50.2. Common Base and Common Gate Amplifiers: Analysis (Part B)

So, let me go to the corresponding circuit here yeah. So, we do have common base circuit here. So, this is the common base amplifier and this is the corresponding small signal equivalent circuit. For to save some time what I have done is that I have drawn this circuit, but I will explain that what the things I have done are. If we see here the bias circuit in the small signal equivalent circuit, we have dropped this DC current and we consider only the resistance here R1. So, likewise here at the emitter side, the DC current we have dropped and we have considered only this R2 here. Then this is the small signal which is getting coupled through the capacitor. So, we simply have shorted it, signal it is coming to the directly coming to the emitter and then at the base, we do have DC voltage.

In this chunk, we explore the small signal equivalent circuit for a common base amplifier. Essentially, this method simplifies the analysis by removing direct current (DC) components and focusing on alternating current (AC) signals. The resistances R1 and R2 represent the transistor’s input and emitter resistances, respectively. By considering only these elements and the coupling capacitor, we can analyze how the signal behaves without the complexity of DC biasing. This approach is crucial for understanding the small-signal parameters and their impact on circuit performance.

Consider this process like focusing on a small portion of a large painting. Instead of analyzing all the vibrant colors (DC signals), you concentrate on the lines and shadows (AC signals) to understand how they affect the overall picture. This allows for a deeper understanding of the artwork without getting lost in the myriad of colors.

So, by considering this small signal equivalent circuit, let us see what are the now what are the conditions we are getting. First of all we do have vbe, base node it is grounded and emitter it is having some signal and that signal is incidentally vin. So, we can say vbe is essentially 0 – vin. So, and vin it is same as ve. So, in fact, ve is vin.

In this chunk, we dive into the voltage relationships in the small signal equivalent circuit. The base voltage (vbe) is effectively grounded, meaning it doesn't fluctuate with the input AC signal. Meanwhile, the emitter voltage (ve) is directly related to the input voltage (vin). This simplification allows us to express various voltages in the circuit as functions of the input voltage, establishing foundational equations necessary for analyzing the amplifier's performance.

Imagine a water tank system where a faucet (input voltage) controls the flow of water (signal). The base of the system is grounded (stable level), while the water level in the tank (emitter voltage) rises and falls with the faucet's adjustments. This relationship helps us clearly see how the faucet's position affects the tank level without worrying about other variables.

On the other hand gmv is now it is g(m)(–vin) and the current flow here was from collector to emitter.

Here, we analyze how current flows through the circuit by applying Kirchhoff's Current Law (KCL). KCL states that the total current entering a junction must equal the total current leaving. In this case, the voltage-dependent current source (gmv) plays a crucial role, and by expressing the relationship based on the input voltage (vin), we can better describe how the small signal behaves at the collector terminal. This sets the stage for deriving further characteristics and performances of the amplifier.

Think about a highway junction where cars (current) can either enter or exit. Just as KCL ensures that the number of cars entering a junction equals those leaving, our circuit analyzes how current flows and gets divided. This ensures we maintain balance in our electrical system while varying conditions.

So, this relationship it is helping us to get the voltage gain expression. So, what we are getting is that voltage gain of this circuit, it is (vo/vin). In fact, you may approximate that this 1 it is very small.

This chunk focuses on deriving the voltage gain expression from the previous KCL analysis. By relating the output voltage (vo) to the input voltage (vin), we find that the gain can be quantitatively expressed. The approximation simplifies our equations, indicating that the circuit behaves similarly to a common emitter amplifier, with the key difference being in the phase of the signals (in-phase vs out-of-phase). This relationship is critical for designing amplifiers that meet specific amplification criteria in practical applications.

Consider a microphone amplifying sound—what you speak (input voltage) translates into louder sound from the speakers (output voltage). The ratio of the loudness you hear to the sound you make represents the voltage gain. In this case, the mic's design is like an amplifier, where minor adjustments can lead to significant changes in output, demonstrating how efficient amplification can work.

So, to get the input impedance what we can say? If I stimulate the circuit by say vin, and then if we observe the corresponding current here iin, then if I take the ratio of that will be giving us the input resistance.

Input impedance in amplifiers affects how signals are processed. In this segment, we discuss the procedure to calculate the input impedance by applying a stimulus voltage (vin) and observing the resultant input current (iin). The ratio of voltage to current gives us the input resistance of the circuit, which helps us understand how much the circuit will influence incoming signals and how it will respond to different connected sources. This aspect is critical in designing circuits that match input and output stages for effective performance.

Imagine a sponge absorbing water (input impedance). The sponge's capacity to soak up water (input resistance) dictates how much you can pour before it can't take any more. In electronics, just like the sponge, the input impedance indicates how much signal an amplifier can handle without overflowing or distorting the signal.

So, basically we like to say that this voltage gain it will not remain so, nice.

When considering real-world scenarios where source resistance is present, a critical problem arises: attenuation. The gain between the input and output may remain unchanged, but when you factor in the source resistance within the circuit, it causes a drop in voltage levels arriving at the amplifier. This attenuation significantly impacts signal integrity and quality, making it essential to correctly analyze and design circuits to accommodate these factors for practical applications.

Think of a team relay race where one runner (source) must pass the baton (signal) to another. If the first runner is too slow (high source resistance), the baton reaches the next runner with less energy (attenuation), affecting the overall race time (signal gain). This example demonstrates how initial conditions can heavily influence outcomes, emphasizing the importance of understanding resistance in circuitry.

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