54.2.5. Voltage Gain Calculation

Let us assume that the performance requirements are given namely voltage gain it is given output swing it is given and input impedance is given to us and then, we need to find the values of different parameters.

In circuit analysis for a Common Gate Amplifier, we start by outlining the essential performance criteria we need to meet. This includes setting targets for voltage gain, output swing, and input impedance. Before beginning to calculate component values, it's important to confirm that the specifications we’re aiming for fall within the achievable limits of the circuit design.

Think of it like planning a road trip. Before you set off, you need to know your destination (output swing), how quickly you want to arrive (voltage gain), and how many passengers (input impedance) your vehicle can carry. Just as you wouldn’t start driving before checking your fuel levels and vehicle capacity, you shouldn’t begin circuit calculations without confirming these parameters.

The voltage drop across this resistance, it should be more than 4 V and the so that will ensure the +ve swing of the output voltage it is at least it is 4 V.

To ensure that the output can swing positively to the desired level, we calculate that there must be a minimum voltage drop across a specific resistor in the circuit. If our required output swing is ±4V, then the positive side must have at least a voltage drop of 4V provided by previous stages of the circuit. This drop is crucial because it indicates that the output voltage must be capable of reaching its target level without clipping or distortion.

Consider a water tank that needs to maintain a specific level of water (output voltage). If the inflow (voltage drop across the resistance) is too low, then the tank will never fill to the desired level. Just like the tank needs enough inflow to maintain a steady level, the circuit needs enough voltage drop to achieve the required output swing.

The gate voltage of this MOS transistor should be sufficiently low to ensure that the device remains in saturation.

For the MOS transistor to function properly and provide the desired gain, we have to ensure that its gate voltage is low enough. This is crucial because if not maintained, the transistor risks transitioning out of the saturation region, which affects the linearity and performance of the amplifier. Specifically, the gate voltage must be significantly lower than the drain voltage, allowing the transistor to remain fully 'on' during operation.

This scenario is similar to traffic flow at a busy intersection. Just as traffic needs to be regulated to keep cars moving smoothly, the voltage levels at different points in the circuit must be carefully controlled to maintain the performance of the amplifier. If a car (voltage) goes too fast through a red light (exceeding voltage limits), it can cause disruptions, just as exceeding gate voltage levels can disrupt the transistor's function.

We can say that the required values for the resistors were found, and the resulting gain is 5, although this is lower than expected due to device limitations.

After going through the calculations and adjusting parameters, we determine specific values for resistors to achieve the desired input impedance and maintain an acceptable output swing. In this case, the calculated gain of the circuit is found to be 5, which means the output will be 5 times the input signal. However, it is noted that this gain may fall short of initial expectations due to the inherent limitations of the MOSFET being used.

Imagine a team setting a target to run a marathon at a certain pace. After training, they find they can achieve a pace that's a bit slower than expected due to various factors (like fitness level or weather). They adjust their expectations and plan accordingly. Similarly, in circuit design, while aiming for higher gains is important, practical limitations often determine the final achievable performance.

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