52.3. Common Gate Amplifier

In the next slide, we have common gate amplifier having practical bias arrangement. Now; obviously, finding the operating point of the MOS transistor is slightly different from BJT and that may be one important exercise we must try to do with this.

This section introduces the common gate amplifier and highlights that the process of determining its operating point differs from that of a BJT (Bipolar Junction Transistor). The common gate amplifier is a configuration often used for MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors) and is important in understanding its biasing and operational characteristics.

Think of a common gate amplifier like a water tap (the transistor) which allows water (current) to flow from a tank (the power supply) to a garden (the load). In different tap designs (MOSFET vs. BJT), the way you access and control the water flow can change (different methods of finding the operating point).

So, we can say at the gate node we have 6 V getting generated by this R and R from this 12 V, supply V supply. So, we have 6 V here and then that is coming in series with the Thevenin equivalent resistance of R ⫽ R which is 50 k. And then, we have the MOS transistor and then we have R, I should not say R, I should say rather different resistance and it is its value it is 4 kΩ.

In this section, the working of the common gate configuration starts with the generation of 6 V at the gate node using a potential divider formed by two resistors. The Thevenin equivalent resistance of this divider is 50 kΩ. A MOS transistor is connected, and its bias resistor is 4 kΩ. The goal is to determine the drain-source current (I_DS).

Imagine you're filling a pool (the transistor) with water. The level of water entering the pool (6 V at the gate) is controlled by a network of hoses (resistors). The size and length of these hoses affect the total pressure (Thevenin equivalent resistance) that ultimately determines how fast the pool fills (how much current flows).

If I say that I current is flowing and that is creating a drop here at the gate of course, the current is 0. So, we can say that the 6 V it is directly coming here. So, V is 6 V and source voltage it is such that V it is supporting the required current here which is exactly producing this IR drop.

Here, the voltage at the gate (V_GS) is confirmed as 6 V, and since no current is flowing, this voltage directly corresponds to the voltage drop across the transistor. The section emphasizes that the gate-source voltage must support the necessary current for proper operation, highlighting the relationship between voltage and current in biasing the MOS transistor.

Think of the gate voltage as the pressure provided by a water pump that pushes water (current) through a pipe. If the pressure is too low (insufficient V_GS), water won't flow properly. We need just the right amount of pressure to ensure smooth flow.

So, we can say that this 6 V ‒ this 4 k × I that produces the V. If I say that I equals to into V and V is 6 V ‒ V of 1 V ‒ this IR drop which is 4 k into I.

This section involves setting up an equation to calculate the drain-source current (I_DS). By analyzing the voltage drops across the resistances and the transistor, the equation relates the gate voltage and the current flowing through the circuit. It showcases the fundamental principles of how to find the operating current for a MOSFET.

Consider the current through a pipe connected to a water source. The difference in water levels at different sections of the pipe (voltage across the resistors) determines how much water can flow through (current). Here, the goal is to calculate how much water flows (how much current) based on the pressure (voltage drop).

So, I as I said equals to 1 mA and hence g of the transistor which is having an expression of √ . So, that gives us g = 2 m/V. The other parameter we are looking for it is r small signal parameter.

In this section, the calculated DC current (I_DS = 1 mA) allows for the determination of small signal parameters, such as transconductance (g_m), which denotes the response of the MOSFET to input variations. The section focuses on finding the transconductance using its formula and highlights its significance in small-signal analysis.

Think of a musician (the transistor) amplifying sound. The louder the music played (the input), the more she sings back (the output). Transconductance as a measure shows how powerfully she can increase sound volume based on what she hears, representing her responsiveness to sound differences (input) during a performance.

The gain of course, from source node to drain node. Now, if I want to know what will be the voltage gain from the primary input to the primary output after considering this R = 10 kΩ, then we need to calculate what will be the corresponding input impedance.

This chunk addresses the calculation of the voltage gain from the source to drain of the MOSFET. It emphasizes the importance of resistance values in determining gains and how they affect overall circuit performance. The relationship between the input and output voltage features prominently here.

Imagine a speaker system. The gain is like adjusting the volume; how loud you can make the sound (voltage gain) is affected by how much power (input impedance) or resistance is present in the wires connecting the speaker to the power source. More resistance could mean less volume, hence lowering the efficiency.

So, the corresponding swings on the +ve side; however, it is 3 V. So, anyway for this design that is what the operating point and that is a corresponding signal swing.

In this section, the maximum and minimum voltage swings at the GPIO (general purpose input/output) terminal in the common gate configuration are discussed. The potential limits for how high and low the output voltage can go during operation are analyzed.

Think of an elevator. When you push the button to go up, the maximum height it can reach depends on the building's structure (the upper boundary). Similarly, the output voltage swing has upper and lower limits depending on operating voltage and circuit parameters, akin to how an elevator cannot go higher than the top floor.

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