53.1.1. Common Base and Common Gate Amplifiers (Contd.): Numerical Examples (Part C)

So, dear students welcome back after the break. And before the break we were discussing about Common Base amplifier and Common Gate Amplifier.

This chunk introduces the topic of Common Base and Common Gate amplifiers. The speaker welcomes students back and refers to a prior discussion about these amplifiers. Understanding these amplifiers is essential as they serve different uses in electronic circuits, particularly in signal amplification.

Imagine a common base amplifier as a booster in a mobile phone that amplifies the weak signals received from towers. The common gate amplifier acts similarly but might be used in a different scenario, like enhancing signals in a network. Each amplifier has its unique role, much like different devices that help us communicate better.

It is very important to keep this capacitor sufficiently large so that the base node particularly for the signal should be working as a ground. This is important specifically for common base.

This chunk emphasizes the significance of using a sufficiently large capacitor at the base node of the amplifier circuit. The capacitor helps keep the base node at AC ground, which is necessary for proper signal processing. If the capacitor is not adequate, the amplifier might not function effectively.

Think of the capacitor as a sponge that absorbs fluctuations in water levels. If the sponge is too small (a small capacitor), it cannot handle all the changes, leading to overflow (poor signal handling). A large sponge (large capacitor) manages these fluctuations much better, maintaining stability.

Now we are going to talk about the performance of the common base amplifier without this capacitor.

In this chunk, the focus shifts to analyzing the performance of a common base amplifier when the capacitor is removed. The speaker aims to understand how critical parameters such as voltage gain and input resistance are affected when the capacitor is absent.

Removing the capacitor can be likened to removing a filter from a water pipe; without it, impurities (noise) may enter the water (signal), affecting the purity and quality of what comes out. This illustrates how the absence of a capacitor can lead to degradation in performance.

If we have the capacitor connected, the expression of the voltage gain has been enlisted here.

This chunk highlights that when the capacitor is present, there is a specific expression for voltage gain, which can be used for further calculations. Understanding this expression helps in quantifying how much the amplifier boosts the input signal.

Imagine the voltage gain as a magnifying glass that increases the size of an image. Just as different lenses magnify images differently, various configurations in our circuit utilize the connected capacitor to enhance the signal to varying degrees.

So, the summary is that this input resistance is getting modified here, and we also have this R.

The input resistance of the amplifier changes significantly based on the configuration of the circuit and the presence of the capacitor. When characterizing amplifiers, it's crucial to understand how these resistances affect overall performance, as they can influence how much input current the amplifier will receive.

Consider input resistance like the width of a funnel. A wider funnel allows more liquid (current) to flow into a container (the amplifier). Similarly, higher input resistance means the amplifier can accept more signals effectively. If input resistance is limited, less signal can flow through.

If we are not using this capacitor, then the input resistance is quite large now compared to if I use the capacitor.

The absence of the capacitor leads to a significant increase in the input resistance of the amplifier. This change indicates the performance degradation we discussed earlier, as the ability of the amplifier to accurately reproduce input signals is compromised.

Think of this scenario like a doorbell that only works when powered. When the power is on (capacitor present), it rings effectively. If the power is off (capacitor absent), it doesn’t ring at all, leading to high input resistance and a signal that’s weak.

The gain of the common base amplifier from emitter to collector is significantly reduced.

The reduction in voltage gain when the capacitor is removed is emphasized. This point connects directly to the previous discussions regarding how critical the capacitor is for maintaining performance in amplifiers.

Imagine a powerful speaker system that only works when connected to its power source. When disconnected, it becomes quiet and ineffective. The same principle applies here; without the capacitor, the amplifier loses its capacity to boost signals effectively.

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