51.2.1. Effect of Source Resistance on Output Impedance

In this bias circuit we do have I which is given as 1 mA. So, for the time being in this numerical examples we are considering this emitter bias it is ideal. So, we do not have any associated conductance of this bias circuit, in later examples we will be replacing this ideal bias by resistive bias.

In the analysis of the common base amplifier, we assume an ideal emitter bias, meaning it doesn't have additional resistance that would affect the circuit's behavior. This is important for simplifying calculations in this example but might change in later examples where we look at more realistic scenarios with actual resistive biases.

Think of it as starting a car without checking if there are any issues under the hood. This initial assessment simplifies the journey (our calculations) but later, as we drive (analyze more complex examples), we might need to consider those underlying issues (the resistances) and how they affect performance.

So, if we see the effect of the input voltage applied here, even though this circuit is having very low input resistance. Since we are considering that R = 0. So whatever the input voltage you are applying here, almost the entire signal is reaching the emitter.

When the source resistance R is set to zero, it means there is no additional resistance limiting the current from the input source to the emitter. As a result, nearly all of the input signal voltage comes through to the emitter, allowing for effective amplification. However, this also reveals the importance of input resistance and how it impacts the signal strength; in real-world applications, some resistance can significantly affect performance.

Imagine a water pipeline delivering water to a plant. If the pipeline is wide open (R=0), the plant receives all the water (signal) it needs for growth (amplification). However, if you partially close the valve (introducing resistance), the plant gets less water, which means it cannot thrive as well—illustrating how resistance affects input performance.

Now, if I consider R = 0 as we have discussed earlier. So, this emitter node it is getting grounded and the impedance will be seen here it is only r. So, the net output resistance it is R = r. Now, R it is 3 k and r it is 50 kΩ.

With the source resistance R set to zero, the output impedance of the common base amplifier is essentially determined by the small signal parameters of the transistor (represented as r). Since R is much lower than r, the output impedance closely approximates r, which is a much larger value. This means the output is capable of handling more load without significant voltage drop, leading to efficient behavior in the circuit.

Consider a strong battery supplying power to more devices. If the battery has a high internal resistance (like high R), it might struggle to power all the devices effectively (output impedance). However, if the battery has a low internal resistance (like low r), it powers everything smoothly and efficiently—this is similar to how output impedance influences circuit functionality.

The input capacitance for this circuit can be examined, as it includes C from the emitter-base junction and C from the base-collector junction. If C is much higher than C, the input capacitance can be approximated by C.

Input capacitance plays a crucial role in determining how quickly the common base amplifier can respond to changes in the input signal. The total capacitance seen by the input signal can be influenced by the capacitances associated with the transistor. If the emitter-base capacitance is significantly larger than the base-collector capacitance, we can simplify our calculations by ignoring the smaller capacitance, leading to more efficient frequency response characteristics.

Think of input capacitance like the size of a bucket being filled with rainwater. If you have a large bucket (C) and a small cup (C), the time it takes to fill the bucket (react to input changes) is mainly determined by the bucket's size. This analogy helps illustrate how larger capacitance can dominate the circuit's ability to respond swiftly to signals.

Now, if I compare again the CE amplifier and CB, then and if I consider practical value of the source resistance, because of the input impedance being low, we are expecting significant attenuation of the signal as it travels from the input to the emitter.

When comparing the common base (CB) amplifier to the common emitter (CE) amplifier, if we take into account practical values of source resistance, we find that the low input impedance of the CB amplifier leads to significant signal attenuation. As the signal travels from the source to the emitter, the lower the input impedance, the more the signal gets reduced in strength, which compromises overall voltage gain.

Imagine trying to fill up a balloon with a thin hose vs. a thick one. With the thin hose (low input impedance), even if you have a powerful pump (source signal), not much air (signal strength) gets into the balloon (emitter). The thicker hose (higher impedance) would let in much more air, thus inflating it better—this demonstrates the impact of resistance on performance.

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