17.7. Numerical Examples

Students welcome back to the topic of Analysis of non-linear circuit containing MOSFET after the short break. So, we are discussing about what will be the generalized method whether the signal or the input is applied to the NMOS or PMOS. And, we are discussing about the situation when the circuit contains the PMOS and the on the other hand the load as you can see here it is PMOS, and the load it is connected to ground and so and so.

In this section, the professor welcomes students back to the analysis of non-linear circuits that include MOSFETs (either NMOS or PMOS). The objective is to understand how the input signal behaves based on the type of MOSFET used. It sets the tone for exploring different scenarios and understanding how these circuits function depending on the configuration.

Think of a MOSFET as a traffic light. Depending on whether it's red (NMOS) or green (PMOS), it controls how the cars (input signals) can flow in a circuit. Just like a traffic system can manage vehicles differently based on the signal, this analysis shows how electrical signals are manipulated based on the type of MOSFET.

Now, let us see some numerical not numerical different situation, if the voltage it is changing at the gate and then what happens.

The focus shifts to different scenarios that arise when the voltage at the gate of the MOSFET changes over time. This is crucial as how the input affects the output can significantly vary based on these changes, mirroring the behavior of circuits featuring BJTs which were discussed earlier.

Picture a dimmer switch in a room—if you gradually increase the voltage (like turning the knob), the brightness of the light (output) changes accordingly. Similarly, when varying the gate voltage in a MOSFET circuit, the output response changes, which is what we will explore.

So, as we have discussed so for a given value of V_in, we can draw the I-V characteristic or the device or in you can say that output port I-V characteristic.

This paragraph discusses creating an I-V characteristic graph, plotting the output current against the output voltage for given input conditions. These I-V curves represent how the MOSFET responds to various gate voltages, with intersections indicating valid operating points.

Imagine plotting your monthly savings on a graph, where the x-axis represents the amount you saved (current) and the y-axis shows how much you can afford to spend (voltage). The intersecting points on that graph represent your financial balance, similar to how we find valid operating points on the I-V characteristic for a MOSFET.

Now for a given V_in, say V_in1, how do you find the current? So, then we need to consider the input to output characteristic namely I versus V_out.

To find the output current for a given input voltage, the relationship depicted by the input-output characteristics must be understood. The professor explains that this characteristic follows a square law dependence, typical of MOSFET behavior.

Think of this relationship like a water tank. The more water (input voltage) you pour in, the higher the tank fills (output current). The connection between the amount you pour and how high the water rises can be represented on a graph, similar to the input-output characteristic curve.

So, now if I vary this voltage, say if I increase this voltage to some other value say V_in2, that gives us different current say maybe at a higher value like I_DS2.

When the input voltage is increased, the professor notes the increase in current flowing through the circuit. The output voltage then adjusts accordingly, demonstrating how changes in input lead to varying output states.

This is akin to adjusting the throttle of a car. As you press down on the gas pedal (increase input), the car accelerates faster (output current increases). Such relationships in electric circuits can help us understand and predict the various behaviors just as we do with vehicles.

So, as you can see that if V_in is higher than threshold voltage V_th, then we can see that this characteristic it is going up or down and making this intersection point of the device characteristic with the load line going up and down.

The section explains that if the input voltage exceeds the threshold voltage, the MOSFET enters different operational regions, affecting how the output voltage behaves in relation to the input voltage. Understanding these regions is pivotal to predicting circuit functions and ensuring efficient operation.

Think of this concept like a driver needing to go uphill. If the car has enough power (more than the threshold), it can move up with ease (entering the saturation region), but if not, it struggles and might stall (move to the triode region). The threshold voltage is the tipping point that determines the car's ability to ascend smoothly.

The slope of this line namely that gives us the gain; that means, if I vary this input by some amount how much the corresponding effect will be observing at the output that gives us the gain.

The professor explains that the gain of the circuit is determined by the slope of the load line on the I-V characteristic graph. This slope reveals how sensitive the output response is to changes in input signal, a key aspect for using MOSFETs in amplifiers effectively.

Consider this gain like a volume knob on a speaker. When you turn it a little (input), the sound gets significantly louder (output). The way the volume increases compared to your small adjustment is analogous to the gain resulting from the slope of the load line.

What we like to say that; so, we already have discussed the input to output transfer characteristics. So, and then if you consider that input we are changing with respect to a DC voltage nothing, but say V_IN.

This part elaborates on the effect of superimposing a small AC signal over a DC voltage input. The analysis demonstrates how the circuit responds to such combinations, which is prevalent in real-life applications where signals have a DC offset.

Imagine listening to music on a speaker. When you adjust the volume (DC level) and then add a beat (small signal), the beat becomes part of the sound you hear. Similarly, in electronic circuits, we often mix DC signals with small AC signals, affecting how the output behaves.

So, we have analyzed a non-linear circuit containing MOSFET. We have discussed input to output transfer characteristics, the ability of the circuit to amplify signals, and the gain expression.

In this conclusion, the professor summarizes the analysis done, highlighting the key aspects of non-linear MOSFET circuits including their characteristics, operating principles, and the importance of understanding their gain for practical applications.

Wrapping everything up is like reviewing the key points of a recipe after cooking. By understanding how the various ingredients (concepts) work together, one can appreciate the final dish (circuit) and replicate it in the future.

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