27.8. Summary of Changes

So, interestingly depending on the value of this R, we can see that v it is rather a small fraction of v. Whatever it is this output voltage we said equals to –g × R × v.

This chunk discusses how the value of the emitter resistor (R) affects the output voltage (v). When we talk about amplifier gain, we often see that a certain circuit condition results in the output voltage being a fraction of the input signal. The key equation represented here shows that the output voltage is inversely affected by the gain factor that involves R. This means if R has a significant value, the gain decreases, showing a reduced output relative to the input.

Imagine you are trying to fill a balloon (the output voltage) with water (the input voltage), but you have a narrow pipe (the resistor R) that's limiting how fast you can fill it. If the pipe is very narrow (large R), it takes much longer to fill the balloon even if you have plenty of water flowing from the tank (the input signal).

In fact, the main motivation of putting this R is to stabilize the operating point of the circuit in case if beta is changing. So, you can think of that R, it is desensitizing the circuit or rather its operating point it is getting desensitized against the variation of this beta.

This chunk highlights the purpose of the emitter resistor (R) in stabilizing the amplifier's operation against changes in beta (β), which represents the transistor's current gain. Resistors can help ensure that variations in beta do not drastically affect the circuit’s performance. This is particularly important in maintaining a steady operation point for the amplifier, despite fluctuations that may occur in circuit parameters.

Consider a governor in a car (the emitter resistor) that maintains a steady speed (the operating point) regardless of how much pressure you put on the gas pedal (variations in beta). Even if the car’s engine responds differently at different speeds, the governor helps ensure that the car goes at a consistent speed despite these changes.

Apart from the voltage gain open loop voltage gain, we do have two more important parameters namely input resistance and output resistance of the model.

In this section, the focus shifts to two additional significant parameters of the amplifier: input resistance and output resistance. Both of these resistances affect how the amplifier interacts with other components in a circuit. Input resistance is concerned with how much the amplifier resists incoming signals, while output resistance pertains to how the amplifier manages signals flowing out. Understanding these resistances is crucial when designing and integrating amplifiers into larger circuits.

Think of input resistance like the funnel at the top of a water tank: it controls how much water can enter. If it’s too narrow (high input resistance), not enough water gets in. Meanwhile, output resistance is similar to a hose's diameter at the bottom of the tank: if it’s too narrow (high output resistance), not enough water can flow out when you want to use it.

But if we recall the expression of the voltage gain, it was only this much. Now, we do have additional factor here which is in fact degrading the gain of the circuit.

This part discusses how an added factor - the emitter resistor - can diminish the overall gain of the amplifier. In circuits, any extra resistance can detract from the effectiveness of the amplifier, meaning it won't amplify signals as much as it could without that resistor. This degradation shows the trade-offs involved in circuit design, balancing stability and performance.

Imagine you are trying to boost the volume on a speaker (amplifier gain). If you add extra foam around the speaker (the emitter resistor), while it may look nice and reduce feedback noise (provide stability), it can also muffle the sound (reduce gain). The sound isn't as loud or clear compared to if the speaker was open and unobstructed.

One consequence is that of course, there will be a dc current flow here so, that practically increases the power dissipation.

This chunk indicates that in circuits, while striving to achieve lower cutoff frequencies, there are often trade-offs such as increased power dissipation due to DC current flow. This means the circuit may become less efficient when striving to achieve desired performance characteristics like lower frequencies.

It's similar to trying to keep a car running efficiently at very low speeds. While you may achieve a very gentle speed (lower frequency), you might end up using more fuel (power dissipation) in the process compared to when the car is cruising at higher speeds.

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