linearize these characteristics in the vicinity of a specific operating point (Q-point), thus simplifying the analysis.

Imagine riding a bicycle on a winding road. The path can be highly curvy (non-linear) but if you keep your focus on a small segment of the route, it becomes easier to navigate straight (linear). In electronics, we'll focus on a small area of the non-linear curve to analyze it more simply.

Whenever we are talking about linearization, probably we need to fix one point called quiescent point or Q-point and with respect to that Q-point we may try to linearize.

The Q-point is crucial for establishing an operating point around which we can linearize the circuit's behavior. By determining this point, we ensure that our small signal linearization is valid over a range of variations, allowing for more accurate predictions in performance.

It's similar to tuning a musical instrument. You establish a baseline note (Q-point) from which you can accurately adjust and play other notes. In circuit design, finding the Q-point allows engineers to adjust input signals meaningfully without losing the intended performance.

In summary, linearization is essentially capturing a small signal response around the Q-point so that we can analyze the small fluctuations instead of the complete waveform, allowing for a more manageable analysis.

This segment summarizes the purpose of linearization, emphasizing how we can reduce complex behaviors into simpler, manageable analyses by focusing on small signal variations around a fixed operating point. This helps in effectively understanding and designing circuits.

Consider a chef trying to adjust a recipe. Rather than changing every ingredient at once (complex changes), they might focus on just one element (small signal), like the amount of salt, to taste and improve the dish gradually. In electronics, this approach allows for careful adjustments leading to better circuit performance.

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