16.2.2. Example Circuit

In this module namely week-2 modules, we are going through this non-linear circuit containing only one transistor and as I said that previously we have covered circuit containing one BJT. And, today we will be going through similar kind of circuit containing one MOSFET.

In this section, we understand the basic configuration of a MOSFET circuit. It is the continuation from earlier discussions about circuits using BJTs (Bipolar Junction Transistors). The focus here is to analyze how we can use a single MOSFET to perform similar functions, particularly in amplifying signals. The circuit uses the concept of applying direct current (DC) voltages to the transistor to analyze its behavior.

Think of a MOSFET in this circuit as a water valve. Just like you control the flow of water through the valve using a handle, you control electrical current in a circuit using the voltages applied at the MOSFET's gate. This helps students visualize how the input voltage can affect the current flowing through the circuit.

We are assuming that the device it is in saturation region which is equivalent to active region of operation of BJT; namely, in the channel if you see the drain end the channel pinch off it is happening. And if that condition is satisfied, in other words if V_DS is more than V_GS - V_th then the pinch off is happening at the drain end.

The saturation region is a crucial operating condition for MOSFETs. In this state, the transistor is capable of amplifying signals effectively. For the circuit to work as expected, the voltage across the drain-source (V_DS) must exceed a certain value determined by the gate-source voltage (V_GS) minus the threshold voltage (V_th). This condition allows the transistor to behave like a current source, amplifying the input signal effectively.

Imagine you have a balloon that you can inflate. When you apply enough pressure (voltage) at the input of the balloon (gate), it expands (allows more current) until it reaches a threshold where it can hold more air (saturation). If you don’t apply enough pressure, it becomes flat and doesn’t function properly.

The expression of the current I_DS of the device can be given by this formula, where K is the trans conductance parameter. Assuming this condition is getting satisfied namely the device is in saturation.

The current flowing through the MOSFET in saturation is described by a specific equation that incorporates key parameters like trans conductance (K). This parameter is critical as it determines how much current the MOSFET can conduct for a given gate-source voltage. Understanding this equation is crucial for predicting the behavior of the circuit under different input conditions.

Think of K as the size of a water pipe. A larger pipe allows more water to flow for the same water pressure. Similarly, in our MOSFET, the trans conductance determines how much current can be conducted for a given gate voltage, just like a wider pipe allows more water to flow.

If you compare the common emitter configuration based on the bias condition at the base we are suggesting to analyze the base loop to find the base current. Whereas, for this circuit the dependency of the current on V_DS or V_GS is different from the BJT circuit.

The analysis of MOSFET circuits differs from BJTs. In a BJT, the base current is significant for operation, and any resistor used would impact its performance due to the current flow. In contrast, MOSFETs have very high impedance at their gate, meaning they draw negligible current. This stark difference leads to the unique behaviors of each type of device, which must be understood for accurate circuit analysis.

Consider a BJT as a door that requires someone to push (base current) it open. In contrast, the MOSFET is like an automatic door that opens with just a signal (voltage at the gate) without any push required. This illustrates the fundamental difference in operation and analysis!

To find the operating point or to find the solutions point for given condition what are the steps we need to follow...

Finding the operating point of a MOSFET involves several steps, including determining the drain current, calculating voltage drops, and applying Kirchhoff's Current Law (KCL). We analyze how the current through the circuit creates voltage drops across resistors and subsequently determine output voltages. This systematic method helps establish the steady-state conditions under which the MOSFET operates effectively.

If we think of this process as following a recipe to bake a cake, each step—from mixing ingredients (finding the current) to placing it in the oven (determining voltage drops)—is necessary to achieve the final product: a well-functioning circuit that behaves as expected.

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